# Turning 3100 RPM in 6th at 80 MPH ??



## KretzJ (Jan 8, 2011)

Good morning everyone,

First, I tried a search on several keywords but couldn't find anything specific to my question... if this is a dupe please let me know and sorry upfront if that's the case.

So we bought our Cruze for a well-featured commuter car here in Phoenix... so far, with perhaps a 75/25 split of highway to street driving I'm averaging just 26.2 MPG (after 301 miles as of this morning). I'm a bit worried about that as I'm hearing about people getting far better results.

This morning I noticed I was turning 3100 RPM on the highway with the cruise control set spot-on at 80 MPH. When I drove six-speed Corvettes (MT), when I was at 80 MPH I was turning 1800-2100 RPM and netting 32-34 MPG. I know ratios are different between the two, but 3100 just seems abnormally high for a six-speed transmission.

Is 3100 RPM at 80 MPH normal for the Cruze? If so, no wonder I'm below expectations for fuel economy... ????

Thanks in advance everyone... 

-jk


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## SilverCruzer (Nov 30, 2010)

Kretz,
Yes, that is the result that myself and many others get as well. 
If you want the 36 mpg, you need to drive steady around 65. You can get close to 40 mpg with speeds 50 to 65 mph, but the economy declinces sharply after 70 mph.


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## KretzJ (Jan 8, 2011)

SilverCruzer said:


> Kretz,
> Yes, that is the result that myself and many others get as well.
> If you want the 36 mpg, you need to drive steady around 65. You can get close to 40 mpg with speeds 50 to 65 mph, but the economy declinces sharply after 70 mph.


Wow... 65 MPH on I-17 North here in Phoenix during morning rush-hour means I will be bug-splatter... 75 is barely keeping up with traffic and 80 means you're safe.

Turning 3100 RPM at 80 MPH seems wholly unnecessary... especially with six forward gear ratios...


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## SilverCruzer (Nov 30, 2010)

KretzJ said:


> Turning 3100 RPM at 80 MPH seems wholly unnecessary... especially with six forward gear ratios...


But is only 1.4 liters, and the car is north of 3,000 lbs. You won't get the same engine/road speeds as other cars with larger engines.
For your commute, a mid-size with 2.4 liter might have been a better choice. I know that I can actually do better than EPA MPG ratiing with a mid-size in that configuration, but with this car, very tough.


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## robertbick (Jan 1, 2011)

You can't compare RPMs of a small 4 cyl to that of a much larger V8 no matter the tranny. That's a no-brainer. No one drives 80mph around where I live unless you want constant speeding tickets. Like others have said, anything over 65mph and your MPG is going to the crapper.


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## Knightslugger (Jan 11, 2011)

the faster you go, the harder it is to push the car through the air. i remember watching a top gear where they drove a Bugatti Vyron to it's top speed, and the following year, tested their claim of an even higher top speed of 253 mph... but in order to achieve that higher top speed, *they needed an extra 350 hp to do it.

*and it was like, 17 mph higher... or something.

so top gear, and as slow as you go for MPGs.


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## KretzJ (Jan 8, 2011)

robertbick said:


> You can't compare RPMs of a small 4 cyl to that of a much larger V8 no matter the tranny. That's a no-brainer. No one drives 80mph around where I live unless you want constant speeding tickets. Like others have said, anything over 65mph and your MPG is going to the crapper.


Welcome to the west... I leave the house at 6:15 to head to work and I do between 65 and 80 the entire way.. 31 miles each way... on I-17 southbound. 

Wanna talk about lousy gas mileage... I'm keeping up with dually crew-cab pickups pulling trailers... at 80 MPH... I'm guessing they're feeling lucky when they get 10 MPG, even with diesel. 

I understand the physics behind the situation... c/D, weight, ratios, etc. I was just less than impressed with the highway results so far. In my 2002 V6 Isuzu Axiom XS, with 4-speed automatic transmission and RWD, with cruise set at 80 MPH I'm turning 2400-2600 and the instant MPG readout is telling me 24-25... and that vehicle is closer to 4,500 pounds.


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## Knightslugger (Jan 11, 2011)

instant isn't accurate. heck, even the average isn't spot on.

it's a 1400cc engine. Kawasaki makes a Concours 14, and a supersport 1400 (Zx14). both motorcycles use a 1400cc engine! it is a CRAZY SMALL ENGINE, so it has to do more work to produce enough power to overcome things that slow it down.

it makes peak torque very early, and HP isn't great. what you guys need is a motor that makes peak power later in the RPM range. a peaky motor is what you're going to get. then you'd be complaining about how there's no power beboping around town doing 30 mph... when you're dealing with a set displacement engine, it's a tradeoff. always. more power down low means less power up top, and vice versa. it will always be.


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## SilverCruzer (Nov 30, 2010)

Ironically for some (those buying this for long commutes), this car and some other compacts have better economy with city driving than highway. Your overall DOLLAR savings is much greater switching from a larger car to a compact for heavy city driving than making that switch for interstate driving.

I also used to live out west, but in higher densitities so my 40 mile commutes resembled city driving more than "Open Arizona" driving .


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## KretzJ (Jan 8, 2011)

So, I've never pretended to be a physicist or an automotive or aerodynamic engineer, but I believe that once two vehicles are in motion at a set speed (say 80 MPH), and factoring out (unfortunately) items such as rolling resistance, the amount of energy required to keep that vehicle at that speed is (mostly) a function of aerodynamic drag and weight.

In my past I drove a 2000 Corvette Convertible, with a standard 345 HP LT1 V8 and 6-speed manual transmission. I now drive the 2011 Cruze with 138 HP turbo 4-Cyl and 6-speed automatic transmission.

Specs for both cars (c/D, weight, Axle Ratio) are below. Weight is almost the same while the Corvette certainly has an advantage on drag (the quoted .29 is suspicious as I believe that is the rating for the coupe.. I seem to remember the convertible was dirtier than the coupe but I can't find anything in my quick searched to back this up). What I notice first was the axle ratio difference... 3.87 for the Cruze versus 3.14 for the Vette.

While the aero is different, the weight is almost a wash, but the Cruze sporting a 3.87 gear is NOT going to help out RPM's at highway speeds.

Anyone know if someone's planning aftermarket gear sets for the Cruze? 

And my example above regarding my Isuzu Axiom... the 24-25 is proven via actual calculations based on miles and gallons... I referenced the "instant" reading for simplicity but I've done the data gathering and math. Sad that I can get (figure downside) 23 MPG in a 4,500 pound CUV and 26 in a Cruze...
*
2011 Chevrolet Cruze
*- .31 c/D 
2010 Chevrolet Cruze - Top Speed

- 3,200 pounds curb weight (estimated for LTZ, LT is 3,102)
2011 Chevy Cruze Specs and Features | Chevrolet

- 3.87 - Axle Ratio
2011 Chevrolet Cruze Standard Equipment and Specs


*2000 Chevrolet Corvette
*- .29 c/D

- 3,217 pounds curb weight
2000 Chevrolet Corvette Standard Equipment and Specs

- 3.15 - Axle Ratio (3.42 for the auto transmission)
2000 Chevrolet Corvette Standard Equipment and Specs


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## 70AARCUDA (Nov 14, 2010)

...actually, (at least for the Canadian LTZ models) GM publishes an _estimated_ curb weight of 3,177 lbs; so, adding in 170 lbs for a driver, that adds up to almost 3,350 lbs.

...below are the _estimated_ curb weights published in the 2011 Canadian Cruze brochure:

- LS...3056 lbs / 1386 kg
- Eco..3009 lbs / 1365 kg
- LT...3146 lbs / 1427 kg
- LTZ..3177 lbs / 1441 kg

...I believe that cD = 0.31 number is for the Eco model -- not the other models -- with it's lowered body stance, shuttered grill opening, and rear deck spoiler aerodynamic enhancements. 

...this older Canadian article puts the Eco cD at 0.298: http://www.nationalpost.com/Preview+2011+Chevy+Cruze/3843733/story.html

...I'm guessing a cD ~ 0.33 for the standard Cruze.


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## KretzJ (Jan 8, 2011)

70AARCUDA said:


> ...actually, (at least for the Canadian LTZ models) GM publishes an _estimated_ curb weight of 3,177 lbs; so, adding in 170 lbs for a driver, that adds up to almost 3,350 lbs.
> 
> ...below are the _estimated_ curb weights published in the 2011 Canadian Cruze brochure:
> 
> ...


Negative... the .31 is for the Standard Cruze. If you follow the link I provided the material is dated 2009, long before the ECO model was announced. 

I think the ECO is actually under .30 (YEP, .298 according to GM: GM News - United States - News)

Also, the weight of the Corvette was curb weight... no driver included... so the weights were apples-to-apples.


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## 70AARCUDA (Nov 14, 2010)

KretzJ said:


> Negative... the .31 is for the Standard Cruze. If you follow the link I provided the material is dated 2009, long before the ECO model was announced.
> 
> I think the ECO is actually under .30 (YEP, .298 according to GM: GM News - United States - News)
> 
> Also, the weight of the Corvette was curb weight... no driver included... so the weights were apples-to-apples.


...Thanks! I stand corrected.

...however, I get 0.33, not 0.31, when I divide 0.9 into 0.298 (0.331 = 0.298/0.9) following the statement below quoted from that GM release:

_"As a result of the aero enhancements, aerodynamic drag was *reduced by 10 percent* over a non-Eco model, with a coefficient of drag of *0.298*."_

...and, I wonder how close those Canadian weight 'estimates' are to real-world values, ie:

Eco = 3,009 vs. 3,002 (CA)
1LT = 3,223 vs. 3,146 (CA)
LTZ = ?,??? vs. 3,177 (CA)

...the LTZ is stated to have about 150 lbs of additional sound deadening materials over the LT models, so the numbers-game varies.


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## KretzJ (Jan 8, 2011)

I agree... I think the weight and aero are "almost" a wash...

I'm still stuck on why the Cruze has such a tall axle ratio... 3.87? I'm not sure if it's the same when comparing FWD to RWD... I'm assuming it would be, except perhaps a bit less parasitic loss because there's no u-joints/driveshaft/torque tube in the way.

I was joking before... now I'm wondering if someone WILL come up with aftermarket gear options for the Cruze. Even bumping that down to 3.73 would help lower top-end RPM's without sacrificing first gear acceleration that much. With my ratio of highway driving to city, I'd jump at 3.73.


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## 70AARCUDA (Nov 14, 2010)

KretzJ said:


> I'm not sure if it's the *same* when comparing FWD to RWD...


...it's not, drag racing analyses show FWD power requirements are different than RWD requirements, due to weight transferance characteristics.


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## shawn672 (Oct 31, 2010)

KretzJ said:


> I was joking before... now I'm wondering if someone WILL come up with aftermarket gear options for the Cruze. Even bumping that down to 3.73 would help lower top-end RPM's without sacrificing first gear acceleration that much. With my ratio of highway driving to city, I'd jump at 3.73.


It's been done before but it's super expensive. I think Bog Warner made a cobalt 6 speed transmission but it was over $5,000 if not more. If one does come out, it won't be worth the price


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## Knightslugger (Jan 11, 2011)

Final Drive shouldn't be that hard to replace...


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## 70AARCUDA (Nov 14, 2010)

Knightslugger said:


> *Final* Drive shouldn't be that hard to replace...


...did you mean *AXLE*, as in the old context: 

*FINAL = GEAR × AXLE*

2.887 = 0.740 × 3.870

...but, I agree the axle gearing shouldn't be too hard (or costly) to change.


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## Knightslugger (Jan 11, 2011)

sure, why not... Final Drive, final gear, final Axle... all the same thing if you ask me. "The last two gears that see the wheel" ratio.


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## 70AARCUDA (Nov 14, 2010)

...3,100 RPM at 80 MPH sounds correct:

*MPH = [60/(G·A)]·[RPM/rpm]* = [60/(0.746·3.87)]·[3100/802] = *80.3 MPH* ...or, 25.9 MPH / 1K RPM

...where:

G = 6th gear ratio -- 0.746:1
A = Axle ratio -- 3.87:1
RPM = Engine revolutions-per-minute -- 3,100 RPM
rpm = Tire revolutions-per-mile -- 802 (P225/45R18)

...which _is_ rather "buzzy" -- _if_ that's the point you're making.

...lowering the axle ratio from 3.87:1 down to 3.73:1 reduces engine speed by about 124 RPM at 80 MPH:

*RPM* *= [MPH·rpm]·[(G·A)/60] =* [80·802]·[(0.746·3.73)/60] *= 2,976 RPM* ...or, 26.9 MPH / 1K RPM

...the "key" to high fuel economy is *NOT* to drive at 80 MPH...(wink,wink)


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## CRUISE-CRUZE (Nov 8, 2010)

Knightslugger said:


> the faster you go, the harder it is to push the car through the air. i remember watching a top gear where they drove a Bugatti Vyron to it's top speed, and the following year, tested their claim of an even higher top speed of 253 mph... but in order to achieve that higher top speed, *they needed an extra 350 hp to do it.*


1001 HP, who is going to buy a car like this?


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## 70AARCUDA (Nov 14, 2010)

...and HP and MPG are _inversely_ proportional.

...the steeper one goes UP the quicker the other goes DOWN.


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## RS LTZ (Jan 5, 2011)

I just ran an excel sheet using the formula 70AARCUDA gave earlier. 

Assuming the transmission stays in 6th gear, every 5MPH increase, increases the engine RPM's by 193.

Use that information when you think of speeding up 5-10 MPH, it really does have a significant effect on mileage.


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## racer114 (Nov 7, 2010)

This is the exact problem I've had since my tranny reprogram from the service bulletin. I'm now at 2550+ at 70mph. I haven't had it to 80 much, but 3100 sounds about right. I'm also getting around 26mpg now at this speed. Before it was 35.6mpg.


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## spacedout (Dec 7, 2010)

I agree 3000 rpm at 80 sucks though especially for a 6speed. my 4speed auto 2004 cavalier runs the exact same RPM at those speeds with very similar MPG(28mpg). 

All of the OP comparisons are with cars with much more torque(even the Isuzu Axiom). The Cruze uses such low gearing so they can not only get the car up to speed quicker/safer but also it doesn't fall on its face once it hits a slight incline/grade at speed. 

The Cruze only makes 148lb-ft torque, a 2000 corvette LS1(not LT1 like the OP stated) makes more than 330+ Torque, which means at any given RPM it makes at least twice the power. Of course with that much more power it can be geared to run much lower RPM at 80mph without sacrificing driveability.


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## Knightslugger (Jan 11, 2011)

CRUISE-CRUZE said:


> 1001 HP, who is going to buy a car like this?
> YouTube - Top Gear - Bugatti Veyron top speed test - BBC


that's the one. quite ridiculous actually. extra 100 MPH required 750 more HP.


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## KretzJ (Jan 8, 2011)

spacedout said:


> I agree 3000 rpm at 80 sucks though especially for a 6speed. my 4speed auto 2004 cavalier runs the exact same RPM at those speeds with very similar MPG(28mpg).
> 
> All of the OP comparisons are with cars with much more torque(even the Isuzu Axiom). The Cruze uses such low gearing so they can not only get the car up to speed quicker/safer but also it doesn't fall on its face once it hits a slight incline/grade at speed.
> 
> The Cruze only makes 148lb-ft torque, a 2000 corvette LS1(not LT1 like the OP stated) makes more than 330+ Torque, which means at any given RPM it makes at least twice the power. Of course with that much more power it can be geared to run much lower RPM at 80mph without sacrificing driveability.


But, the torque figures you're quoting are PEAK torque. Once a vehicle is in motion (held to a constant 80 MPH), peak torque isn't in play as the engine running at a lower RPM... especially the Corvette under 2,000 RPM. I do get your point, however, and I see the overall relevance.

What I'm concerned about are the comments (one posted above in this thread, in fact) where people who have had their ECM flashed are now experiencing what I'm seeing from the factory... higher RPM and reduced gas mileage.

Don't get me wrong... I'm still happy with my Cruze, but living here and driving 65 on the highway aren't compatible thoughts... 70 is the unofficial minimum in the SLOW lane... and no, I'm really not making this up. Where I live (I-17 and Carefree Highway for you Phoenix area residents)... the legal highway speed is actually 75.


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## KretzJ (Jan 8, 2011)

racer114 said:


> This is the exact problem I've had since my tranny reprogram from the service bulletin. I'm now at 2550+ at 70mph. I haven't had it to 80 much, but 3100 sounds about right. I'm also getting around 26mpg now at this speed. Before it was 35.6mpg.


Hey Racer... have you talked to your dealership about your findings, post FLASH? I wonder what their response would be???

Honestly, I wonder what a Cruze would show on the EPA test cycle with the new transmission FLASH... I'll bet it's lower than the current window sticker.


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## racer114 (Nov 7, 2010)

Not yet. I take in in Monday when the stock markets are closed and I have a day off.


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## Spyder (Dec 26, 2010)

CRUISE-CRUZE said:


> 1001 HP, who is going to buy a car like this?


I would if I had the million it takes to buy the thing and the ten grand a year I'm sure it take to insure it.


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## 70AARCUDA (Nov 14, 2010)

...looks like the 6T40 6-speed automatic in the new 2012 Buick LaCrosse will have _different_ axle gearing (*2.64:1* vs. 3.87:1) than the 6T40 in our Cruzes:

AXLE = *2.64:1*, so: Final = (2.64×0.75) = *1.98:1*
_6th = 0.75
_5th = 1.00
_4th = 1.45
_3rd = 1.91
_2nd = 2.96
_1st = 4.58
_REV = 2.84

...the above numbers are from this GM news release: Buick News - United States - News

...and, that's gearing for a 3,835 lb vehicle with 2.4L (182 hp) engine.

...with _that_ gearing, the 1.4LT Cruze engine would be only turning 2,117 RPM at 80 MPH(!):

MPH = [60/(G·A)]·[RPM/rpm] = [60/(0.75×2.64)]·[2117/802] = 80.0 MPH


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## sedanman (Dec 10, 2010)

Knightslugger said:


> that's the one. quite ridiculous actually. extra 100 MPH required 750 more HP.


That's not ridiculous. That's called the law of diminishing returns. It applies to a lot more than cars as well.


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## 70AARCUDA (Nov 14, 2010)

...the HP required to shove something through air increases as a SQUARED function of VELOCITY! 

Aerodynamic Drag - Craig's Website at Backfire.ca


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## sedanman (Dec 10, 2010)

KretzJ said:


> Don't get me wrong... I'm still happy with my Cruze, but living here and driving 65 on the highway aren't compatible thoughts... 70 is the unofficial minimum in the SLOW lane... and no, I'm really not making this up. Where I live (I-17 and Carefree Highway for you Phoenix area residents)... the legal highway speed is actually 75.


You should not be buying an econobox if you need performance for daily driving. They really should have offered a Cruze SS with a direct injected turbo 2.0 like the Chevy Cobalt had. 260hp and 260 lb ft of torque.

Really is it so hard to make a nice interior, nice exterior styling AND a high performance drivetrain without breaking $25k base price?


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## Knightslugger (Jan 11, 2011)

then they wouldn't call it a cruze...


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## montgom626 (Jan 16, 2011)

I drove an 2010 AUDI A4 2.0 T (loaner). Ran it at 80 mph sucked fuel at 25 mpg. 
What did I expect? The faster I go, the more fuel it uses. I have never found a car that does the opposite. Speed costs money, how fast do you want to go and how much do you want to spend? 
RGM


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## racer114 (Nov 7, 2010)

The point here is that the Cruze isn't marketed as an "econobox". It is actually a very nice car. I just think it may be underpowered and the fuel economy won't be what GM says due to it working too hard. I also live in an area where the average freeway speed is 80 or stopped, one of the two, and you have to be able to go back and forth between the two to avoid getting run over buy a F-250 or a Suburban. T


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## robertbick (Jan 1, 2011)

racer114 said:


> The point here is that the Cruze isn't marketed as an "econobox". It is actually a very nice car. I just think it may be underpowered and the fuel economy won't be what GM says due to it working too hard. I also live in an area where the average freeway speed is 80 or stopped, one of the two, and you have to be able to go back and forth between the two to avoid getting run over buy a F-250 or a Suburban. T


GM does not state the fuel economy, the EPA does that. If you didn't realize that your MPG's would not come close to the EPA figures stated when you are driving 80MPH, that is not GM's or the EPA's fault. Why do people always try to pass the blame on others???


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## spacedout (Dec 7, 2010)

robertbick said:


> GM does not state the fuel economy, the EPA does that. If you didn't realize that your MPG's would not come close to the EPA figures stated when you are driving 80MPH, that is not GM's or the EPA's fault.



I agree, EPA hwy MPG are NOT rated at constant 80mph speeds. 

Driving at 78-80mph in a 2010 GMC terrain(2.4 ecotec 6speed auto) I was getting 23mpg over a 1,300 mile trip. The GMC is rated at 32mpg hwy but at those speeds that little motor had to work really hard. I have got 33mpg with the GMC, but it was at a more leisurely 55-65mph speeds. 

Looking at the map on the link below you will see most states interstate hwys in the US are 65mph or 70mph. Guess thats why the EPA doesn't rate cars at higher speeds on the Hwy. 

Map of maximum posted speed limits on rural interstates


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## SilverCruzer (Nov 30, 2010)

robertbick said:


> GM does not state the fuel economy, the EPA does that. If you didn't realize that your MPG's would not come close to the EPA figures stated when you are driving 80MPH, that is not GM's or the EPA's fault. Why do people always try to pass the blame on others???


Because the EPA rating is more often than not, what the MFG says it is. The MFG tests it according to guidelines, submit to EPA for approval, and in most cases, the EPA uses those figures without testing it themselves, rubber stamp if you will.
Somewhere in these postings a person gave me an EPA link on this methodology. I would do the same if I can find it.


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## spacedout (Dec 7, 2010)

SilverCruzer said:


> The MFG tests it according to guidelines, submit to EPA for approval, and in most cases, the EPA uses those figures without testing it themselves, rubber stamp if you will.



found a bit of info on the new method they use for testing, yes you are correct the EPA only tests 10-15% of the cars to Confirm MFG results. 

How Vehicles Are Tested

Regulatory Announcement: EPA Proposes New Test Methods for Fuel Economy Window Stickers | Fuel Economy | US EPA

Your Mileage Will Still Vary


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## montgom626 (Jan 16, 2011)

Anyone notice that summer gas has more BTU's and gets better mileage?


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## 70AARCUDA (Nov 14, 2010)

...yup, just about every *ecomodder* driver can tell you the extact differences in BTU's (wink,wink).


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## montgom626 (Jan 16, 2011)

70AARCUDA said:


> ...yup, just about every *ecomodder* driver can tell you the extact differences in BTU's (wink,wink).


Oh yeah, I know what you are talking about

I think I can get 10% better mpg in the summer. Winter fuel sucks and the cold weather with extended warm up is bad too


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## booradley (Dec 20, 2010)

I too had the re-flash and I just looked at my MPG and it is 23 MPG. Pre re-flash it was 28. Now it is only 19 degrees out and I am short tripping. Plus I had driven on the turnpike at 75-80 MPH which impacted the score. I just wanted 28 out of this car and I get that easily with the new Ford Focus 4 cylinder I drive for the local Ford Dealers, on their new car swaps. A new Focus with zero miles gets much better mileage than this Cruze.


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## shawn672 (Oct 31, 2010)

I'm pretty certain the new flash killed the mpg. I'm also averaging 24mpg combined highway/city 50/50 mix but I am not babying the throttle either so I feel like it's normal for my driving style.


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## racer114 (Nov 7, 2010)

Silvercruzer,

Exactly.

Roberbick,

I know this. It doesn't matter. My other vehicles do very well at 80.


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