# MPG drop



## MPGeez (Oct 8, 2015)

All of a sudden my MPG has dropped by 10mpg at cruise. I used to get 60 at 50mph. Now it's 50 at 50. Only major change was a new set of Walmart Goodyear assurance tires in stock size. 
Is that the probable cause? We're the Oem Tires that much better?! Or is there a checklist to go through to find an issue? I'm at 55k miles. Mpg got worse at 45k along with the tire swap.
Any ideas?


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## mechguy78 (Jun 6, 2016)

From personal experience Walmart sets the tire pressures low. They have set pressures for each rim size. I'd take a look there if you haven't already.


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## MPGeez (Oct 8, 2015)

mechguy78 said:


> From personal experience Walmart sets the tire pressures low. They have set pressures for each rim size. I'd take a look there if you haven't already.


Oh yeah first thing I do is set my own pressure.
That was 10k miles ago so I've pumped it up a few times.


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## jblackburn (Apr 14, 2012)

Have you seen this?

http://www.cruzetalk.com/forum/201-...-mileage-loss-fix-clean-map-sensor-again.html


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## MPGeez (Oct 8, 2015)

jblackburn said:


> Have you seen this?
> 
> http://www.cruzetalk.com/forum/201-...-mileage-loss-fix-clean-map-sensor-again.html


Haven't seen that. That's job #1 now.


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## TX CTD (Oct 24, 2014)

My wife had a set of those put on, I wasn't happy about it but the car needed tires and one got hole too big to patch and it was Sunday. 

I've been pretty impressed with them so far. They are quite and got some of the mpgs back that we lost with the last set of off brand tires she had. No problems in the rain at speed either. I run them at 45psi and had them rotated yesterday and they came back at 37psi. They are a tad bit more comfortable on the half mile of bad road we have to take everyday, but the 50 mile avg was about 5mpg from where I was expecting it to be.


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## VtTD (Nov 5, 2013)

Someone posted something a while ago about cleaning the MAF, or whatever it's called in our cars.



MPGeez said:


> Oh yeah first thing I do is set my own pressure.
> That was 10k miles ago so I've pumped it up a few times.


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## JDH (Dec 24, 2014)

It is Walmart. Did they put the Assurance Fuel Max back on or plain Assurance?


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## MPGeez (Oct 8, 2015)

JDH said:


> It is Walmart. Did they put the Assurance Fuel Max back on or plain Assurance?


I bought the non fuel max.
I didn't think tires could add 10mpg and still don't.
I cleaned my map sensor and will monitor it. I'll know on my first drive if that was it.


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## JDH (Dec 24, 2014)

Wife bought tires for hers not knowing the difference. Lost an average of 8 mpg.


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## CruzeDan (May 17, 2015)

The tires that originally came with the diesel Cruze will yield you the best milage. You will lose milage with just about any other tire due to weight and tread compound. Matter of fact the OEM TPC rated FuelMax tires on our cars are different then the retail version. I can guarantee the tires are the cause of your lost milage.


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## obermd (Mar 3, 2012)

CruzeDan said:


> The tires that originally came with the diesel Cruze will yield you the best milage. You will lose milage with just about any other tire due to weight and tread compound. Matter of fact the OEM TPC rated FuelMax tires on our cars are different then the retail version. I can guarantee the tires are the cause of your lost milage.


Not all of it. The OEM Fuelmax tires only add about 3-5% to the car's efficiency. Something else is at work here and I suspect it's the MAF sensor as we've seen this sensor cause up to 10% drop in efficiency when dirty.


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## Cruz15 (Mar 17, 2016)

According to CR the Cinturato P7 All Season Plus should give better fuel mileage than the Assurance Max.
It is rated as the lowest rolling resistance tire in our category of tires. They weigh exactly the same. Dia is .2" larger, I would have to put a pair on and test.


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## MPGeez (Oct 8, 2015)

I saw about 3mpg from cleaning the MAP sensor. I'll try cleaning the MAF.
I'm noticing a larger than normal swing in mpg at cruise. I'm seeing 5-15 mpg swings on mostly flat road now.
That is not a tire problem to me.


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## TX CTD (Oct 24, 2014)

That's exactly a tire problem. They don't roll quite as well and they weigh more so it takes more power to turn them. 

That could also have a little something to due with the weather. I don't notice it much in the CTD, but my Beetle TDI looses a noticeable amount of power in the afternoon when its 100 degrees. It only started with 90hp so even 5hp is a significant drop. The CTD is deleted and tuned and i don't really need all the power it has to get around.


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## sailurman (Sep 29, 2016)

CruzeDan said:


> The tires that originally came with the diesel Cruze will yield you the best milage. You will lose milage with just about any other tire due to weight and tread compound. Matter of fact the OEM TPC rated FuelMax tires on our cars are different then the retail version. I can guarantee the tires are the cause of your lost milage.


That is an opinion.
I'm telling you. the Pirelli Cinturato Strada AS from Discount Tires are wonderful, ride better, are quieter, and are delivering slightly better mileage than the OEM Goodyears did. This is my second set of Pirellis. My first was on my Sonic 1.4T Hatch, and they really woke that car up in the corners. I believe these are the P7s, but numbered for Discount only.


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## Cruz15 (Mar 17, 2016)

sailurman said:


> That is an opinion.
> I'm telling you. the Pirelli Cinturato Strada AS from Discount Tires are wonderful, ride better, are quieter, and are delivering slightly better mileage than the OEM Goodyears did. This is my second set of Pirellis. My first was on my Sonic 1.4T Hatch, and they really woke that car up in the corners. I believe these are the P7s, but numbered for Discount only.


According to CR they have less rolling resistance than the fuel max. (Pirelli Cinturato)


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## TX CTD (Oct 24, 2014)

The OEM Fuel Max is a different tire from the average off the shelf Fuel Max, it weighs 17lbs.


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## Cruz15 (Mar 17, 2016)

TX CTD said:


> The OEM Fuel Max is a different tire from the average off the shelf Fuel Max, it weighs 17lbs.


Wow thats seriously crazy.
Is it built differently or just less tread to start or both?


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## jblackburn (Apr 14, 2012)

Cruz15 said:


> Wow thats seriously crazy.
> Is it built differently or just less tread to start or both?


Both. 2/32" less tread, slightly narrower, and lighter construction.


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## MPGeez (Oct 8, 2015)

TX CTD said:


> That's exactly a tire problem. They don't roll quite as well and they weigh more so it takes more power to turn them.


You must not understand what I said.
Cruising I can watch the mpg fluctuate from 60-45 and all over in between. 10k miles ago the fluctuation was 57-63 in the same circumstances.
That huge variance is NOT attributable to rolling resistance (a constant). It's a variable. There is something causing the PCM to add and subtract fuel in larger increments. Load isn't fluctuating while this is happening.
I believe I have a dirty or faulty sensor. The tires may drop he mpg a few but they won't cause that.


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## mkohan (Dec 19, 2015)

Did you have the emission recall done recently? I had the recall done at the beginning of the month. Prior to recall, hand calculated mileage was about 1.5-2 MPG better than DIC mileage. Today I filled up for the second time since recall work. Hand calculated mileage 5 MPG better the DIC. On the first tank I noticed a difference but chalked it up to the manual regen. Maybe there is a calibration problem when the computer is reset during recall work. just wondering


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## TX CTD (Oct 24, 2014)

If you think a 5lb heavier tire isn't a variable then I can't help. It could also be sensors or wind or the fuel mix or the season of the year.


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## MPGeez (Oct 8, 2015)

TX CTD said:


> If you think a 5lb heavier tire isn't a variable then I can't help. It could also be sensors or wind or the fuel mix or the season of the year.


The heavier tire will impact mpg, I'm not disputing that. Rolling resistance is a constant ONCE APPLIED. It will not cause mpg to VARY more than before but will cause it to LOWER.
There is no wind. I watch both directions and average them to get flat terrain mpg, I've tried multiple brands of fuel and I've gone through all four seasons over the 10k miles I've had the issue.
Again this has been going on for 10k miles. It's not a simple variable, im not tooting my own horn but I'm rather knowledgable with vehicles. 
I've narrowed it to a mechanical issue (not a tire problem!).
I'm looking for Cruze diesel specific oddities that can cause this.


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## MPGeez (Oct 8, 2015)

mkohan said:


> Did you have the emission recall done recently? I had the recall done at the beginning of the month. Prior to recall, hand calculated mileage was about 1.5-2 MPG better than DIC mileage. Today I filled up for the second time since recall work. Hand calculated mileage 5 MPG better the DIC. On the first tank I noticed a difference but chalked it up to the manual regen. Maybe there is a calibration problem when the computer is reset during recall work. just wondering


No, haven't done the recall. But I am calculating actual mpg. And it correlates quite well to the Dic.


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## Cruz15 (Mar 17, 2016)

15 mpg is pretty big range. Did you check all four tires to make sure they are all exactly the same tire? Could be voltage on something in the fuel system? Have you had any cel s from the fuel system before? Has dealer checked for wondering voltages on the critical sensors like maf, map and oxygen sensor? Im stabbing here. Its the stuff I would look at anyway.
Have you checked how your oil pump is running how about oil pickup?


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## starspangled6.0 (Apr 16, 2016)

Perhaps you've been watching the MPG drop during a regen cycle?


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## obermd (Mar 3, 2012)

Cruz15 said:


> According to CR the Cinturato P7 All Season Plus should give better fuel mileage than the Assurance Max.
> It is rated as the lowest rolling resistance tire in our category of tires. They weigh exactly the same. Dia is .2" larger, I would have to put a pair on and test.


The FuelMax give better fuel efficiency. I rode on both.


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## MPGeez (Oct 8, 2015)

Finally took it to the dealer after a couple CELs. They supposedly did the PCM update/recall and told me the mpg should be fixed and the codes were part of it. They also said they did a manual regen.

On the drive home I had a regen immediately. The next morning I popped the same cel.
Great job guys. 

After doing research the code for the EGT sensor has a lot to do with the fuel trims. And is what I keep popping now. Each time I clear it the mpg drops. If it's set the mpg is 75% better.

Ordered a new EGT #2 sensor to fix it myself since the dealership can't do a simple job and doesn't even do what they say.
Will possibly report back in a week with progress.
Just goes to show you that even in a new to you unknown vehicle the best mechanic is still yourself. Just wish this site had more info but when the car is rare and most aren't diy I guess it's expected.
If only there was a reasonably priced self tuning/datalogging setup this would have been over a long time ago.


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## Taxman (Aug 10, 2017)

TX CTD said:


> If you think a 5lb heavier tire isn't a variable then I can't help. It could also be sensors or wind or the fuel mix or the season of the year.


Tires are only variables when you put them on or take them off. 

While they're on the car and maintaining steady inflation pressure, they're a constant. 
Or, in other words, OP is not running alongside the car at 50mph switching tires when his constant velocity fuel consumption is changing substantially.


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## oldestof11 (Apr 3, 2016)

All of this talk about tires...

I went from stock setup getting 45ish mpg @ 80mph to 36mpg @ 80mph with 245/35/20s on Nankangs. 

Tires may be partly to blame but not all of it.


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## raffmanlt (Oct 4, 2014)

Trying to figure mine out also. I have had a steady decline for a long time. Before and after tire change. Wierd mine didnt have the special tires when I bought it new. Currently avg of 30 mpg instead of 40 mpg us. I just cleaned map and it really needed it but doubt that was the total difference also. Just took off underside plastic havent put back on yet wonder what that will do to mpg?


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## Patman (May 7, 2011)

New tires overall will kill your MPG for a little while till broken in(drag created by new tire tread unlike the "old tires" ) given all else is OK.


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## AlbertaDiesel (Aug 30, 2017)

MPGeez

You mentioned in one post you were planning to clean the MAF. Did you ever end up doing it?


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## MPGeez (Oct 8, 2015)

AlbertaDiesel said:


> MPGeez
> 
> You mentioned in one post you were planning to clean the MAF. Did you ever end up doing it?


I took out the air filter and checked over the MAF. Both looked relatively new inside still. The MAF had zero buildup and was so clean inside there was no reason to do it.


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