# Odo Miles LS vs LTZ vs Eco



## cruzeman (Mar 13, 2011)

WOW very interesting..........


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## limited360 (May 6, 2011)

This is not how it works, the actual tire size is programmed into the BCM from my experiences...(if not tire size but number of ABS tooth counts per rotation. They couldn't legally sell a car knowing the miles are being misrepresented.


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## 70AARCUDA (Nov 14, 2010)

limited360 said:


> This is not how it works, the actual tire size is programmed into the BCM from my experiences...(if not tire size but number of ABS tooth counts per rotation. They couldn't legally sell a car knowing the miles are being misrepresented.


...yes, (1) it's measured via ABS counts per rotation on front wheel; and, (2) they're allowed a "5% tolerance" on both MPH and "milage" values.


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## Quazar (Apr 28, 2011)

limited360 said:


> This is not how it works, the actual tire size is programmed into the BCM from my experiences...(if not tire size but number of ABS tooth counts per rotation. They couldn't legally sell a car knowing the miles are being misrepresented.


As 70AARCUDA mentioned, they have a tolerance, which they are within. They don't reprogram the computer for every tire size if they fall within tolerance. As for "legally", as long as you are within specifications there is nothing "illegal" about it. Less then one percent difference is completely tolerable for wheels.

That it counts rotations, creates the slight difference, Car and Driver's data confirms that the mileage is off. http://www.caranddriver.com/var/ezf...lication/aa4d3fb0ba817b1b2471aedffb210f44.pdf

I tried to measure myself and my data seemed to confirm this on the eco, from there I just did the math. Its not the first time I have seen this on a vehicle.


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## robertbick (Jan 1, 2011)




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## Jim Frye (Mar 16, 2011)

Even tires of the same stated size, say P225/50R17, from different manufacturers, or different models from the same manufacturer, can have different revolutions per mile.  Hence the tolerance that Mr. Cuda detailed. 

In the olden days, when speedometers were driven off of the output shaft of the transmission, you could purchase "correction" gear boxes that would bring the speedo back into an accurate range as a result of different tire sizes.


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## 88ls1blazer (May 3, 2011)

GM cals the modules of the car with the revolutions per mile of the specific tire. I know a little bit about tuning and have seen this first hand with numerous GM cars, but the one that comes to mind is the pontiac G8. They were available with an 18" wheel or a 19" wheel, the only difference in calibration were the speedometer segment of the tunes depending on which wheel you had.


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## FoSHO99 (May 16, 2011)

Yes, the easiest way to confirm would be to contact trifecta or ask the differences there. I know we always changed it based on the gearing and when you got into different tire sizes you would use a tire size calculator to adjust from there, but i always used the same size tires or did not care about 1-4mph differences as well.


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## toydriver00 (May 4, 2011)

I have an LT. I have not gone into a tire size calculator to figure out things. However, I can tell you that I was noticing that radar'd street signs state that I am doing 37 when my car is doing 35. I noticed that on several of those signs. So, I asked a cop friend of mine to do radar me coming down a road at 35 mph. My car was doing 37 again when it read 35. So, I am not going to complain about it because for every 37 miles I drive, my car only registers 35 of them on the odo. I guess if it were the other way, I would complain.


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## jsscooby (May 11, 2011)

toydriver00 said:


> I have an LT. I have not gone into a tire size calculator to figure out things. However, I can tell you that I was noticing that radar'd street signs state that I am doing 37 when my car is doing 35. I noticed that on several of those signs. So, I asked a cop friend of mine to do radar me coming down a road at 35 mph. My car was doing 37 again when it read 35. So, I am not going to complain about it because for every 37 miles I drive, my car only registers 35 of them on the odo. I guess if it were the other way, I would complain.


the only downside to that, is it looks like you're speeding.


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## jaygeo1 (Nov 10, 2010)

*So, let's see.......*

If my tire pressures are over spec, my speedo reading will show *slower *than it should and visa versa....and then as my tire tread wears down........  No wonder there are tolerences....


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## toydriver00 (May 4, 2011)

jsscooby said:


> the only downside to that, is it looks like you're speeding.


 
I keep that in mind when I am driving. I learned my lesson some time ago about speeding. I rarely drive over the limit. Most cops also have a certain tolerance for speed. In most cases it is 10%. So, if I drive 36 but really 38, I am ok.


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## 70AARCUDA (Nov 14, 2010)

...interesting, GM is "known" for speedometers "under-stating" MPH.

...here, a local speedometer shop that calibrates the AZ Highway Patrol cars (Fords and few "old" Chevys) states the GM electronic speedometers _consistently_ show 2 MPH above what you're actually doing at all speeds, ie: there's a 'consistent' offset of +2 MPH from 10 MPH all the way up to 80 MPH.

...and, my *ScanGaugeII*™ _confirms_ that offset in both our 2009 Vibe and 2011 Cruze.


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## Quazar (Apr 28, 2011)

70AARCUDA said:


> ...interesting, GM is "known" for speedometers "under-stating" MPH.
> 
> ...here, a local speedometer shop that calibrates the AZ Highway Patrol cars (Fords and few "old" Chevys) states the GM electronic speedometers _consistently_ show 2 MPH above what you're actually doing at all speeds, ie: there's a 'consistent' offset of +2 MPH from 10 MPH all the way up to 80 MPH.
> 
> ...and, my *ScanGaugeII*™ _confirms_ that offset in both our 2009 Vibe and 2011 Cruze.


Japanese car companies do this too, specifically Mazda vehicles.


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## Patman (May 7, 2011)

When I look for after market wheels, I always look on a web site that does tire size comparison to see what will happen if I go from 16s to 17 18 or 19. Hopefully each level of Cruze is calibrated correctly for the wheels applied to the car! I just did 216 6016 with the 245 45 18 and the calculator said that when I am doing 60 I am actually doing 61.2 MPH. I sure it could be looked at as whats a MPH or 2.


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## dennisu (Jun 17, 2011)

I checked my '11 LT against my GPS and it was right on the money at all speeds and I am in km/h so it is even a finer tolerance than in mph.


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## 70AARCUDA (Nov 14, 2010)

...*Car* *and* *Driver* "tests" the odometer by "zero'ing" the trip-odometer at the "start" of a measured mile, then drive the car until the trip-odometer is just ready to 'click' over to *1.0* mile. They then "stop" when the odometer reading clicks-over to _exactly_ *1.0* mile distance. They then simply (tape?) measure the ±distance the car is ahead or behind the *actual* 5,280-foot "mile" mark. It's done on the 'measured' area of the race/test track.


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## NBrehm (Jun 27, 2011)

jaygeo1 said:


> If my tire pressures are over spec, my speedo reading will show *slower *than it should and visa versa....and then as my tire tread wears down........  No wonder there are tolerences....


A very good point also. I think you guys are reading too much into this, it is almost impossible to have a perfectly accurate speedo which is why there are tolerances in the Specs from the factory and why you also normally get a 5MPH "grace area" from the local PD. You're speedo can change just from a shift of temperature during the day as the air pressure in the tire changes. I think you would be hard pressed to find a 100% accurate speed on any car. Also wheel size really doesn't factor in in speedo calibration, it is the tire size that changes it. You can have a 14 inch wheel and a 20 inch wheel and if the outside diameter of the tires are the same they will read the same. I'd be surprised if the full spread of Cruze wheels has very different overall diameter, I'd be willing to bet they are pretty close


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## NBrehm (Jun 27, 2011)

Here ya go, a little quick math yields this info. with the EXACT same calibration on the computer, using 215/60/16 and 35 PSI as a base at 65mph you get these results

215/60/16 reads 65.00MPH diameter 26.16"
225/50/17 reads 64.26MPH diameter 25.86"
215/55/17 reads 65.38MPH diameter 26.31"
225/45/18 reads 64.54MPH diameter 25.97"

4 different wheel sizes and almost every tire is practically identical but seeing the differences there I would almost bet GM has just 1 tire size in the computer which may account for the differences mentioned above. All in all they are not going to make a huge difference if you want to bolt on 18's to your LS, just get the correct tire size.


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