Tire size and MPG's.

[TJNE]

I have an 07 classic 2500hd Duramax. Stock tires are LT245's, when I bought the truck (used) it came with LT285's. Is the size diff. enough to affect mpg's? Also how much mph's do think my speedo is off?

[Mark Rinker]

Depending on brand, you are probably about 6% off, so when your speedo reads 60mph, you are actually going 63.6mph.

Your mileage could be better, or worse, depending on the kind of driving you do. Most likely its lower under normal driving conditions. If you do lots of highway miles, it might be better. Probably not enough difference one way or the other to matter much, based on my calcs running stock and 285 rubber on my fleet. I think we squeeze a little extra mileage out of the Kodiak and K3500 by running taller tires, but they spend alot of time on cruise control running down the highway to offset the additional fuel burned in stop and go circumstances. It takes more energy to get the larger tire rolling from a stop, and there is more mass and weight as the tire increases in size.

You can reflash for tire size changes with many handheld tuners, or have a custom tune made with that tire size in mind. www.kennedydiesel.com (http://www.kennedydiesel.com) took care of the last truck that I had 285s on, and the speedo was then spot on with my GPS.

[JohnC]

If your speedometer is not corrected for the tires your calculated mileage as well as the mileage from the DIC will be lower than actual.

[TJNE]

Went to dealer for an injector clean and recalibrate for tire size, $200 later No Change In MPG's.

[DmaxMaverick]

Went to dealer for an injector clean and recalibrate for tire size, $200 later No Change In MPG's.

This is what you got for $200:

An "injector cleaning" will almost always do absolutely nothing on a Duramax engine.

The dealer will not and cannot calibrate the speedo for tire size. They can recalibrate the PCM tire size for ABS, which essentially does nothing, functionally.

Upsizing to 285/75/16 tires can and does effect mileage. Most often, for the worse. Tire air pressure and driving habits will have the greatest effect on mileage, within a respective tire size. The tire diameter isn't so much the major contributing factor. The friction of the tire with the road surface (rolling resistance) can be optimized with tire size/tread choice and air pressure. A 285 tire will have a significantly higher friction rate than a 245 or 255. A 255/85/16 tire has roughly the same diameter as a 285/75/16, yet it has a much lower friction rate, all else being equal, and will also have less mass.

Your actual ground speed will be about 6-8% faster than indicated speed on the speedo. I've seen variations of 2% on vehicles of the same year, with the same tire/wheel. Calibrate and verify it with a GPS, or highway markers on a straight stretch of highway (curves and lane changes skew results, markers or GPS). A greater distance traveled for your test will increase its accuracy. You will need an aftermarket solution for speedo calibration.

[jbplock]

Here's a handy tire size calculator ...

http://www.miata.net/garage/tirecalc.html

As DM pointed out running 285/75/16's vs stock 245/75/16's will yield a speedometer reading 7.8% low.

[TJNE]

I wish I knew this $200 ago.


This is what you got for $200:

An "injector cleaning" will almost always do absolutely nothing on a Duramax engine.

The dealer will not and cannot calibrate the speedo for tire size. They can recalibrate the PCM tire size for ABS, which essentially does nothing, functionally.

Upsizing to 285/75/16 tires can and does effect mileage. Most often, for the worse. Tire air pressure and driving habits will have the greatest effect on mileage, within a respective tire size. The tire diameter isn't so much the major contributing factor. The friction of the tire with the road surface (rolling resistance) can be optimized with tire size/tread choice and air pressure. A 285 tire will have a significantly higher friction rate than a 245 or 255. A 255/85/16 tire has roughly the same diameter as a 285/75/16, yet it has a much lower friction rate, all else being equal, and will also have less mass.

Your actual ground speed will be about 6-8% faster than indicated speed on the speedo. I've seen variations of 2% on vehicles of the same year, with the same tire/wheel. Calibrate and verify it with a GPS, or highway markers on a straight stretch of highway (curves and lane changes skew results, markers or GPS). A greater distance traveled for your test will increase its accuracy. You will need an aftermarket solution for speedo calibration.