Home Forums Gear Hunting Trucks and Four-Wheel Drives formula for getting MPG with bigger tires.

I know my truck's MPG is wrong because I put on larger tires than factory. I went from 245X16 to 285X16 on my Siverado.I know that my tires are turning less Revs per mile, so my odometer is not giving me proper miles per gallon. Please help figue the proper MPGs. Thanks, Mark. <img src="/ubbthreads/images/graemlins/confused.gif" alt="" />

When looking at the numbers on a radial tire (I'm more familiar with passenger car tires) there is usually a number like 245/75/16. The first number, "245" for instance, is the width of the tread in millimeters. The next number, which you didn't provide, (something like 75 or 85) is the ratio of sidewall height to tread width- say 75% of 245 millimeters. The final number, "16" is the rim diameter, 16 inches.

For instance, if you had a 245/75/16 tire: tread width is 245 millimeters or 9.64" wide. Sidewall height is 75% of that or 7.23" tall. The rim is 16" tall with a sidewall height of 7.23" both above and below the rim for a total of 30.46" tall.

If you then went to a 285/85/16 tire: Tread width of 285 millimeters is 11.22" wide. Sidewall height is 85% of that or 9.53". Rim diameter is still 16" tall but now has 9.53" of sidewall height both above and below the rim for a total of 35.07".

Divide the total height of the old to the new or vice versa. The ratio between the two in this example tells me that the new tire is only turning 86.84% as many revolutions as the old one did over the same distance. (The odometer and speedometer are linked to how many times the tire revolves.) Another way of putting it is that you are now doing 15.1% more miles and more miles per hour than what your odometer and speedometer are showing.

Here's a website that does the math for you:
http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireMath.jsp

Bob

After a lift and larger tires on my Jeep I used my GPS to figure out milage. Worked really well and told me exaxtly how far off my speedo was.

Why not just take it in and get the speedo calibrated. Might save you a speeding ticket in the long run.

Phil

That is how I calc the difference...........

Don't use the manufacturers indicated tire size, go out sid and measure the Circumference of both and then figure the difference.

your average 31 inch tire is 97 inches around and it revolves 653 times in one mile.

your average 33 incher is 101 inches around and it revolves 627 times in one mile.

a difference of 4%

Fat N Slow,

I come up with:

31 X 3.14 = 97.43" vs. 33 X 3.14 = 103.71"
giving a difference much closer to 6%.

Although I've used that technique too, I've often wondered about the accuracy of it. Some questions that spring to mind are: how much are the tires compressed due to the weight on them & and the air pressure in them (which in itself could vary depending on their temperature)? You could take them off to measure them, although once they are mounted on the vehicle they will then be compressed. But then again, would that be cancelled out due to the centrifugal force standing the tire up when the tire is rotating at highway speed?

If a 31" tire is compressed 1/2" versus a 33" tire compressed 1/2", is that an apples to apples comparison since 1/2" is a greater percentage of the total height on the shorter tire?

By the way, my boss likes instant answers and results and gets on me all the time for "overthinking simple problems". <img src="/ubbthreads/images/graemlins/grin.gif" alt="" /> (I also originally carried out the original figures to a bunch of digits, then shortened them to only five decimal places, then down to only 2 since it didn't affect the results much. <img src="/ubbthreads/images/graemlins/grin.gif" alt="" /> <img src="/ubbthreads/images/graemlins/grin.gif" alt="" /> )

Circumference isn't really much better than manufacturer's data. "Loaded rollout" is THE most precise way to do it. However, maufacturer's numbers or measured circumference work okay enough for comparisions.

You can calculate all you want, but all it will give you is the percentage difference from before.

Most speedos over indicate a few miles. Do a speedo check (run five miles at 60 miles an hour, and measure time. Then calculate actual speed. Divide actual speed by indicated speed, which will give you your %. Multiply your measured MPG by this % and you will have your actual, calibrated MPG. JMO, Dutch.