Milometer, MPG vs. Tire Size

[PompeyChris]

I hope this is not a dumb suggestion, but how does the milometer work out mileage?
If 17 in. tires are stock, does the milometer get out of whack if you put on taller tires (larger circumference)? They would cover more ground per revolution. And if the milometer is wrong, then so are any MPG calculations you do based on it.
Please educate me...

[cruiser88]

Four Wheeling Calculators

Look on down that screen....I THINK that is what your asking for??

[spaldam]

Bigger tires will off-set your speedo (and how many miles you've traveled) and thus mess up gas mileage calculations, so adjustments will have to be made for larger tires.

Also your wheels are 17" (mine are actually 16"). The actual tires are closer to 31" or 32" stock.

[DEWFPO]

The tire size is what determines your diameter (revs per mile), not the wheel size. You can put several different sized tires (all with different diameters) on the same wheel.

DEWFPO

[PompeyChris]

Sorry, I meant tire size NOT wheel size. I was wondering when forum folks go on about miles per gallon, whether they were comparing like to like. Cruiser 88: Thanks for the great calculation site.

[MajorMiner]

What I have done for years is to check the odometer using the mile markes on the side of the road. Set you trip meter or write down the odmeter milage as you pass a mile marker and use at least ten markers (more markers are better). Then calculate the difference. Mile markers are accurate.

[Brock]

depending on tires it can be a big difference or minimal... per my Garmin....since adding 33" tires to my oem alloy wheels 17"...I am only off about 1.7mph at 60 mph......so not worrying about it.

[ckline]

Besides the poor mpg with a lift and bigger tires, my speedometer reads accurate up to 60mph. I was fortunate enough to be driving next to a Dallas PO the other day. We were stopped at a light and I asked him if it was alright for us to get up to 60 in a 35 to test my speedometer. He agreed. With his radar on, my detector picking it up, we checked from 30 every 5 miles an hour. He read exactly what my FJC read.

[KightOwl]

So what is the correction value?

Stock tires turn 638 Revs/mile.
New tires turn 617 Rev/mile.
Tire size calculator
If I use to go 250mile per tank of gas and now I get 200miles per tank (just for example). Should I just adjust the miles per gallon by 3.5%?

I feel like an idiot. I don't get it.

[DEWFPO]

Originally Posted by KightOwl

So what is the correction value?

Stock tires turn 638 Revs/mile.
New tires turn 617 Rev/mile.
Tire size calculator
If I use to go 250mile per tank of gas and now I get 200miles per tank (just for example). Should I just adjust the miles per gallon by 3.5%?

I feel like an idiot. I don't get it.

Using your numbers above, multiply the 200 miles per tank you now get by 1.035. You get 207 miles (actual). The same thing with your speedometer. If it says you are traveling at 60 mph, then multiply it by 1.035% to get your actual speed (assuming the speedometer/odometer were accurate before the tire size change). So 62.1 mph would be your actual speed at an indicated 60 mph. Divide your gallons used into the 207 miles number to get your correct mpg.

By the way. 617 divided into 618 comes out to a 3.4% difference.

DEWFPO