Can someone point me to a formula for calculating the gas mileage with my new tires. I went from the stock 265/70/16's(31")to some 265/75/16's (32"). I know that I travel a bit farther than the odometer says because of the increased size, but I don't know how much. This throws off my gas mileage calculation some because I'm not dividing into the right number. Does anyone know of a formula to correctly calculate the true mileage gone with the bigger 32's so that when I divide by the gallons used it will be more accurate?

 

(real miles) = (odometer miles) * (new tire size) / (old tire size)

old tires = (265/25.4*.70*2)+16 = 30.61"
new tires = (265/25.4*.75*2)+16 = 31.65"

(real miles) = (odometer miles) * 31.65/30.61

or

(real miles) = (odometer miles) * 1.034

that is you're going about 3.4% farther than your odometer reports, and that means you're getting 3.4% better gas milage than you're currently calculating.

edit: that also means you're going about 3.4% faster than your speedometer reports. but since speedometers usually underreport the speed, you're probably right on. i usually double check mine with those radar speed limit signs that have been going up everywhere.

 

I'm having real doubts about our educational system, judging by how many times this sort of question was asked

 

Awesome, Thanks!!

 

yeah, i've posted that formula about 3 or 4 times myself. maybe someone should search the posts and just count how many times that's been posted. i'm too lazy to do it myself

 

I was referencing the formula to find the circumference. I thought it's 6th or 7th grade math

 

Circumference is diameter * 3.14

If you're talking about tires, it's more accurate to use (static loaded radius*2) *3.14.

Not particularly useful to figure gearing/tire changes or fuel mileage tho.

 

Or since typical odometers are rarely dead accurate, find a stretch of highway with mile markers and drive ~10 marked miles, noting your beginning and ending odometer readings and determine the error. You can do the same to check your speedometer, driving a few measured miles at a steady 60 MPG and recording the transit time.

I have a gear/tire size/speedo calculator page on-line here...