How much does my gas mileage decrease by putting bigger tires on my truck?

Question

they came stock with 28's and i put 31's on them

Answer 1

Larger tires may give you better gas milage (like an overdrive gear) but they will also throw your speedometer off, so you will have a hard time measuring the gain.  They will also slow your acceleration .

Answer 2

Your actual miles per gallon could actually increase because the tires move the truck farther for every revolution of the wheel.

However, the bigger tire also causes your speedometer and odometer to read inaccurately because they were factory calibrated for the 28 inch tires.

So when you divide your miles driven by gallons used the result will be inaccurate because when your odometer says you drove 1 mile, you actually traveled farther.

Here's the calculation for the old tire verses the new tire...

Circumferance = 2 x Pi x Radius. (radius is half the diameter of 28)

or                         = 2 x 3.146 x 14

or                        = 6.298 x 14

Circumference = 88.172  verses

Circumferance of the new tire

                            = 2 x Pi x Radius. (radius is half  the diameter of 31)

or                        = 2 x 3.146 x 15.5

or                        = 6.298 x 15.5

Circumference = 97.619

Difference = 97.619 - 88.172 = 9.447

Variance = 88.172 / 9.447 = 9.33%

So, to correct your speedometer or odometer take the miles indicated and multiply by 1.0933 and you will know how many miles your actually covered.

Answer 3

Not too much. I would guess that you would lose around 1-2 miles to a gallon depending on what and how you drive. I know that when I put 36" tires on my 01 Z-71 I was getting somewhere between 10-12 mpg. I think stock it was getting 16-18 mpg if that gives you any reference to go off of.