So I went from about a 28" tire to a 31" tire. It feels like I'm going faster than what my odometer reads but I'm not sure if 3" would make a difference. I think since a 31" is roughly 10% bigger, shouldn't my speed be about 10% more than what the speedometer reads? And I also think the bigger tire would make my odometer read miles faster? So basically I'm wondering how much my speedometer is off and if I'm accruing more miles with bigger tires

 

Yes from 28" to 31" would be about 10% change

At 60mph on speedo your actual speed would be 66mph

Calculator here: OnlineConversion.com - Bigger Tires

Odometer would show LESS miles than actually traveled
i.e. after driving 60 miles on trip odo you would have actually traveled 66 miles

Speedometer and odometer both are pre-set for one revolution of the axle = a specific distance.

The distance a tire travels in one revolution is determined by its circumference.
That formula is 3.14(pi) x the diameter
3.14 x 28" = 88"
3.14 x 31" = 97"

So the 31" tire is traveling 9" farther in one revolution than the 28" tire

Most electronic speed systems use revolutions per mile to calculate speed.
But where the speed sensor is located is also a factor.
On the rear axle then it is a straight revolution per mile, but if it is on the drive line then rear axle ratio becomes part of the calculation, i.e. drive shaft rotates 3.73 times for 1 axle rotation, or 4.10 times for 1 axle rotation, depending on the differentials ratio

The larger tires also changes the differentials ratio in how it performs, a 3.73 ratio would act like a 3.37 ratio when tires are changed from 28" to 31"
If you changed ratio from a 3.73 to a 4.10 then it would "act" like the original 3.73, and if speed sensor was on the drive line then speedo and odo would be correct again :)

 

Wow thanks I'm glad I was right about the 10% difference. RonD to the rescue yet again! Haha