# Tire size calculation

my 2000 nissan frontier xe comes standard with p215/65/r/15 tires the tires presently on it are p215/70/r/15 . what percentage factor do i use to calculate the difference in speed ? just purchased vehicle and i am moving faster than the speedometer indicates. any info is appreciated. thanks.

A 215/65R15 tire has a diameter of 26" and a 215/75R15 tire has a diameter 26.9", roughly a 3.5% increase.

At 50 mph on the speedometer with the larger diameter tires, the actual speed would be 3.5% greater or ~51.75 mph. Since speedometers aren’t that accurate to begin with, I wouldn’t be too concerned about the difference.

Ed B.

Search “tire size calculator”. There are a bunch out there; you might find one that you like. Using a little algebra, you can set up an inverse proportion equation such as New size revs per mile/Old size revs per mile = New speedometer reading/50 mph. Solve for New speedometer reading. Keep in mind that a larger(smaller) tire will make the speedo run slower(faster).

The difference between a 65 series and a 70 series tire of the same size is about 2%, that is also the difference between a brand new tire and a tire that needs replacing of the same size. Slightly less than a half inch difference in diameters in both cases.

http://www.powerdog.com/

Trucks have lots of wheel well clearance and are suseptable to owner whims. Check your spare ASAP and make sure it matches the rest of the tires. The last thing you want to do is use two tires of different sizes on the same axle.

Hook up a GPS and check the speed readout on the GPS with your speedo. You can do the math, but who knows how accurate the speedo was to begin with.

Uncle…good point. Given that speedometers are notoriously inconsistent you may even find that the tires improve accuracy. Either way, it’s a good idea and good info to have. When I up sized a truck by “several” sizes, I put a sticky on the speedo indicating error at 25 and 55 mph as a reminder to myself and other drivers. IMO, one size could still be within the margin of error used by law enforcement.