My Ford Bronco was fitted at the factory with 30 inch diameter tires. That
means its speedometer is calibrated for 30 inch diameter tires. I "enhanced"
the vehicle with All Terrain tires that have a 31 inch diameter. How will
this change the speedometer readings? Specifically, assuming the speedometer
was accurate in the first place, what should I make the speedometer read
as I drive with my 31 inch tires so that the actual speed is 55 mph?
If the Bronco is going 55 mph with 30 inch diameter tires, then it will
go approximately 57 mph with 31 inch diameter tires. Click here
for a spreadsheet showing what happens for different speeds. From observing
the spreadsheet, we saw that the 31 in. diameter tires would increase your
speed.
To find out what the speedometer should read with 31 inch diameter tires
so that the actual speed is 55 mph, we used the following proportion:
Thus, the speedometer should read approximately 53 mph.