There are different diameter 15" and 16" tires. Many truck radial tires on our narrow wheels (with proper inflation for a balance between good tracking and easy low speed turning) end up to be a wider diameter than their stated specs.
I used to check (and calibrate) my speedometer by driving (at 60 mph) 20 miles away on an interstate and 20 miles back. I take my odometer readings at the start and stop of every 10 mile interval. I gave those readings to The Speedometer Shop and they send me an adapter. With the adapter, the speedometer readings are now less than 1% off.
Now that I have a GPS, I'll calibrate on that and send the information to The Speedometer Shop, for them to make an adapter.
