That may be it. Someday I'll know. For now I'll use the tach. The only real problem is in Peoples Republic of Nevada, Eastern California we have to report milage each year to register our vehicles. Mine is always the same.
Go ahead, rub it in...(I don't have a tach), my RPMs guesses are just that, a guess.
My tire size is 235/75r15, the diameter is 28.88".
The original 650r16 was 29.26". There's about 1/3rd of an inch difference, but the amount of tires to choose from is enormous. America's Tire has 44 different tires to select in that size, ranging from about $70-$200/tire. Tire Rack has 39 to select, $85-$210/tire. FWIW, if/when I buy new tires I see that have the Yokohama Geolander in that size, we've had really good wear on my wife's Honda CR-V, nice DEEP tread, they seem to last well.
For now I'll use the tach. The only real problem is in Peoples Republic of Nevada, Eastern California we have to report milage each year to register our vehicles. Mine is always the same. 
