RPM at 60 mph

There are many free speedometer apps for smart phones. They use GPS to calculate your speed and they work pretty well, especially at a constant speed.

Rather than try to measure your tires with a tape measure, do a Google search for "tire size calculator" and type the size into the calculator to get all the dimensions of your tire including diameter.

Once you know the diameter, use one of these calculators to computer your RPM at any given speed. http://www.ringpinion.com/Calculators/Calc_RPM.aspx