If you drive your hot rod, even once in a while, then you depend on all of your gauges to be reliable and accurate. Read About Mechanical Speedometer Calibration - Rod And Custom Magazine ...

Unfortunately, these changes usually throw the vehicle's speedometer calibration way off. If you're fortunate enough to live in a big city, for a price, a specialty shop can recalibrate the speedo.

EXAMPLE A mini-truck originally equipped with a TH400 trans, 8.75R16.5LT tires on 16x6.75-inch wheels (29.6-inch overall diameter), and a 4.10:1 rear axle ratio now has P205/75R15 tires on 15x5.5 ...