Speedo gear 1970 A904 selection

metric tires are rounded up to the next nearest size. A 255 could be as small as 253 or as large as 257, when mounted on a checking-rim that is 70% of the branded size,rounded to the nearest 1/2 inch. So 70% of 255 would be 7 inch rim.So in reality, a 255/60-15 could be as large as 27.14, and as small as 26.95. However, this is less than .7% difference, or .45mph at 65.
The gears can usually get you to within 2% of perfect, which is about 1.3 mph at 65 mph. So at a true 65 the gauge could be indicating somewhere between 63.7 and 66.3.And the tire diameter could kick in it's own error.

One trick I have used, is to just throw something close in there and go for a ride.Then as opportunity presents, I calibrate the current error in percent. This could be done on one of those calibration sections on the hiway, or by radar, or by gps,etc. The % error can be directly applied to the current gear.So if you had a 3% error on a 33 tooth gear this would be a 32.01 tooth or a 33.99 tooth requirement, depending on which way the error was. From that one could surmise that a 1.5 % or less error is not addressable.More teeth slows the speed-O down