You'll need a way to determine just how far it's off. Doing the math works. The GPS is just a suggestion and the easiest way to do it. If you have a phone then use that. I personally don't have smart phones. Having someone follow sometimes isn't the best way. Not only hard to correlate but you also don't know how far off the speedo is you're comparing to. The reason Jim wants a "range" from 45-55-65 is for accuracy reasons. You can have a speedo that reads closer in the lower mph ranges and be less accurate in the upper mph ranges. Jim takes this into consideration when building your reducer and corrects that discrepancy giving you a very accurate speedo throughout the range.
The problem with some of these muncies are the drive gears. Not a lot available and some aren't currently made. A certain drive gear only works with a certain range of driven gears so you just can't stick any driven gear in there you want, even if the math calls out a specific tooth count. Going too far eventually eats the plastic teeth.
As an example, I have a 66 M21 that is this way. It was optioned in a vette with either 4.11's or 4.56's. I'm running it in a 3.55 geared car currently. I simply cannot remove enough teeth on the driven gear to speed up the speedometer enough. It chews up the gear within 10 miles. The correct drive gear isn't produced so the only option is to run the correct driven gear that lives with this specific drive gear, which slows the speedo, then run an external gear reducer to correct the reading.
And yes, I strive to make all my speedometers accurate. MPH and odometer accuracy is important to me. If I were going to live with it being inaccurate, I may as well just disconnect it because it would be no good to me otherwise.
If going with a new trans like an M22Z that are basically modified muncies, Autogear can simply provide the correct drive and driven gears that they make for that trans if you provide the gear ratio and tire height since these transmissions use the larger diameter output shaft. More of those drive gears are available. Makes things easy. I do the TKO swaps here and it's the same deal. Unfortunately some of the drive gears for the old muncies and small output shafts just haven't been available.
