I put a 3.7 rear in my '69 and I now know about the bike computers (thanks!) and I have a correction list taped on my dash, but I want to watch out for a 1500 speedomer in case I trip over a nice one. How can I tell from the outside the diff between the various speedometers? Is there a code?

BTW, I can not get my 2009 reg tag until the 16th, but I have driven on some of the nearby canyon highways and the car runs great. I had a vibration around 65, but I rotated the driveshaft 180* to the pinion and made sure the caps on the new U-joint were centered and it cleared up. I was driving this weekend with my son, we were at 60, top down, windows down, and we could talk without raising our voices. I was surprised that it was not more noisy. nimble in the corners, very responsive the the throttle. Just a great car.

Yes there is a code on the face, but the '79 used a different transmission, so the tpm (turns per mile) will not be the same. Your transmission speedo gear would turn 1376 tpm with a 3.9 rear end so your need a speedo that is 1306 tpm

I don't know if one exists. You might need to go to a transmission shop and see if they can make you an adapter.

I think there is a 1280... that would get you close. What do you have right now? The four digit number off by itself.

Nisonger can do a custom calibration.

The 1500 midgets used 1000 tpm speedometers that were ranged to 120 mph. Your should be a 1376 tpm speedometer ranged to 100 mph. Both would be off with your transmission / differential combination.

Of course.... it also depends on your size tire. A custom calibration may run more than a cable adapter. You need to make some phone calls. Really 1280 which was in some MGB's is only about 2% off. I think that would be the most ecenomical solution. But... double check my arithmetic.

But... then again 1376 is only about 5% off. Right?

The info I am missing is that there is a difference between the speed drive ratio from a 1275 trans and a 1500 trans?

So just matching my 1500-sourced 3.7 rear end to a 1500-sourced speedo driven by a 1275 trans will not give me an accurate reading?

A 3.9 differential and a ribcase transmission will output 1376 revolutions of the speedometer cable per mile (or tpm, turns per mile), that is the 4 digit number on the face of the speedometer. A 1500 transmission with a 3.7 differential will output 1000 tpm.
A ribcase with a 3.7 doesn't match up to a "stock" midget speedometer ratio.

I pretty sure there is a difference because the late 1275's had a 3.9 rear end and 1376 tpm, while the early 1500 with a 3.9 rear end had 1000 tpm.

Based on what Chris is saying, they changed the gearing in the tranny when they switched to a 3.7, but a bit of research may be in order.

How far off are you currently when your speedometer says 60 mph?

Thanks for all the info, folks. As for the error, it's about 7 mph according to my mathematical estimates and assuming the tach is accurate. It's not bad, and I have my cheat sheet, but I do like things to work correctly.

I think I'd try the 1280 tpm from an MGB and see if that doesn't get you a bunch closer. Hit a swap meet and you can pick one up for $10 or so.

Rick Lybarger at United Speedometer in Riverside was the speedo calibration man, back when I was in SO Cal. Might check to see if they're still there. If so, I would expect them to be able to correct or install an adapter for you.