Speedometer Calibration:

This page exists to explain how to alter the mileage readout on a TR7 or TR8 speedometer. I'm sure it applies to pretty much all Smiths odometers, but I don't know for sure. It carries certain risks of damaging things and no one but you is responsible for if something goes wrong. In addition to that disclaimer, what I'm showing here is not illegal. It is, however, very illegal to misrepresent the miles on a car to a potential buyer. In this case, what I've done is to change the speedometer out and match the miles on the replacement speedometer with the miles that are actually on the car. Considering the potential for misuse of this information, the usual disclaimers must be given. You've been duly warned.

With that in mind, we shall continue. This is what I started out with. The speedo I wanted to put in actually had only about 200 miles more than the one it replaces, but I wanted it to be right, so I did it anyway. Here's what I started with:

I did this once already years ago, for my Anniversary edition. I screwed up a little bit and have a digit halfway off. I never bothered to fix it, but I was able to do a better job this time around. I didn't remember exactly how I did it last time, but this time I discovered an easier way to do it, and I had to take it apart anyway to learn that. We took photos as we went along so I can use that learning curve to explain what we're doing with confidence.

So this is the speedometer that we're starting with, from the backside. Note the gray gear that the white lever pushes on to rotate. You may have to look a little harder than that, though!

Taking it all apart, I learned that between each of the digit wheels is a plastic plate with a yellow gear stuck in it. The plate has a tab that you can barely see in the following pictures that engages in a slot in the mounting plate. There is a shaft that the gray gear (mentioned above) drives that goes through the stack and turns the black wheel to the right of the row of digits (pictured by itself from the wheels in each photo.) It's actually a tenths wheel, and it also drives the trip odometer. It has a tiny tab on the inside, that, as it passes through zero, engages the tiny yellow gear, which in turn rotates the ones digit a tenth of a turn. Each digit wheel has this tab. There is an internal gear on the opposite side that the yellow gear drives continuously.

After I put the miles I wanted on the digits, I had too much trouble with the digits moving when I put the stack back in, so I put it together anyway and then tried to disengage the yellow gear by prying the wheels apart at the yellow gear. 

So all the confusing and/or boring education above boils down to the following solution: A jeweler's screwdriver is inserted to the right of the digit I wanted to move, at an angle that would cause it to separate the yellow gear from the wheel to the left of it. That was the ticket to making it all very painless and I discovered that you don't have to take anything apart to do it! The yellow gear is on the top of the stack, as the speedometer is installed in the car. Gently roll the wheel in the down direction with your finger. If it doesn't move smoothly, try adjusting the angle of the screwdriver. This is slightly scary stuff here (okay, compared to what I just did, it was a breeze!) so be extremely gentle.

And then I'm done. All that sweat and stress that I went through is now to your benefit. I hope you like it.