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I have a feeling it's probably more trouble than its worth, but is it possible to calibrate the MR2's speedometer to accurately reflect speed once you've upgraded from the 14" or 15" wheels to 17"?

In my case I have a 92' MR2, so mechanical speedo.
 

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What tyre size do you have?

With a mechanical you could change out the gear on the end of the speedo in the gearbox if you know what size it is however this is going to be very hard to achieve.

You can either figure out what speed the car is actually doing and just compensate in your head or do what I did and remove the needle off the dial and place it back on in the right spot. I don't really recommend this as it's really hard to get right. Took me 10 attempts to correctly position it. Driving up and down a straight bit of road with the clear plastic cover off the gauges, pulling off and putting it back on each time I stopped. Now the needle sits at -20 when stopped and goes even further when going in reverse. All for 3kph... :| However I forget what the needle does on a non messed with dial when going in reverse as I was 99% of the time looking backwards and paying attention to my surroundings.

My MR2 is a 1990 that came with stock 14s. 205/60R14 Speedo ran bang on accurate with the 15s. 225/50R15 and was about 3kph out with the 17s. 245/40R17.
I fixed it for the 17s but I'm back running the 15s so it's out again. Not an issue as I know it's 3kph out the other way compared to before I changed it.
 

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You cannot recalibrate the speedo. You can change the offset, as described above, but then the speedo will only be right at one speed.

However, speedos are always factory calibrated to read about 2% high. That means that you can go up one tire size and have it read dead on. If you put low profile tires on a larger wheel, you can get close to the original circumference. Look at the tire specs for actual outer diameter.
 

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I calibrated my speedo when I switched to an alternative speedo sensor by changing one of the resistors on the speedo: https://www.mr2oc.com/61-3sgte-turbo/659730-alternative-speedo-sensor-not.html

Unfortunately the pictures seem to be missing from that thread, I’ll see if I can get them back.

-edit- restored the pictures, apparently postimg.org isn’t much better at keeping pictures online than photobucket is
 

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True, if you have an electronic speedo. It is just the mechanical speedos that cannot be recalibrated.
 

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Let's think about this for a minute. The mechanical speedo has a little stepper motor that moves the needle and it is getting a voltage that is generated from the rotations of the cable. So somewhere on the board between the cable and the motor there should be a resistor that you can change to recalibrate the speedo - JUST LIKE THE ELECTRIC SPEEDO. Just nobody has tried this like nobody really has tried to recalibrate the electric speedo. rmeller you're the electrical engineer, tell me if I'm wrong on this.
 

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The mechanical speedo has a spinning magnet next to a metal plate. The magnet induces a current in the plate, which produces a magnetic field which interacts with the magnet. The plate is constrained by a spring and holds the pointer that you see on the dial. So, the spinning magnet drags the plate with a torque that is proportional to speed, and the spring sets the calibration. It really is all mechanical. There is often an additional reed switch that picks off a pulse train, so that you can have cruise control and an ECU speed limit.
 
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