MG_Midget_74
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I have a 1974 Midget. The rear end has been changed, are there any companies that recalibrate the speedo?
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Speedometer correction
- Thread starter MG_Midget_74
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dklawson
Yoda
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Almost any speedometer shop can re-calibrate it. The most common company names you will hear are Nisonger, Palo Alto Speedometer. APT (gaugeguys.com), and MoMa.
Recalibrating the speedometer so the needle reads correctly is "easy" compared to what is involved for the odometer to read correctly. For the odometer and trip meter to work correctly, mechanical bits inside the gauge have to be swapped out in addition to re-calibrating the needle. Therefore, it is expensive.
If your speedometer is in otherwise good condition (i.e. it looks nice and works properly) a less expensive option is to install a ratio box. As its name implies, it is a little gearbox and it is about the size of a pack of cigarettes. A cable from the transmission goes into the ratio box and a second cable leaves it and goes to the speedometer. Inside the box, gears convert the turns-per-mile coming from your transmission to the turns-per-mile your gauge needs. You provide the vendor some measurements and a new cable and they provide the modified cable(s) and ratio box.
The link below shows ratio adapters and information on how to order them.
https://www.gaugeguys.com/speedcorrect.htm
Recalibrating the speedometer so the needle reads correctly is "easy" compared to what is involved for the odometer to read correctly. For the odometer and trip meter to work correctly, mechanical bits inside the gauge have to be swapped out in addition to re-calibrating the needle. Therefore, it is expensive.
If your speedometer is in otherwise good condition (i.e. it looks nice and works properly) a less expensive option is to install a ratio box. As its name implies, it is a little gearbox and it is about the size of a pack of cigarettes. A cable from the transmission goes into the ratio box and a second cable leaves it and goes to the speedometer. Inside the box, gears convert the turns-per-mile coming from your transmission to the turns-per-mile your gauge needs. You provide the vendor some measurements and a new cable and they provide the modified cable(s) and ratio box.
The link below shows ratio adapters and information on how to order them.
https://www.gaugeguys.com/speedcorrect.htm
OP
MG_Midget_74
Senior Member
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It's for the wife and her math sucks and she knows it.
dklawson
Yoda
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I agree, replacement speedos would be the easiest and best option if you can find the right one for your combination of final drive and tire size.
Moving over to the U.S. there are a few different thread sizes. Two of the common sizes are 5/8" and 7/8" threads. The ratio boxes I mentioned earlier typically have one of those threads. That's why when you ask a shop to make a ratio converter for you, you typically supply them a new speedometer cable for your make and model of car. They will cut the cable where you tell them to and install the ratio box at that location with fittings/nuts appropriate for the ratio box.
EDIT: So you send them one cable, they return it to you cut into two cables. Each will have fittings for your car on one end and fittings for the ratio box on the other. One standard cable becomes two custom cables to support the ratio box.
Unfortunately, no. There are many different standard sizes. Most pre-1970 Smiths cables have a 3/4-26 threaded nut on the gearbox end and M12x1 nut on the speedometer end. Sometime in the early to mid-1970s different makes & models started introducing cables that were held onto the gearbox with clamping forks and plastic fittings that snapped onto the speedometer at the other end.
Moving over to the U.S. there are a few different thread sizes. Two of the common sizes are 5/8" and 7/8" threads. The ratio boxes I mentioned earlier typically have one of those threads. That's why when you ask a shop to make a ratio converter for you, you typically supply them a new speedometer cable for your make and model of car. They will cut the cable where you tell them to and install the ratio box at that location with fittings/nuts appropriate for the ratio box.
EDIT: So you send them one cable, they return it to you cut into two cables. Each will have fittings for your car on one end and fittings for the ratio box on the other. One standard cable becomes two custom cables to support the ratio box.
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