MAJOR Speedo Problem

[The Humungus]

Hey guys,

My 89' Stang has a speedo calibration problem. I switched to 17 inch rims last year which threw the speedo off making it read that I am going faster than I really am. So, when I'm really going 100kmh the speedo is telling me that I'm going 120kmh. Not too bad of a problem.

Now, I just switched out the stock gearing for 3:73's. I wasn't sure if this would throw the speedo out even more or reign it back in. Turns out, it throws it even further off.

There are speedo gears that compensate for having 3:73's, but not for 17 inch rims. What solution is there? Maybe I should try a speedo compensation gear for a more extreme gearing like 4:10's? Surely someone else has run into this problem.

 

[BlownFiveLiter]

The wheel size has no determination on the speedometer calibration if you keep the overall circumference of the tires the same. What size tires are you running?

 

[jrichker]

See T5 Speedo calibration for a calculator to get the correct speedo gear for your application. They also sell the speedo gears and other transmission parts.

Or if you can't find an exact match, there are other solutions.

The stock range of speedo gears work correctly with a limited range of tire sizes. The 23 tooth white gear works good for some guys, but self destructs in a few months for others.

See http://www.corral.net/tech/gearcalc.html for more help

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap.

Please excuse the typo (theirs, not mine) in the following picture - it should read Custom ratio adapters, not Custom radio adapters.

See Speed Correction Devices (Ratio Adapters) for more info.

 

[The Humungus]

See T5 Speedo calibration for a calculator to get the correct speedo gear for your application. They also sell the speedo gears and other transmission parts.

Or if you can't find an exact match, there are other solutions.

The stock range of speedo gears work correctly with a limited range of tire sizes. The 23 tooth white gear works good for some guys, but self destructs in a few months for others.

Thanks for the link jrichker. That pretty much solves my problem.