Speedometer Calibration?

I have one of those aftermarket Speedos and just checked the speed against another car. I was reading 10 mph slower than them. Can I calibrate for this. Maybe a different Speedo gear in the TC?

 

I have a Stewart-Warner correction box that is setup for 10% faster or the gears can be reversed for 10% slower. use to be common 40 years ago ,not so much now. you could buy gears for different values of correction. Handy little item.

 

Use your GPS and check it at different speeds to see what the overall correction should be. just a suggestion. mike

 

The speedometer driven gear is matched to a certain series of available drive gears.
You'll need to pull the speedometer driven gear out.
It has number on it that we will need to know to identify the series and the ratio.
If your speedo is reading slow then you need to install a matching driven gear for a lower ratio differential.

If you change the tire diameter that will also effect your speedometer gear selection.

 

Is it really useful to know the truth ??

 

Just get smaller tires. Problem solved.

 

Just re-shimmed the output / Speedo housing. Not ready to do it all over again. Guess I'll have to learn subtraction and that other math deal.

 

The driven gear is easy to remove.
Just remove the threaded speedometer cable sleeve from the TC and then pull the driven gear straight out from the rear bearing cap.

 

Oh, ok Ken. I will get that soon. On another project now but will do that next. Will you be at the Willys reunion in Fulton in Sept? Thanks

 

Talking about GPS units - All the times i used GPS to measure speed and checked the cars speedometer (modern cars, not only my jeeps), the GPS was about 10% slower then the "real" cars speed. I realy dont know why.

 

GPS doesn't lie

 

I had a speedometer shop reset mine. They use a 90 degree gear reduction at the transfer case. They could of got it perfect if they added another inline one. It's only 5 off, so I opted to not buy it.

 

Just to clarify this: mechanical speedometers are never off a set speed they are off a %, so if they are off 2 mph at 15 mph they will be off more at a higher speed.

 

So the right way is as oldtime says. Change the gear in the tc. Thanks mcruff.

 

Gear availability is the problem.