The Stovebolt Site Forums The Shop Area The HiPo Shop Speedometer reading low even after speedo gear change

Hi,

I did a T5 trans swap and referred to the online calculators as well as the tables and info at various websites to determine which driven gear I needed for the driving gear that I have with my tire/rear end gearing. The speedo still reads low. Improved, but still too low.

Speedo drive cable was cleaned, inspected, lubricated, and re-checked again later- no issues with the cable.

I then went with the very smallest driven gear available for use with my driving gear, and the speedo still reads low throughout the range, with it reading about 6 mph low on the highway now. It should be reading way high, at this point.

Any suggestions as far as what might be going on inside the speedo itself? Can the magnets lose power over time and require re-magnetizing? Speedo behaves perfectly otherwise - no bouncing or erratic movement...very smooth and quiet.

Any suggestions appreciated...

Speedometers can be affected by age and can be repaired or calibrated. There are probably speedometer repair shops near you. If not, there are many you can find with an online search.

This is one of the best videos about how these work. I would think the springs would being worn have more an affect than the magnets. Your should try getting a estimate on number of revolutions per mile. jack it up . roll the tire and see how many times that cable turns. A little math or online calculators can tell you how many spins the cable is doing and is it at where its supposed to be. The fact that it is even across the range seems to be more of a ratio being off gear or otherwise and that should be easy to get closer. finding the right gear of course.

What makes it work...

To clarify: is it off by 6mph across the board? Or is it off 6mph at the speed you drive on the highway?

I second the plan to turn the wheels and count the revolutions of the cable.

What does the odometer read. Is it accurate to the mile? If it is right then the speedometer needs repair.

Thanks guys᠁ the speedo is off by 6 mph across the board. Actually, it starts out at about 10 mph low, and at about 65 it is 6 mph low.I’ll have to check the odometer accuracy- I didn’t think about that. They don’t have those “measured mile” markings out where I am- do they have them anywhere anymore?

Interstate highway mile markers are usually pretty close. 10 in a row would be even closer.

Use Google maps to get the distance from point A to point B and then drive it and compare the numbers. They're quite accurate.

Thanks guys- hopefully the rain will let up next weekend and I can give that a try.