I have a bit of an odd, and very difficult to search for speedometer problem I'm trying to solve. I have a 1981 RX7.

My speedometer consistently reads 10 km/hr high. At 50km/hr it reads 60, at 80 it reads 90, 100 it reads 110, etc.

Unfortunately, searching online only gets me results for speedometer gears not matching tire diameter (reading X% off), which is not the same problem.

Has anyone experienced anything similar and found a solution? I don't want to start pulling the speedometer apart and risk completely destroying it before I have an idea where the problem lies.

 

What size rims and tires do you have?

 

^this all the way

 

Could be your cable just needs to be lubbed. But I suspect tire size is an issue. Stock tire
diameter is about 23".

 

Tire size and gear ratio gives a % error. not a constant 10km/hr across the sweep.
A 10% error at 100 will read 110. at 50 will read 55..
Your needle placement is 10 km/hr off..

 

Be very careful futzing around with the speedo. I ruined one trying to fix it. Its a very delicate
instrument under it all. There are shops you could send it to to get it professionally fixed.

 

The best way to check for speedometer error is to use the odometer. Drive the car for a known distance (5-10 miles) and see what the odometer registers. Odometers are gear driven and do not slip (unless you have a VDO speedo) and do not loose magnetic calibration like the speedometer can.

If the miles driven and the miles recorded by the odometer are the same, the speedometer is defective. Lubing the cable will not solve this problem. The speedometer needs to be repaired/replaced.

If the miles driven and the miles recorded are different, determine the percentage difference. If speeds are off by the same percentage, the speedometer is fine. In this case the error is caused by how the speedometer is being driven. This can be caused by the diameter of the rear tires, rear end ratio or the drive/driven speedometer gears in the transmission.

Some basic cause and effect rules are as follows:

Taller rear tires or a higher rear gear ratio will cause the speedometer to register miles and speed low. In other words the speedometer will register speeds that are slower than your actual velocity.

Shorter rear tires or a shorter rear gear ratio (4.88 for example) will cause the speedometer to register miles and speed high.

 

That is within range. The tolerance is to be up to 10% high.

Check the odometer versus distance markers. At least in the US the markers are consistent and over a long enough distance, any error here or there will become a minimal issue.

My FB's speedometer is 5% fast compared to the odometer, once corrected.

 

I guess it depends on who makes the speedometer but my experience repairing and calibrating speedometers in the 90s was that 10% is too much. None of the new replacement speedometers we installed or the old ones we repaired left the shop with that much error.


10% speed error is 3mph at 30, 6mph at 60, 9mph at 90 etc.


The domestic speedometers I used to calibrate/recalibrate had a 2% +/- speed error tolerance at 60mph. For Canadian metric speedometers the tolerance was the same at 100kph. In fact most GM mechanical speedometers have calibration marks on the instrument face that are in the 2% range.


Mechanical Odometers will be spot on with no error unless they are damaged or are improperly driven by the car.

 

My car has 205/60r13 tires, which adds an additional 2% error to the speedometer, I just left that out to prevent confusion.

As rk970 mentioned, it's a 10km/hr error across the sweep. I suspected it might be a misplaced needle, but wasn't certain. Unfortunately, I didn't get out to check against the odometer before winter, but I don't remember any extreme readings from the trip odometer.

Does anyone have any tips on resetting the needle? Is there a good procedure to find zero, or is guess and test the way to go?

I have a spare from an 85 but I'll have to swap faces so it doesn't look goofy, which leaves me right back at resetting needles (and cracking open the odometer on top of that).

 

I would live with it, its never going to be as accurate as modern digital speedo we have on new
cars, ever.