speedometer question

I just bought a 1982 xj750 , I don't think the speedometer is accurate. I took the bike out a couple of times . Last night when I was going up the road I got it up to what said was 50 mph and scared my wife half to death, she said it felt like we were going 100. So then I took it out on the highway and got about 150 yards behind a semi and kept it up to his speed and it read between 35-40 mph, (he had to be doing at least 65 mph or so. Any suggestions on how to correct and check out this speedometer??

 

The first thing to do is easy enough ... take-off the Speedometer Cable and give it a thorough cleaning and cleaning-out.

Disconnect both ends. Withdraw the Drive Cable. Check ends for squared corners not rounded-off. No bends or kinks. Wash Drive Cable in cleaning solvent to remove old grease and waxy lube residue.

Flush-out Drive Sheath with solvent until its cleaned-out of old greases and oils.

Oil the Drive Cable and the Sheath.
Re-assemble and test for Drive Cable spinning freely within sheath.

Inspect Wheel Unit and Instrument attachment points for Square (not rounded-out) Cable Drive insertion point at where the cable connects.

If everything checks ... hook-up and test the clean cable.
If the Instrument is still inaccurate ... you'll have to check the Cable Drive Unit at the front axle or the Instrument itself.

 

You really should check this out!

You can set you speedometer on 60mph and time the interval between mile posts on the highway.

If indeed your off you can get the wife to follow in the car and see what speed your going at different speeds on the bike.

When you get up to speed give her a sign so she can see what you were really doing.

Check at different speeds to see if the error if an even 10mor 20 mph off at all speeds or is it a variable amount bases on how fast your going.

 

I've had other speedo's apart but can't say yours is this way. But I've seen some that they engineer a 2 magnetic cup design. The cable spins 1 cup directly, and the needle drive mechanizm is driven off the other cup to readup the needle. This makes overspeeding the speedo or pegging it as some would say, harmless to the speedo, as a direct drive or gearing would be destroyed.

If the Magnetic Cup to the needle drive side gets some dirt under it, it changes it's weight, and won't spin at the rate it should for accurate indictation. Spraying it out cleaning it fixes the speedo.

It's worth taking it apart to see if this is the case.
Good Luck.