The speedo on my 83 iron head sportster reads about 20%  fast, ie when it is showing 100 kmh, I'm only doing about 80.,  The speedo was replaced about 6 years ago, b efore I got the bike.    The funny thing is that the odometer does not seem to be that far out.

Anyway, I bought a second hand speedo drive from a 16", wheel (my wheel is the original 19" mag) figuring that this would make the speedo read faster, as the 16" wheel would have been turning faster at a given speed.

I fitted the bloody thing, and it hasn't made any difference.

With British bikes, there are different ratio drives, and also different ratio speedos.

Does anyone know what the situation is with Harleys?

Are all the speedos the same?

Can't work this one out.

 

Dave.

 my heritage is out but not by that much

A lot of bike speedos are out - usually by up to 10%. I'd get a Sigma bike computer and work out the exact speed difference at 50, 60, 80, 100 and 110 (or whatever) and just remember it.

or take a gps with you on a squirt, and see how far it is really out.

I think the 83 is one of the last years of the ironhead & i would say they have a front wheel speedo drive. to find out what ratio your front wheel drive is remove the front wheel take the speedo drive off ( good idea while off, to clean and lubricate ) then unscrew the drive from cable. find some wire that will fit into the cable drive & bend it 90 degree. now rotate the wheel hook up part of the drive one turn slowly & count how many turns the cable pick up turns. normal would be 2 to 1 ratio, but i have seen 1 to 1.. then check what ratio the speedo was made to run with.

Any variation in accuracy between odometer and speedometer is due to instrument quality.
There are indeed different drives and speedometers, each with different ratios, so some crazy combos are possible.
It is very common for a replacement speedometer to be selected on the basis of looks, and not compatibility.
Assuming you have the correct(this means ratio and wheel spacing fit) drive(you may not), it is likely you simply have the wrong speedometer(as in ratio), which explains speed error, and that it is a cheap unit, which explains internal inaccuracies.

Yes Suz, I have a GPS and at 100 my speedo shows 92 at 113 my speedo shows 123....can be frustrating, I ride by my GPS which is mounted on my 6" risers, so directly in front of my face