spedometer reads high but odometer reads low?

[Rema1000]

I have a bicycle computer on the GS, which I added to doublecheck the spedometer.  I found that the spedometer is a little bit high; for example, when the bike speedo says 74, the computer says 71 (mechanical speedo is reading about 4% high at 71mph).   I had expected the spedometer to be reading more like 10% high based on other postings here on GSTwins.

I recently started using the computer as a second trip odometer, and found that it never added-up with the mechanical odometer.  Strangely, the OEM odometer is about 5% low by comparison (!).  So when I hit reserve, the bike says 120 miles, but the computer says 126 miles.  I can understand why the speedo would be inaccurate, but the odometer should be reliable.

All I can think of, is that either these two errors are completely unrelated; or else maybe the odometer is is reading 5% low to begin with; and the spedometer is reading 10% high; but that because it receives its input from the odometer, it is starting-off 5% low, and subsequently ends-up  only 4% higherl than actual speed.

But the silver lining to this confusion, is that it appears that I'm getting better gas mileage than I had previously thought .

[96gs]

You put your reserve on at 120 miles? I can 200 miles before I hit reserve. It might just be because you have a bigger than stock tire.  :dunno:

[PAC]

I wouldn't trust the bicycle speedometer either.  Are bicycle speedometers meant to handle speeds of 70+mph?  I do trust my GPS, as it has proven to be about as accurate as anything.

I need to make a graph of real vs. indicated speed sometime, but basically I've found that my speedometer is accurate up to about 40mph, then gets worse and worse.  I haven't checked the odometer reading, but I think it's pretty accurate.

From memory...
Indicated = Real

40=40
50=48
55=51
65=58
70=62
75=66
80=70
85=74
95=83

And that's as far as I went.

And 120 miles/tank?  That seems wrong.  Normal acceleration and such I get about 205 miles/reserve.  The worst ever was 185 miles/reserve and that was after lots of high-revs and agressive riding...

[Rema1000]

I have a '92 (smaller tank).  The earliest I ever hit reserve was at "120 miles" (which I now believe was really 126 miles).  So after that, I always filled-up by 120 miles (didn't want to have to hit reserve in heavy traffic).  So I really never found out how far I could usually go.  

Regarding the tire, the front is a 110/70-17 Kenda, but I have a better replacement tire just waiting for a rainy afternoon.  So I'll see soon-enough if the tire is the cause.

<EDIT>
The computer is a Sigma Sport, rated to 183mph.  It's sold as a cyclocomputer, but is used with go karts and other motorized vehicles.  But I'll do a GPS test just to be sure.