2000 M750 speedo off by ~12%?

[motoxmann]

so for a while now I've questioned the accuracy of my speedometer. then just yesterday I went on a highway trip out of state, and though I was cruising at 80mph according to my speedo, I was getting passed an aweful lot, a lot of which were box trucks and tractor trailors, which annoyed me. then I passed a radar speed sign thing which said I was going noticeably slower than my speedo indicated speed. so I installed a speedo app on my phone. I have a phone holder on my handlebars, so I was able to monitor it closely right next to my bike's speedo. sure enough, it's off by 12%, indicating an incorrect cable spinning ratio somewhere. ie: 100mph indicated = 88 actual, 80 indicated = 70 actual, 60 indicated = 53 actual. 45 indicated = 40 actual, 30 indicated = 26-27 actual.
I know lots of manufacturers make their speedos read a tad higher than actual speed, but 12% is a HUGE variation. 1-3% is the norm with most companies.

what could possibly be causing this? my cable is freshly cleaned and lubed, and the needle in the speedo is rock solid at anything above 15mph. the speedometer itsself I'm pretty sure is original, as is the "transmission assembly" on the front wheel that the cable connects to which it reads from.

does anyone know what the spin ratio is supposed to be for my bike? IE: # of rotations of the cable per 1 revolution of the front wheel?
knowing this would help me determine if the transmission assembly is incorrect or if the speedometer is incorrect

oh, and yes my front tire is stock size

[ducpainter]

The mechanical speedos were all off by at least 10%. I think part of it is they used the same unit regardless of tire circumference.

It isn't right, but it's typical.

No fix I'm aware of. You can reduce the error by changing from a 60 to 70 series front tire.

[Howie]

Mine is 10% high. 

[ScottRNelson]

My 1993 888 SPO was off by nearly 20%, which was very annoying.  I don't know if it was off by that amount at all speeds, but at freeway speeds it would indicate 80 mph and everyone was passing me.  I just lived with it.

My KTM 990 Adventure was off about 10%, but I discovered a way to fix it on that bike.  There is a mode in the speedometer that lets you set the front wheel size, so I "lied" and said that the 21" front tire was actually 19".  The speed is within 1 mph at all speeds I've checked, comparing it to my GPS.  The odometer is now off about 5%, but I feel that it's much more important to know how fast I'm going than to know exactly how far I've gone.

You'll just have to live with it on your Monster - sorry.  Or use some form of GPS device to know your actual speed.

[Speeddog]

Mine's about 10% high with a /70 front tire, '98 M750.

[motoxmann]

hmm, ok, thanks guys. maybe I'll see if I can find some kind of "fix" for it.
I'm a wiz with math, so now that I know the almost exact 12% difference at all speeds, I can calculate my actual speed no problem and will get used to it pretty quickly, but I would strongly prefer it to be accurate so I'll see if I can come up with anything. I'll post up about it if I do.

[carbmon]

Make a new speedo face?