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I've just been out for a run and yet again trying to work out the correct mph /km per hr figure for my R80 ST .

At low speeds , say 2000 or 3000 rpm she seems to do 29km per hr (18mph) per 1000 revs in top,5th .
But then I wound her out on a straight to 7200 in top (no, I don't do that often !) and the speedo read 155 km / 96mph.

Which is wrong ? Both ?

Confused in Oz .

So for legal purposes, I see we are talking about a track day here?

They'll both be wrong no doubt, but one more right than the other... I don't know. You need a GPS and you'll figure it out very quickly- once it gets wound up a bit, my R90 speedo runs on about 10% optimism.
After rebuilding my G/S speedo, I tinkered with the spring preload and got it pretty well lined up with the GPS readings between 60-100. Between my old car and old bikes, I really appreciate a GPS for this reason- I like to be able to read a very steady and very sensitive digital speed and not try to decipher and translate a dancing optimistic needle.
You could probably also figure it out with tyre circumference and final drive ratios and and your tacho reading. Don't ask me how though.
The speedo works off magnetic resistance. The odometer is geared (and accurite). Your tacho works off an electronic signal from the ICU or something.

At 7200 rpm in top gear, its all relative anyway, the faster and further you went, the older we all were when you got back, you young sprite.

Yep, I forgot to mention , it was a mate's long driveway.... all private .

I really should put one of those bike speedos on her but I can't work out the relationship at all... it's all over the place . If I assume the tacho gives an accurate (electronically determined rpm ) the speedo is wild, very optimistic at lower speeds but maybe around expectations at the top end ?

An ST is specced at 106mph top speed but they didn't give the rpm . From this it would be right at the top of the redline. I just moved into redline today and it got there quickly.

It doesn't sound like your speedo is very happy. In the spirit of preventative maintenance to avoid expensive and annoying failures, It might be time to open her up and give it a clean and a lube and see what is going on. It is not nearly as daunting as you might imagine and once spread out in front of you will make perfect sense. There are a few good write-ups around the place, and adv's resident expert Wirespokes is usually very helpful with his knowledge if you discover something that needs a second opinion. Once they actually fail things seem to get pretty ugly inside. My G/S one was a slaughterhouse.
After his old speedo died mid-trip, Charlie had a bicycle speedo hooked up for a while. Seemed to work pretty well once it was calibrated.

Unwelcome but sage advice Hans ... I'll have a look around .

A bicycle speedo might be worth owning too.

Calculation of MPH per 1000RPM assuming 3.44 (31/9) final drive ratio and 4.00 tire


Overall transmission ratio in 5th = 3.44 x 1.5 = 5.16

Tire Circumference = 81.69"

Tire Revs@1000rpm in 5th = 1000/5.16 = 193.8

MPH per 1000 RPM in 5th = 193.8 x 60 x 81.69 / 12 / 5280 = 14.99


Edit

When I checked R80ST's (European version) were 3.36 final drive so that would work out at 15.35

Mine is marked 37/11.

I posted the question on ADV and got a few good replies .http://www.advrider.com/forums/showthre ... st19634883

Any recommendations for an affordable GPS that works well for clocking motorbike speeds. Not really interested in knowing where I am, but accurate speed would be nice and good speedos may be more expensive than GPS these days.

From the ADV thread I reckon I got my 171 km/hr or 106mph at 7200 rpm . Good stuff if it's true. Not bad for 30 years old bike !