85mph spedo 140mph spedo compatibility?

[TELLURIDER]

I am wondering if there are any compatibility issues with swaping an 85mph 82 R100RS speedometer with an older 140mph speedometer. I used to own a 77 R100S and I now own the 82 and I think it is lame that the spedometer only goes to 85. Would the speed and odometer be acurate? Has anyone done this before?

Thanks

[sumran]

The speedometers are interchangable. The issue that matters is the gear ratio of the speedometer. That is printed on the dial near the bottom. You will see a notation that says "W=1.078" or some other number. The proper ratio is determined by your final drive. If you have a 32/11 final the 1.078 is the correct speedo.

The 85 mph units tend to be more accurate that the 140 mph units from what I have read. I changed mine because the odometer was broken. The 140 mph is definately reads higher at the same RPM than the 85 mph unit. I'm old and slow so I would have kept the original if all had been working.

[lbrackr756]

Can you believe that the 85mph speedo was the result on a US Gov't mandate to decrease the upper limits of speedos due to the 55mph laws passed then? Some bureaucrats thought this would make folks slow down since the needle would be higher towards the upper end of the scale. They did that on cars, too. I'd leave the 140mph speedo in just for originality factor. The final drive / speedo ratio is the determining factor of accurateness, though.

[TELLURIDER]

Nope other way around I was thinking about changing my 85 mph spedo for a 140 but I decided to leave it alone for now. I hardly ever get on the interstate anyway its all twisty mountain roads around here (Sigh) ;)

[Bob_M]

The speedometers are interchangable. The issue that matters is the gear ratio of the speedometer. That is printed on the dial near the bottom. You will see a notation that says "W=1.078" or some other number. The proper ratio is determined by your final drive. If you have a 32/11 final the 1.078 is the correct speedo. .

My 85 MPH speedo stopped working at about 120,000 miles. I switched to the 140 MPH model and suddenly my RPM at speed was about 700 lower. No wonder I got a performance award at 60 MPH!
Take a good photo of the old speedo for the file when you swap em out.:deal

[lkchris]

My 85 MPH speedo stopped working at about 120,000 miles. I switched to the 140 MPH model and suddenly my RPM at speed was about 700 lower. No wonder I got a performance award at 60 MPH!
Take a good photo of the old speedo for the file when you swap em out.:deal

BMW speedos read high by design.

[PHMarvin]

Hi,
Kent is right in that most BMW bike speedometers read high by design. However, for some reason the 85mph speedos were surprisingly accurate. At least they were on both my wife's '81 R65 and my '81 R100RT.

[Bob_M]

the 85mph speedos were surprisingly accurate. True enough, but sometimes stuff breaks.

My speeding ticket was quite a while ago, but the LEO had me clocked at 75 while I read 60 (or 65) on the speedometer. I had thought the gearing on my bike was changed because the tach read much higher at any given speed than on my other bike. Kent is often right and he actually diagosed the errant speedo.

[sumran]

for some reason the 85mph speedos were surprisingly accurate...

Your experience with the 85 MPH speedos is the same as mine. Perhaps 85 MPH didn't leave enough room for "optimism".

[breyfogle]

Your experience with the 85 MPH speedos is the same as mine. Perhaps 85 MPH didn't leave enough room for "optimism".

Part of the law that imposed 85 MPH speedometers also mandated more accurate speedometers. When the law was repealed, so was the accuracy requirement.

Also IIRC, German law (or maybe the EU) requires speedometers to read some percent high and specifically bans speedos that read any amount low. BMW is simply selling speedometers that comply with current regulations.

[sumran]

Thanks for the info. Interesting to see how various governments function and how it impacts daily life. Seems like accurate information would be valued and that manufacturers could use appropriate instruments. Personal responsibility and judgement are usually undervalued. No political agenda, just interesting to watch the machinery.