Firstly, thank you knowledgeable members in advance. I have an opportunity to buy a /7 speedometer (white markings, W=1.244) to replace the speedometer on my '86 R80RT (green markings). Are they compatible? The current speedometer on my '86 RT is the wrong ratio! 🙁
Hank,
Snowbum's website has a very detailed article on final drive ratios and corresponding speedometer W numbers. See https://bmwmotorcycletech.info/ringgears.htm, about half way down the article.
The MAXBMW fiche lists a W=1.244 speedometer and a 37/11 (3.36) final drive for your bike as factory delivered. The fiche shows several speedometers with different W numbers for the /7 series, and specifically a W=1.244 for '76 and '77 R75/7 models. So it looks like that speedometer will fit and work on your bike. The next step is to check the actual final drive ratio on your bike. The ratio should be stamped on the final drive casting just aft the fill plug for the drive shaft housing. If you have a 37/11 ratio, you should be good to go.
As to your current speedometer having the wrong ratio, it should have it's W number printed on it's face. And these 140 MPH speedometers are notorious for reading high. On my 1995 R100RT, it reads almost exactly 10% fast. I have verified this through calculations base on a calibrated tach and GPS readings. One of my winter tasks is to try to correct this error in my speedo. Let the forum know this instrument swap works out for you.
Andy
ABC# 15601
1995 R100RT
As to your current speedometer having the wrong ratio, it should have it's W number printed on it's face. And these 140 MPH speedometers are notorious for reading high. On my 1995 R100RT, it reads almost exactly 10% fast. I have verified this through calculations base on a calibrated tach and GPS readings. One of my winter tasks is to try to correct this error in my speedo. Let the forum know this instrument swap works out for you.
When all is correct, the tach and speedo needles should hold at the same angles while in top gear. So if the tach needle is pointed at the 10 o'clock position, then the speedo needle should also point at 10. This is a quick, easy way to see if they are off, and exactly how far.
Also nice to know if a cop is following you. ?
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Thank you Andy & Richard. Yes I'm aware of the stamped final drive ratio - 37/11. My concern is whether the plastic cast of the '86 R80RT instrument housing will "fit" the /7 speedometer. I supplant the analog speedo with a GPS-based digital one (as can be seen in the accompanying foto). These Moto Meter speedometers are terribly inaccurate at hwy speeds!
I remember many years back I tweaked my speedo so it was more accurate at critical cruising speeds. Back then there was no gps, so I had to compare with our car, precisely piloted by my wife. I think the error was close to 10%. I then disassembled the speedo and used an electric drill to spin it. Wide open with the drill was right in the desired 55-70mph range, as I remember. I could then tweak and check with the drill until satisfied with the result.
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