RPM's at 70

[EGRG]

I've read in one thread, that at 70 mph Rpm's are about 5000.  Mine operates slightly higher close to 5500. Are these values correct?  My speedometer trip meter does not reset, so I was wondering if a possible speedometer inaccuracy and trip meter failure are related.  

[Bob_Roller]

The speedometers on these bikes can be off by a bit (some, a lot), but your readings aren't too far off .

If you can get on an empty road, time how long it takes to travel a mile at 60 mph, using road side mile markers .

[Rob Valdez 79 R65]

I've read in one thread, that at 70 mph Rpm's are about 5000.  Mine operates slightly higher close to 5500. Are these values correct?  My speedometer trip meter does not reset, so I was wondering if a possible speedometer inaccuracy and trip meter failure are related.  

I would say you are about right on, according to my GPS:

http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1168796938

[nhmaf]

What is your final drive ratio? (It is stamped on the top, flat silver part where it bolts onto the black painted swingarm near the right shock absorber mount).  It should be either "32/9" or "31/9", which would be either 3.56:1 or 3.44:1 ratio final drive, respectively.  Your speedometer ratio (W:number in small print at the bottom of the speedometer face) should either be "1.318" for the 32/9 or "1259" for the 31/9.

Assuming a fairly 'standard' tire circumference of 79.3 inches, 70 MPH should be right around 5000 RPM with the 32/9 and 4850 with the 31/9.... but, the speedometers often do run ~ 3% or as much as 5% fast, which could account for a few MPH at that range.  Different profile tires could also shift the RPM point a little.

Things are probably OK on your bike - 'cept for thetripmeter thing you mentioned.

[Barry]

I've read in one thread, that at 70 mph Rpm's are about 5000.Mine operates slightly higher close to 5500. Are these values correct?

5500 RPM is too high for 70MPH. Assuming standard 4.00 x 18 tire fitment on a 32/9 final drive you should be doing a theoretical 79.8 MPH at those revs and on a 31/9 you should be doing 82.4 MPH . There are a few variables in the calculations like rolling circumference being a few percent less than unloaded measured circumference but this is more than offset by the speedo being deliberately calibrated to read high. 5500 is still way too high for an indicated 70 MPH. Even metric tires shouldn't produce that amount of error.

The attached spread sheet calculates revs vs MPH in every gear. You will need to use the correct final drive ratio and again it assumes standard tires but you can measure your own and substitute the figures. If you don't have Microsoft Excel or Open Office to open the file I can do the calcs if you know the final drive ratio and tire size

[Yabby]

5000 at 70mph

[bruce_launceston]

I just fitted a new rear tyre and compared to the one removed it is a full 20mm (3/4") taller overall than the one I removed. Even that would throw the revs out.

[Barry]

I just fitted a new rear tyre and compared to the one removed it is a full 20mm (3/4") taller overall than the one I removed. Even that would throw the revs out.  

It sounds and I suppose looks like it ought to make a big difference but in fact it's just short of a 3% change or about 148 RPM at 70MPH.

[EGRG]

I will check the ratios and report later today.
Thanks for all the recommendations

I will have my son drive with me on his pickup to compare speedometers.

[Lucky_Lou]

We had another thread on a similar subject, mine works out at 15mph per 1000rpm 5000 = 75mph as i recall its a 3.44 on the final drive ratio.
Lou

[Dizerens5]

To check a rev counter (tacho) - mine on the 79 reads far too low, just over 3,000 at 60 mph, though the engine seems very happy and pulls like a locomotive from around 2,000 --  on the dial! I want to check it but this seems easier said than done. Accessory stores like VWH sell small tachos, not expensive, advertised for 0 - 8,000 rpm, for 4-6-8-12 cylinder with alternator connection. What does this mean? Does it have a triple connection for the 3-phase output? And is the reading affected by BMW's dead spark - do you set it for 4-cyl and then divide the reading by half, or is the meter fooled by the dead spark into giving a correct reading? Or have nothing to do with the spark? Or is it just not usable?

[EGRG]

Ok. Final drive ratio: 32/9
Speedometer: W1.318
Rear Tire is a 4.00-18 Michelin Macadam with a circumference of 81.5 inches.
At 70 mph on the companion vehicle  the tach reads about 5250 the speedometer reads 67 mph.
I have a bad odometer as My trip meter started working but every traveled mile the odometer registers 10 miles. So I will probably have it checked by Overseas in Austin.
I may also need to check tach as i may be getting a bad reading.

[Barry]

DoubleZero

You have the correct speedo ratio for your final drive.

Crunching your numbers gives 4825 RPM at 70 MPH so there is a problem somewhere. Looks like your speedo is reading low which is unusual as well as the Tacho reading high. Tacho's are normally pretty accurate given a clean signal from the coil.

[EGRG]

I agree Barry, I will have the speedometer fixed as the odometer is running wild. And will have to figure out a way to compare tach readings.
In my business we install very accurate speed sensors that are placed on flywheel housings of engines so as to be very close to the ring gear teeth.  But it would involve drilling and tapping a 3/8' hole close to the flywheel.

[Bob_Roller]

I checked the speedometer/tachometer readings this morning on the way to work .

I know my speedometer reads 2 mph high (85 mph speedometer) at 72 mph indicated, the tachometer reads 5300 rpm.