1975 R90S Speedo ratio question

1975 R90S Speedo Ratio
Speedo currently at Oversdeas being re-built. Usually the interior faceplate has the ratio marked but whoever did the re-build 20 years ago obliterated it. There were some other "unusual" mods from that re-build. The re-builder thinks it is 11/12 but called me for confirmation.
This 1975 R90S has a 33/11 final drive - is that standard for this model/year? I can't think of anything else that would impact the speedo calibration as I am sure the 5 speed gear box is standard (I am unaware of any non-standard 5-speed gearboxes).

It does appear to be a standard rear drive around that time period. My '78 /7 has a 33/11 rear drive. Check Snowbum's page here:

https://bmwmotorcycletech.info/ringgears.htm

About half way down, for 33/11 he shows that the ratio should be "W 1.112"...that's what my /7 has. That is darn close to what the rebuilder is seeing "11/12".

I found the old Palo Alto receipt from August 2003.
Hand written note says " W=1066" and "correct ratio =1066" and ""Note: mileage is (something) as before as there is no closer ratio gearing. Speed is corrected to the 1066 ratio."
I'm not sure how the W=1066 and 11/12ths relate to each other.

EDIT: Kurt - thanks for Snowbum link. I think 1.112 is correct. Not sure how Palo came up with 1.066 as this R90S has 33/11 final drive. I do know Palo had the original owner count tape revs on cable walking the bike a certain distance which would be subject to error. I figure the difference between 1.066 and 1.112 to be 4%.
Eric

I am unfamiliar with the 1.066 ratio...not on Snowbum's page. The ratio is related to the final drive so that speedometer is compatible with the number of turns of the final drive.

You probably saw the Palo Alto ratio test form:

http://www.paspeedo.com/ratio.htm

One is supposed to provide this info to them when they work on a speedometer. If you were to do that specific test, you would come up with a number that the speedometer is supposed to be. I can't find my notes, but I think that if the ratio is 1.112, that means that the speedometer cable will make 1112 revolutions in one mile. Notice in the Palo Alto form, one marks off 52 feet and 9.5 inches. that is 52.79 inches which is 1/100th of a mile (5280 feet per mile). So, I'm thinking if you did that test and pushed your bike exactly the 52.79 inches then multipled the number of turns by 100, you would have your speedometer ratio.

Thanks again Kurt. I'm sure no one had a Snowbum chart in 2003 to tell them that 1.112 is correct. I guess the walk test yielded 1.066 but that included some human error + some tire pressure/tire size anomalies. Still, it's within 4% of 1.112

I'm very grateful for your fast reply and link to Snowbum's page on this. A blazing fast answer to something I knew nothing about.

Might not have been a Snowbum page, but there are plenty of specification sites and manuals including BMW and aftermarket that would listings for these things. But hopefully you can find something that will work for the ongoing rebuild.

MAX fiche shows the following variations:

# Part Number Description
'X' means as necessary (could also be length in meters). Each
01 62 11 1 358 118 INSTRUMENT CLUSTER - MP 3,56W=1,318 2.35 1 NANLA
01 62 11 1 358 121 INSTRUMENT CLUSTER - KM 2,91W=0,67 2.35 1 NANLA
01 62 11 1 358 122 INSTRUMENT CLUSTER - MP 2,91W=1,078 2.35 1 NANLA
01 62 11 1 358 119 INSTRUMENT CLUSTER - KM 3,00W=0,691 (from 09/74) 2.34 1 NANLA
01 62 11 1 358 120 INSTRUMENT CLUSTER - MP 3,00W=1,112 (from 09/74) 2.34 1 NANLA
02 62 12 1 358 123 SPEEDOMETER - KM 3,00W=0,691 0.53 1 NANLA
02 62 12 1 358 124 SPEEDOMETER - MP 3,00W=1,112 0.84 1 NANLA
02 62 12 1 358 125 SPEEDOMETER - KM 2,91W=0,670 0.73 1 NANLA
02 62 12 1 358 126 SPEEDOMETER - MP 2,91W=1,078 0.73 1 $460.08ADD TO CART

To answer your main question, yes the stock rear drive ratio for the R90s is 33/11 (3:1) and the number on the speedo should be 1.112.

RPGR90s said:

To answer your main question, yes the stock rear drive ratio for the R90s is 33/11 (3:1) and the number on the speedo should be 1.112.

Click to expand...

Thanks.