tach gear ratio

[mcclelland7640]

i have a 72 r75/5 i just took my speedo in to be rebuilt. and was asked to find out the ratio for the tach so it could be properly calibrated. so how do i do that as there were no numbers on the speedo

[schrader7032]

Palo Alto has a form on their website specifically for this:

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

What you're trying to determine is how many times the cable will turn in 1 mile. For example, if the number of the face of the speedo was 1.112, the cable would turn 1112 times in one mile.

Typically, an R75/5 would have come with a 32/10 final drive which should be mated with a 1.186 speedometer. See if the test gives you something like that. Snowbum also discusses it here:

http://bmwmotorcycletech.info/ringgears.htm

However, things could have changed or you have a different arrangement. The specs here show some variations in final drives:

http://www.bmbikes.co.uk/specpages/R75-5.htm