The 4-digit number in the lower corner of the speedometer face should be chosen to matchup with the final drive ratio of your R65. IT is possible that someone swapped the rear drive on your bike, so the best was to check for a match is to look at the top of the final drive, on the flat surface near where it bolts onto the swingarm. You will see a numeric ratio of either 31/9 (most 78-80 bikes had these) or else 32/9 (most R65LS bikes had these and many 81-84 R65). The final drives are all more or less interchangeable on all the dual shock BMW bikes, and sometimes different final drive ratios were offered as dealer options, so it is good to check/confirm.
The W1318 speedometer is intended for use with the 32/9 (3.56) rear drive
The W1276 speedometer is intended for use with the 31/9 (3.44) rear drive.
And, if you want even more info on all BMW airhead speedos, this is Bob Fleischer's page on the subject, which is actually relatively concise and has a useful chart:
http://bmwmotorcycletech.info/ringgears.htm
