8 replies to this topic

[ken22]

hi every body i have a problem, last year i renovated a barn find rsp mini cooper, it passed the mot but on driving home i noticed the speedo was showing really low for the speed i was travelling. I compared it to another car and apparently 30 mph was only showing around 20 mph on the cooper. Now i,ve read that different gear ratios can make a slight difference but not this drastic can any body shed any light on what the problem could be,and what gearbox number should be on the casing so i can check if its been changed or not. Although it doesn,t look like its been messed with.I might add that the speedo finger doesn,t flicker which would indicate a dodgy cable or something slipping thanks

[nicklouse]

Casing number won't tell you much.

Best bet it have a look at the drive under the point the cable attaches as thst is easy to access.

You are running the or react wheels?

[ken22]

doesn,t the casing number tell me  if the right gearbox is fitted

[cal844]

Yes, the correct for the period gearbox will be a DAM 5626 however the diff and gearing may well be different

[Spider]

doesn,t the casing number tell me  if the right gearbox is fitted

 

No only roughly what era it was made.

 

The only sure way to find out is to remove it and check the teeth number.

 

You can get a rough approximation but putting the can in top gear and measuring how far it moves (when pushing by hand) for one engine rev, but if you've got Economy Drop Gears, this too is out the window.

 

From this, you might be able to make a guess, following checking your present Speedo Drive ratio, where you need to go with it. It is unlikely to be right, only better than what it is.

 

Keep in mind too, that as well as different Final Drive Ratios, there were also different Speedo TPM (turns per mile) fitted too.

[tiger99]

No. Parts could have been substituted over the years. You need to check your final drive ratio. There are several ways of doing it。I will describe one. Mark a front wheel and the crankshaft pulley. Take out the spark plugs. Select top gear. Roll car forward (or back, for a second check), counting turns of crankshaft and wheel. For good accuracy I suggest 19 turns of the engine, at least. Now divide engine turns by wheel turns and that is your FD ratio.

Now look up the Somerford web site or elsewhere for the correct speedo drive gear and pinion for that FD.

You will need at least a gasket and if I remember correctly, a locktab, if the worm gear has to be changed. The pinion is easy.


Edit: Moke Spider just beat me to it!

Edited by tiger99, 15 May 2017 - 11:57 AM.

[Guess-Works.com]

or.....

 

at 60mph your RPM is near enough 1000 x your final drive ratio......

 

Also it may be worth checking the odometer becuase that is more accurate than the sweep of the needle... so drive a measured distance and see what that reads... likewise at 60mph is should take you 1 minute to drive 1 mile.

[ken22]

Thanks everybody but this gear ratio and final drives goes over my head, think i,ll confine it to a corner of the garden to slowly rot away.

[cal844]

Thanks everybody but this gear ratio and final drives goes over my head, think i,ll confine it to a corner of the garden to slowly rot away.

No don't let it rot, just do a basic time distance calculation