10 replies to this topic

[Womak1992]

I'm having difficulty getting my speedo accurate. A previous owner had changed a few things around. It currently reads about 10mph to slow so reads 30mph when doing 40. Id like to try to make it accurate but don't really understand how to. The car is a mini thirty and original engine had a final drive ratio of 3.1 and when it got changed had a metro engine with a final drive of 3.65. The wheels were also changed from 12" to 13", as well as the clocks being changed from mini thirty to mini fourty ones. Is there anyone that could tell me how to fix this issue? Thanks

[Dan]

You can either have the speedo head re calibrated to match what's there now, or change the speedo drive in the gearbox to match the new FD. The change in wheel size won't make much of a difference (what size tyres are you using?), but the final drive will. Changing the drive gear is the best way to go.

Edited by Dan, 15 November 2014 - 11:17 AM.

[Womak1992]

Tyre size is 175/50/13.

[Ethel]

Guessworks has a ratio calculator and some info on speedo pinions on his site. Go off the mileometer - it's direct drive, so a more reliable indicator of mismatched gearing, speedos can under or over-read if they are faulty.

[Guess-Works.com]

This is what you would have had originally....

 

3.1 Final Drive, 145/70x12 tyres, 1242 TPM speedometer, and a 7/17 speedo worm and pinion drive

 

This is what you probably have now....

 

3.6 Final Drive, 175/50x13 tyres, 1300 TPM speedometer, and assuming it has the original metro Speedo worm an pinion of 5/17

 

which would mean when your speedo reads 20% under, which is near what you state...however, if your gearbox has a 3.4 final drive that will be under reading nearer what you are experiencing, 40 actual speed, when speedometer reading 30...

 

So you need to find out for sure, what final drive you have....

 

If you do have a 3.6 then you need to change the worm drive in the gearbox from a 5 tooth to a 6 tooth...

If you have a 3.4 then you need to change both the speedo worm and pinion drives to a 6 tooth worm and 16 tooth pinion

 

http://www.guess-wor.../Tech/ratio.htm

Edited by Guess-Works.com, 15 November 2014 - 11:38 AM.

[Womak1992]

Thanks for the replies. How would I know for certain what final drive I now have? Guessing on how he seems to of changed bits I would guess that it is the 3.6 but better to be sure first. Any recommended places to buy worm drives from?

[Ethel]

you could take the spark plugs out and roll the car in 4th gear (1:1). The ratio of engine revolutions (rotor arm) to wheel revolutions will be the same as the final drive.

[Steve220]

I'm having difficulty getting my speedo accurate. A previous owner had changed a few things around. It currently reads about 10mph to slow so reads 30mph when doing 40. Id like to try to make it accurate but don't really understand how to. The car is a mini thirty and original engine had a final drive ratio of 3.1 and when it got changed had a metro engine with a final drive of 3.65. The wheels were also changed from 12" to 13", as well as the clocks being changed from mini thirty to mini fourty ones. Is there anyone that could tell me how to fix this issue? Thanks

Is it always 10mph out? I'd at motorway speeds does it read 60mph? Are you doing it against GPS?

[Womak1992]

Ok thanks ill give that a go. Its roughly 10 mph give or take a couple out as tested with someone in a lead car changing speed.

[Steve220]

Could it just be the needle has been moved with changing the clocks? Or dial background?

[Womak1992]

The dial background all looks level and straight an the needle seems to be sitting where you'd expect it to at stationary