7 replies to this topic

[Kellym]

Hello after some more advice :) I am changing my 10" wheels to 12" do I need to recalibrate the speedo?? X

[Captain Mainwaring]

Yes...or more correctly pick the correct drive gear for the bottom.

You may find that some 12'es with low profile tyres are pretty close to the rolling radius of 10'es

to be honest I don't know if all mini heads have the same turns per mile and the drive gear changes or if there is a mix and match that goes on.

[Carlos W]

There are some good bits in the tech section of the Guessworks website

 

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

[Tamworthbay]

Changed my tyres to A048rs, box and speedo are original and its now reading 67 at 60mph. Has helped to off set the dire FD ratio of a City E but doesn't help with a limited mileage policy! I have looked into changing the speedo drives and its doable, you just need to work out how much it is out, then look for a FD ratio that fits that and get the speedo drives from one of those boxes.

[carbon]

The rolling diameter of 12 inch mini wheels is not a lot more than 10 inch.

 

Most speedos read 5% to 10% fast, you may find that with the larger wheels your speedo now reads more accurately.

 

If you have (or can borrow) a GPS you can use this to check the speedo readings.

[The Principal]

speak to these guy's they can recalibrate your speedo without any major engine/gearbox work - http://www.speedycab...alibration.html

 

just need a few measurements based on rolling circumference of your new wheels and tyres etc

 

I ve used them before not the cheapest but good work

[Captain Mainwaring]

Good to know but may not actually be necessary if he changes the drive gear.

[Guess-Works.com]

Going from stock 10" tyres to stock 12", there's about a 5% difference, but if you go to the more popular 165/60x12 tyre then the difference is nearer 3% which can sometimes mean you will not need to alter the speedo or drives...

 

I believe your vehicle is a 1980, which would originally have 10" wheels, drum brakes, central 90mph speedo and a 3.4 final drive.. The gearbox should have a 6 tooth worm and 17 tooth pinion, if you swapped that onto 12's then I would suggest changing the 17 tooth pinion to a 16 tooth, this will speed up the rotation of the cable to offset the slowing down of the wheel rotation by the correct amount..

 

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

 

Speedy cables will recalibrate the speedo, however, IMO as there are so many speedo drive combinations for the A series transverse gearbox then there is little point.