Altering wheels and speedo calibration

[carmadgaz]

The other day I wrote about my latest purchase - A 1968 Volvo 131 Amazon.

It has been noted following a conversation with my neighbour (following me back from the auction) that the speedo seems to under-read by a chunk (GPS verifying it tomorrow on the way to SS) .

It has been fitted with Minilight alloys, would this account for it? (I'm guessing yes). more importantly, how do I get it reading right? (or do I just cover the numbers and write fresh on myself?)

The speedo is one of those that goes left to right rather than round on a dial.

[calibrax]

Speedo is supposed to under-read slightly.

As long as your rolling radius is the same as the original, then it should be correct. So, if you fit bigger wheels than the originals, you need to fit lower profile tyres to compensate. There's a good check here that will tell you how much the speedo will be out when you compare two different setups...

http://www.miata.net/garage/tirecalc.html

Fitting lower profile tyres than you have now (i.e. reducing the rolling radius) will make your speedo read higher than it is reading currently.

Edited by calibrax on Saturday 12th March 23:39

[carmadgaz]

calibrax said:

Speedo is supposed to under-read slightly.

I thought they overread slightly? Say showing 33mph when you are doing a real 30mph. Mine is showing 55mph but doing a real.

Thanks for the info I shall take a peek

[calibrax]

carmadgaz said:

I thought they overread slightly? Say showing 33mph when you are doing a real 30mph. Mine is showing 55mph but doing a real.

Thanks for the info I shall take a peek

Sorry, yes... they should overread. My mistake!

[Balmoral Green]

calibrax said:

There's a good check here that will tell you how much the speedo will be out

http://www.miata.net/garage/tirecalc.html

These tyre/wheel calculator resources are great, a couple more here...

http://www.tyresave.co.uk/tyresize.html

http://www.carbibles.com/tyre_bible.html

My Bentley is -3.65% out, and a little outside of the max recommended. But the Panda is only out by -0.1%, so almost no change.

You can calculate ideal tyre widths for a given rim width too.

[AndrewW-G]

Simple bodge of this is use sat nav to find true 20,30,40,50,60,70mph then put a small splodge of tipex at the relevant point on the glass dial face