Mk3 tyre size & speedo

[1967 MkIII]

Hi All,

My speedo under-reads by around 8-10 mph, could this be linked to larger tyres fitted?

I've got 155/80 13" while the original spec is 5.20S x 13" from http://www.triumphspitfire.com/tires.html  

Does anyone know how to convert these to compare? The same site says the original rim width was 4.5" or 114 mm whilst I'm on 155s but beyond that I'm lost.

Otherwise any idea how to calibrate the speedo to my tyres?

Thanks
Steve

[Clive]

you say 8-10 mph, is this a constant amount, or does the actual amount change with speed? differences are usually % changes. If so the speedo can be re-calibrated, if a constant smae amount out, it may be possible to re-position th eneedle, but  get decent advice as if you just take the needle off you will be stuffed! It is possible, but needs careful marking up, and I have forgotton how to do it :B

[1967 MkIII]

Yes it's a % amount, at 90 mph it marks around 80, at lower speeds the gap narrows.

The mileage also increases more slowly which is irritating on longer trips as I can't use it to measure distance travelled.

[junkuser]

I have a couple of  4" wheels fitted with 520 x 13s and the rolling radius would appear to be significantly GREATER than that of the 155/80 x 13s on the car.
Can't dig them out at the moment to measure but recently put them on the Spit whilst cleaning up the wheels that are in use.   No, I did not drive on them.
Have not been able to find anything on the NET comparing radial and bias ply tyre sizes.

[Chambers]

I have exactly the same on my 1500, only about 10% out so I thought it was down to the tyre size.  Would be handy if someone could measure the standards so we could hopefully find some radials that are similar