If a tyre of a different size or aspect ratio is fitted, this will affect the speedo. To check the effect of the tyres on the speedo, measure the rolling radius of the front tyre and rear tyre. (That’s the distance from the centre of the wheel to the ground with the tyres at normal pressure)
Using the MGB as an example:
165 x 14 is the original tyre size, and the speedo is calibrated to that. For a 165 x 14 tyre (or a 185-70 x 14 which is the same overall diameter), this is about 304 mm. Then work out
original rolling radius (mm) x 100
Rear rolling radius (mm)
This is the percentage error in the speedo.
Other sources of speedo error come from changes in the diff ratio or the speedo drive pinion in the gearbox. The only usual cause of these is if an overdrive has been fitted when the car was originally non overdrive. If undertaking this, check this out by comparing the small 4 digit number (usually around 1000 – 1400) on your speedo with other MGB’s which originally had the donor gearbox. (See list below)
This 4 digit no is the number of wheel revolutions per mile that the speedo is calibrated to and is different between overdrive and non-overdrive. (Sorry, it’s bit of info that keeps escaping me.)
I do recall that when an overdrive is retrofitted, the mph speedo reads very close to half of km/h (eg 50mph means 100km/h)
Speedo gear ratio changes for overdrive boxes were as follows:
1st ratio: all 3 synchro cars (made 1962 – 1967. Note that shipping & showroom time can mean that the year of manufacture and first registration can differ, so cars registered early 1968 can still be 3 synchro)
2nd ratio: 4 synchro chrome bumper (late 1967 to 1974)
3rd ratio: rubber bumper cars.