Ok then I await the laughter from all!

If you decided to change your tyre size on a car, say from 18" to 19" does that affect the speedo reading at all.

And if so would a manufacturer do anything to tweak it depending on what size option you went for when you purchased the car.

Say for example the standard wheel was an 18" on a vehicle that was provided off the production line then you get x% variance, so 50mph might be actually 53mph or whatever. But if a factory option was 19" then could that variance be different.

Am I just being thick here?

Yours Genuinely Puzzled

Tyre profile (effectively sidewall height) is changed in step with the wheel size to give the same rolling radius for all standard wheel and tyre combinations, so speedo accuracy doesn't change.

depends on if you also change the tyre size to match.

i.e.
235/60/18 will give the same overall rolling radius as 235/55/19 as you have reduced the size of the tyre side wall to compensate for the extra 1'' on the wheel size (I may be slightly wrong on exact figures but you get the idea) This will not affect the speedo rating at all

if you increased the tyre size from (for example) 235/60/18 to 235/75/19 then you will get a difference in readings, not to mention the possibility of other mechanical issues due to the increased rolling radius (tyres scrubbing on suspension, possibility of increased transmission strain/wear)

As mentioned above, if you increase the overall rolling radius then it will affect your speed, you will actually be going faster than you speedo says, however, the change will be marginal depending on the increase of size of course..

I used to calibrate tachographs/ speed limiters of Scania trucks, the old trick would be to bring in the vehicle for calibration on drive wheels that have the minimum legal tyre tread depth, we would calibrate then as soon as the vehicle lleft they would fit a brang new set of tryes on the drive wheels, as i'm sure you've seen on the motor way 1-2mph on an HGV makes all the difference when over taking.

alot of customers had a set of drive wheels in the yard which they would put on just for calibration then they would be swapped on return.. As long as they were within the legal limit (1mm of tread across 75% of the tyre) then it was fine to calibrate..

To prove the fact all HGV's should be calibrated to 85kph (56mph) however most are calibrated to 89kph as this is the maximum legal tolerance so if all are calibrated the same HGV's should never be able to overtake when both have their foot to the floor on the flat.. but they do, I would say this has something to do with the difference in circumferance of the tyres..
With HGV's it only makes 1-2 mph difference.. but like I said it depends how big you're going.. MY2011 Freelander 2 GS Santorini Black
MY2004 Freelander 1 Sport Java Black
MY1986 Defender 90 Stornoway Grey - Fully Rebuilt this Year

So right! Commonplace in Nottinghamshire too!  10MY (Sept 09) TD4 HSE Auto in Stornoway Grey (Now Gone)

08 FL2 TD4 SE Manual in Rimini Red (Now Gone)

Try this useful website

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator Jules