Bongo Fury

Hi my 2.0 van should have had 215/65/15 rear tyres but was on 215/70/15s, I never checked against a satnav but I knew it was reading a bit high on the speedo.
Ive just fitted Rx8 alloys with 245/40/18s which according to the tyre comparison sites should only be -1.21% different in diameter compared to the 215/65/15s, well within their +/- 2.5% recommended maximums.
When I drove the van back yesterday I borrowed a TomTom which gave a mph reading of 42 compared to 50 on the dash. This seems a bit excessive. The speedo is a mph dial, with odometer in kms.
Is there any way around this without changing the tyres?

You can reposition the speedo needle on its spindle to get it closer to the real speed. It can be fiddly because the needle is held against the bottom stop by a spring which makes guessing the 0MPH position difficult but with practice you can get it almost spot on at one speed at least. Note this has to be done when fitting MPH dials so this may be the cause of at least some of your inaccuracy

Ah! That makes sense. Just checked another tyre site which said at 70mph on speedo actually would be 69.22 with these tyres.
Thankyou

If the needle is positioned wrong, your speedo reads with the same error whatever the speed. In your case it'll indicate 28 at 20, 38 at 30, 58 at 50. Repositioning should fix for all speeds.

Cheers Chris!

215 70 15 and a speedo out of a pre digital mid nineties british mazda 626 or mx6 which read in miles are exactly right with the satnag.

these are marked on the dial with the letters GA5R. had mine in for years, no corrector chip or face plate needed.

http://igmaynard.co.uk/bongo/forum/view ... tip#p84097.

http://www.igmaynard.co.uk/bongo/forum/ ... a5r+speedo