cheers ill drop the profile on the fronts as they both will need new tyres soon

Depends on the profile on them. Lower profile tyres lower the gearing and increase acceleration and also will make the speedo read a lower speed. Higher profile ones increase the gearing and reduce acceleration and increase the speed your speedo will be showing.

If your Speedo is too fast then you need lower profile ones then you have now. Based on your numbers your about 3-4% out. Depending on the size of the wheels and tyres on it now, one profile level (65 to 60, 60 to 55, etc) should get it pretty much right.

[Edited on 20-08-2006 by DarkBahamut]

cheers mate

if i stuck std size gsi tyres on that would make it right?

Try this website to find the best match. You can see the difference at different speeds too.
http://www.paspeedo.com/calculator.htm

yeah, you need to calculate the rolling radius of the original and try and match it by lowering the profile of the tyres.

i have a tyre size that is slightly bigger than std vectra i think on the van

now at 30 on the speedo im doing 31
40 is 41-42
50 is 53
60 is 64
70 is 75 ish

if i put a lower profile tyre on this would correct it?
by the way its on 14's so i have plenty of profile left even if i did get a lower profile tyre