hears one for 30psi
 

martin you seem to know your maths, so if my tyers are 4.55% bigger than the stock wheels 28mm bigger, what would the difference through gears be in mph,i can't remember what tyers i had on when i found out my clock was 6mph out.

All i need to know is the tyre size

go here http://www.miata.net/garage/tirecalc.html and imput your stock tire size then the size you are using now and it will tell you the difference.

235/45/17's martin, cheers.

You will travel 4.6% faster.

Which to be honest won't make a huge difference. The speedo's over read anyway, so this will make it more accurate

When your speedo reads 60mph your actually doing 61.6mph.

No, this is wrong. For starters on a standard treaded S12 when the speedo shows 60mph, you are doing about 52-53mph.

With oversize tyres, you probably are only doing 56-57mph most.

Thats why when you try to drive per the speedo at 30mph it feels so damn slow, because its only about 25mph.

In my car it doesn't feel like I am running to slow or fast. But if the speedo was right on stock tires then the calculation above is correct.

I've got a GPS sat nav, and a GPS radar detector, and in all 3 S12's Ive had they've shown that.

Even my Nissan Micra, that would show 80mph but was actually 70mph. The only car that was close was my Dads Honda.

The manufactures do it because the last thing they want is a case against them where someone can prove the speedo under reads, so they purposely make it do the opposite.

i've got a graph which was done on the RR, it shows mine at about 6mph out.