I've had a search through the forums and cant find an answer to this proposal.

I know a lot of car manufacturers have this problem but it really niggles me that the speed I see on the dash clocks is not the same as actual GPS speed (in a straight line).

My Zetec is reading 56 mph on GPS and 60 mph on the dash, so about 6% out.

My first check was the drivers handbook for tyre sizes and the actual tyres fitted. My handbook says that for my 1.8 estate (57) I can have either 205 or 215/55R16 which kind of doesnt make sense to me. Surely if you widen the tyre the profile should decrease to keep the same rolling circumference, but what is strange on my estate is I have 215 on the rear and 205 on the front.

In an effort to bring my speedo closer to true I aim to change the fronts for 215's, or I could simply swap back to front and see what happens. This will give me a 2% change.

What I would really like is to get the speedo bang on or just 1mph over just to give the reassurance of knowing the speed I see on the dash is the actual speed for the Police with their laser !

I am also thinking that when my tread runs down to a limit where it needs replacing I might even go up a profile from 55 to 60. If I do this now and swap from 205/55 to 215/60 I change by a little under 5% or if do the bigger profile after I put the 215 on the front I change by just over 3%.

Overall the difference between actual and GPS would be 1%.

I havent scanned the full list of models in the handbook yet but I'm guessing there are alternative tyre size/profile combinations for the different models so I am hoping for example a rubbing tyre on full lock wont happen.

Can anyone see any adverse implications for going to a 60 profile ?

I think you're worrying over nothing. Every car I've ever owned (mostly Fords) has been a bit over-cautious on the speedometer readout. My current Mondeo is about 5-6% off what the satnav says, too. If you're in a 60 limit, drive so it shows 60, then there's no problem with the rozzers.

As for having different sized tyres front and back, that's another issue. I've no idea on the implications of that one!

All cars have a 10% speedo over read.

It's why we allow for 10% leeway on speeding tickets.

Don't put the larger profile tyres on the front. You will reduce the acceleration especially in the mid range which you will notice more at motorway speeds where you will be changing down more often than usual.