Apparently when the speedo reeds 60mph I'll actually be doing 56.9 Mph.

This is kinda interesting because when I'm using tomtom my speed is always different to TOMTOM e.g car speedo 60mph tomtom 58 mph, i wonder how many other speedo's are out ??. And that's using the proper specified size tyre ???..

Fuel consumption will be interesting, yes the tyre will be turning more becuase it's smaller BUT, surely the fact that I'm now running conti eco contacts and not cheap Kuma tyres and have lightweight alloys instead of big heavy steel rims should make a difference.

If a speedo error of just 3.1 mph (at 60mph) is the only downer, which in itself is the best anti speed device (as I'll always be below the limit ;-), it's not as bad as I'd first thought.

Can you please tell me where I can find out if it's street legal, given that as you say; All speedos should read around 5% fast.

I've never heard before that Alloys are actually heavier that steel rims, I'm in to mountain bikes and it's well known that Kevlar (folding tyres) are lighter and stronger than steel tyres, so are you saying that Alloys are weaker than Steel wheels and therefore need more metal ?.

I've been onto a web site that says i'm running at 5.2 % too fast, just .2 % off of your statement, how can that be street illegal, the other question is what about all the others that fit bigger tyres, as well as being illegal ?, there also going too slow which means they'll be actually breaking speed limits ??. E.g someone going from a 195/60r15 to a 205/60r15 will be going 2% slower meaning that they'll be doing 61.2 mph instead of 60?, surely that would mean than "alot" of cars on our roads are not street legal ?.

It's also surprising to learn that Tyres such as the Conti Eco Contact are'nt as fuel efficient, hard wearing and better speed rated than the likes of Kuma Tyres, I should've saved myself some money, £15 a tyre ;-)

Edited by Bigdaveg on 06/07/2010 at 22:05

I don't really need to because someone's already got all the answers and by publishing it they'll not only help me but countless others who have also changed their tyre size.

In the mean time I'll hook up my tomtom and just see exactly how much out it is, bearing in mind that it was 2mph out using the supposed correct tyre size.

Lighter wheels can improve handling by reducing unsprung mass, allowing suspension to follow the terrain more closely and thus improve grip, however not all alloy wheels are lighter than their steel equivalents. Reduction in overall vehicle mass can also help to reduce fuel consumption

The above is the first paragraph in Wikipedia.regarding alloy wheels.Born out by the fact that the winter steel wheels for my Saab are lighter than the summer alloys.

You understand nothing about physics ie rotating mass.

You do not understand how GPS works.

You do not understand about wheel offset.

And you certainly do not consider road safety to be important.

There is such a thing as type conformity and if your car was to be invovled in a serious accident the insurance companies look at this matter for loopholes not to pay out.

Edited by Andy Bairsto on 08/07/2010 at 10:41