Ok, it is simple ratios.

If you put a bigger overall diameter on the car your power at the wheel will be reduced but the potential top speed should actually be higher.

Think about two mountain bikes, one has massive wheels and the other has tiny ones, it will be easier to start the small bike off but the one with bigger wheels will likely go faster.

You should be able to calculate the power loss (ignoring and other mechanical losses) with the following equation:

If for arguments sake we say the car is traveling at 100mph and has 90bhp with 13s on.

Under the same conditions with 17s it would be

17/13 * 100mph = 130mph
13/17 * 90bhp = 68bhp

This assumes the outside diameters are 13 and 17 whcih is never actually the case!

The thing is you i've assumed the same conditions and got a top speed of 130mph but the car would probably never get there because it would require too much power.... which is now reduced :)

I think i have my maths right, its been a while!!!

Mike...

yeah with my 15"s it used to be ok....but now :(

I got a 1.2 8v, 15s on can get 100 in it no probs. Put some 15s on and lower it, it'll look fine.

i got 13" steelies n top out at 100, with 110 downhill. (indicated)

wouldn't the extra weight of the alloys make a difference?

1.2 8v on 16s = :( i top out at 85 and chew thru fuel

Matt

Yes. I just put some 17s on mine, enuff bling bling for me :)

And what about 18's? Am I stupid for wanting this done?

I find it hard to belive? The only reason the car would be slower is if the wheels are bigger in overall diameter. Otherwise it would make no diffrece to the speed? Soooooo if the wheels are bigger and hence slower, then for every one rotation of your wheel the car now does a greater distance. your spedo would then be out of calibration and you would be doing faster than your clocks say. For each one mile per hour you would be doing one and a bit miles per hour.

no coz was drivin with mate on 17s 1,4 16v phoned him at 80mph his was at 80mph too

ICY, You recon its slower with 17" wheels on but maybe its no diffrent at all? To effect the car's speed the rolling diameter of the wheel must be longer than the standard and hence your final drive ratios would be slightly effected. On the other hand your clock would say 85 when your doing much more becouse one revolution of the wheel is covering more distance. It probably all balances out.

had my mates 17's on my car here are the speeds :-
15" - 150mph then off the clock
17" - 150mph then off the clock :D

As long as you keep the rolling radius similar, the only thing slowing you is the width of the tyre.
Ste

ok here my speeds i got on mine
14s top speed on flat 93 downhill 100
15s 90 95
17s 85(usualy under80 tho) 90

Am I not right in thinking that it shouldn't really be affected that much? Yeah, it defo will be with 17's because it's such a big difference but when you go up a size in wheel you come down a size in tyre so - overall the rolling diameter will be near the same on 15's or even 16's:boggle:

Sure I read it somewhere but then it completely goes against what I thought and what everyone says.

your acceleration will be non existent with 17's!! you still feel a bit of a drop on 15's

but your car is slow as anyhow, so acceleration is already non-existent!

will 15" or 17" alloys slow me 1.2 down by a lot?