A post has got me thinking...

[Gatsods]

The post about how fast everyone has been on four wheels got me thinking about mine.

Only been driving for just over a year anyway, but one post I was caught by in particular was about how changing wheel size could have an effect on the speedo.

I have a Corsa B 1.2l 16v (good old insurance.. -.-) and it wears 15" alloys that came off (I believe.. were changed by the owner before my mate who I bought it from) a Vectra.

Will this change the reading my speedo will give significantly (I'd imagine it came on 13/14" wheels) and if so is it likely it has already been adjusted accordingly?

Matt

[samdale]

Depends on tyre sizes for both old and new wheels as to how much the rolling radius has changed.

http://www.willtheyfit.com/

Put in width, aspect and diameter of old and new and see the difference in indicated speeds.

[s3fella]

It's the tyre size that is relevant. By would not think it would make a monkey turd of difference on top speed on yours tbh.

[V88Dicky]

It's not the wheels, but the rolling radius of wheel and tyre that you need to check. Google ' rolling radius converter' or similar, there's some good conversion tables available.

[mgtony]

I think if you've gone up an inch on wheel size, (depending on the size of tyre which could give you the same overall circumference anyway), your speedometer would read about 1mph under what you are doing at about 30mph and about 2mph at 60mph. I think that's about right! but I'm all

[Gatsods]

I'm at Uni at the moment not at home where my car is kept, but they are 205/50 R15's all round iirc (which I'm pretty sure I do).

So I would think they will be overall a little taller than the standard wheels whatever they were (will look this up when I'm next home).

I'm not fussed about my top speed, more about whether it would make the speedometer less accurate. The car will do 100mph, but hardly any more without taking it somewhere like Ehra-Lessien haha!

[HustleRussell]

If the new tyres have a slightly greater circumference, you have probably made your speedometer more accurate! Not a chance it's faster now- somehow I doubt a 1.2 Corsa will hit the rev limiter in 5th!

[Gatsods]

HustleRussell said:

If the new tyres have a slightly greater circumference, you have probably made your speedometer more accurate! Not a chance it's faster now- somehow I doubt a 1.2 Corsa will hit the rev limiter in 5th!

Made it more accurate, ok cool

-sigh- I never meant I expected that the car was faster.. or that the car would hit any limiters or whatever

In my experience it indeed will not hit the limiter! It'll do about 100mph but indeed, has about 400/500rpm to the red line at this point.

Not desperately rapid

[xRIEx]

Googled for the standard wheels/tyres suggesting either 185/60 or 175/65 R14


Rolling circumferences:
205/50 R15 - 72.48"
185/60 R14 - 71.44"
175/60 R14 - 72.12"


Worst case scenario (assuming 185s are standard), your speedo will be reading higher about 1.5% slower than standard, which I think should be less than Vauxhall's margin for error. If anything, you've probably made the speedo slightly more accurate, maybe reading 8% over instead of 10%.