Quick Q. - How does tyre size effect the speedo?

[Gibbo]

Hi there

OK I forget which way round it is.

My car used to have 235/55/17 tyres on it.

Now it has 235/45/17 tyres on it.

Does this now mean the speedo reads higher than I am actually going or lower?

 

[dsb]

i belive the wheels will turn faster therefore read faster.

 

[needmorespeed]

according to the tyre size calulator in the sticky at 60 MPH your speedo will read 64 MPH

 

[Clarkey]

Yes, it will over read.

 

[GlasgowTitan]

I don't think that number makes any difference, am I right? The middle number is the size of the sidewall, so the lower the number, the lower the profile of the tyre. Its the last number that changes the actual size of the wheel, ie. 17" to an 18" would mean the whole wheel is an inch bigger in diameter.

Maybe I'm talking crap though, but thats the way I understood it, the first number is the section width, 2nd number the profile and last the diameter.

Edit: all the above posts were made as I was typing, looks like I am talking pants.

 

[atpbx]

I don't think that number makes any difference, am I right? The middle number is the size of the sidewall, so the lower the number, the lower the profile of the tyre. Its the last number that changes the actual size of the wheel, ie. 17" to an 18" would mean the whole wheel is an inch bigger in diameter.

Maybe I'm talking crap though, but thats the way I understood it, the first number is the section width, 2nd number the profile and last the diameter.

Edit: all the above posts were made as I was typing, looks like I am talking pants.

Taxi!

 

[dsb]

I don't think that number makes any difference, am I right? The middle number is the size of the sidewall, so the lower the number, the lower the profile of the tyre. Its the last number that changes the actual size of the wheel, ie. 17" to an 18" would mean the whole wheel is an inch bigger in diameter.

Maybe I'm talking crap though, but thats the way I understood it, the first number is the section width, 2nd number the profile and last the diameter.

Edit: all the above posts were made as I was typing, looks like I am talking pants.

yep your talking pants, even tyre where makes a slight diffrence.

 

[austinpowers]

It will also accelerate very slighty faster over stock.

Spending all that money on fuel means you have to buy cheaper smaller tyres

 

[needmorespeed]

the profile is a percentage of the width so 45% of 235 is smaller then 55% of 235 so the diameter of the wheel and tyre will be smaller which will make the speedo over read

 

[james.miller]

smaller wheels spin faster at any given speed than larger wheels

 

[Gibbo]

Hi there

So let me understand this then.

Before when I had the stock manufacturer size of 235/55/17's on.

Now it has 235/45/17's on hence making the diameter less.

So before when the speedo read 60mph, it will now read 64mph for the same speed due to their been less overall diameter?

 

[Barry Smalley]

smaller wheels spin faster at any given speed than larger wheels

Thats not strictly correct. The wheels turn at exactly the same speed with whatever tyres or wheels are fitted. The distance travelled changes due to the circumferential length of the tyre. Thats why lower profile tyres are fitted to larger wheels to keep the rolling radius the same.

 

[Barry Smalley]

Hi there

So let me understand this then.

Before when I had the stock manufacturer size of 235/55/17's on.

Now it has 235/45/17's on hence making the diameter less.

So before when the speedo read 60mph, it will now read 64mph for the same speed due to their been less overall diameter?

Correct.

 

[Liverpool-Lad]

Hang on, wont the speedo underread due to the overall diameter being smaller and the fact itll cover less distance in one revolution?

 

[austinpowers]

Yup, but your revs will be slightly higher as well, say it did 60 @ 2000, it should be roughly 2140 ish now for the same speed.

 

[james.miller]

Thats not strictly correct. The wheels turn at exactly the same speed with whatever tyres or wheels are fitted. The distance travelled changes due to the circumferential length of the tyre. Thats why lower profile tyres are fitted to larger wheels to keep the rolling radius the same.

I thought if you're travelling at 60mph *actual* (not indicated) your axle speed would be faster with smaller wheels, due to distance travelled as a result of the circumference of the tire, as you said. Isn't that the reason for the over-reading, the axle's spinning faster?

 

[Gibbo]

Hi there

No need to go too technical fellers. I just wanted to know if my speedo was now over reading due to having lower profile tyres on?

So like when I was do 60mph before indicated I will now be doing 64mph indicated due to the lower profile tyres?

 

[Barry Smalley]

I thought if you're travelling at 60mph *actual* (not indicated) your axle speed would be faster with smaller wheels, due to distance travelled as a result of the circumference of the tire, as you said. Isn't that the reason for the over-reading, the axle's spinning faster?

I'll write a post later - have to pop out now.

 

[Liverpool-Lad]

It sounds like itll underread.

 

[JRS]

Hi there

No need to go too technical fellers. I just wanted to know if my speedo was now over reading due to having lower profile tyres on?

So like when I was do 60mph before indicated I will now be doing 64mph indicated due to the lower profile tyres?

Circumference is lower, so for every revolution of the wheels of the car travels less distance. In your case Gibbo, when the speedo reads 70 you'll actually be doing ~65.2mph.

***edit***

I used the calculator here - http://www.carbibles.com/tyre_bible.html - since I couldn't bothered at this time on a Sunday to do any arithmetic.