tyre specs question

[n15m0]

Hi all,

Just want to confirm something...currently have an R34 with stock 17s wrapped with 225/45/17 rubber...if i was to go with some 18s would the correct tyre specs be 225/40/18 ? or 225/45/18? Bear in mind we wanna keep the speedo readings accurate. Thanks for your input in advance.

[someonestolecc]

So long as the rolling diameter is the same. By this I mean if your previous tyre wheel combo had a diameter of x and your new tyre and wheel combo has the same diameter you're fine.

Search through some other threads or google as people have posted some links to a tyre calculator that will tell you the rolling diameter from inputting your wheel and tyre sizes.

[IanH]

The correct size will be 225/40-18

If the width doesn't change (ie 225 wide) then for every 1 inch bigger take 5% off the profile % (ie 45 to 40)

Try it on this Tyre size calculator to see for yourself - http://www.miata.net/garage/tirecalc.html

Hope that helps........................................IAnH

[blind_elk]

Get hold of the technical brochure for the tyre brand / style you intend to use. It will include the rolling diametr of each size. Then you can compare your current tyres to your proposed new tyres.

[Abo Bob]

Mate, if you going fo r18's you want to go for wider rims. I'd suggest 18x8 on the front and 18x9 on the back with 225/40/18 on the front and 255/35/18 on the back.

You could go to 235 and and 265 with very little change in circumference though.

[IanH]

I found going to 235/265 like Abo Bob said had a good affect on my speedo as it's now exactly right even at 260+kph (compared to GPS) as the oversized tyre takes out the speedos over-reading error (saying 170 when you're only doing 165kph etc).

[Abo Bob]

I was going to say that Ian but I couldn't be bothered typing it at the time.

[n15m0]

thanks guys. Only reason I'm asking is I have been able to source some good 225/40/18 tyres at a good price and the wheel i'm getting will more than likely be 18x8, so I guess I should be alright...thanks again for the info!

[Abo Bob]

I used to have that and I got sick of the lack of traction. I admit it was fun for mini drift but really, if you up the power you will need the grip.