[shwaaboy]

Hello all you awesome i30 fans and please bid me a fair welcome to your tribe!

I have been using this website for years to get answers about my i30, but alas, now I need your help for an answer I cannot find.

I recently put some 18" Rims on my beloved vehicle and have been hunting around for the correct tyre size. However, while the front tyres needed to be a specific size so that the speedo wasn’t out by too much, I’m trying to go slightly wider on the rear wheels so that I can get a more common tyre size that’s much cheaper.

The wheels came off a Lancer Ralliart and had 215/45 R18 tyres on it, which made my speedo out by 3.6% too slow, causing risk to speeding fines.

Front tyres have since been replaced with 225/40 R18 which gives me a total circumference only 0.3% out from OEM specs meaning that my speedo is almost perfect now. Problem is that this is a rare size, like for some Honda Accord, and they cost over $220 each.

What I’m hoping to do it fit some 235/40 R18 to the rear wheels as they are a VERY common size like on the Commodores and Falcons. These tyres are like under $100 

Has anyone has any luck fitting some 235’s to the rear of an i30?

[Surferdude]

The wheels look nice but th eonly way you're going to be able to answer the fitment question is to go talk to a tyre specialist who has a range of sizes in stock and prepared to try some on your wheels.
Even within the stated size, if the tolerance is low, the difference between brands could cause rubbing in some instances.

Even if you manage to squeeze 235's in there you are running a much higher chance of aquaplaning in the wet with fat tyres on a rather light weight vehicle and do not forget that different size tyres front and rear are illegal so you could be asking for trouble there if MR plod is having a bad day and feels like decorating your car with a yellow sticker.

[Surferdude]

Even if you manage to squeeze 235's in there you are running a much higher chance of aquaplaning in the wet with fat tyres on a rather light weight vehicle and do not forget that different size tyres front and rear are illegal so you could be asking for trouble there if MR plod is having a bad day and feels like decorating your car with a yellow sticker.

Two very valid points Craig. 

[AlanHo]

Some Mercedes and BMW cars have different tyre sizes front and rear. Does this ban them from Aussie?

[Dazzler]

Some Mercedes cars have different tyre sizes front and rear. Does this ban them from Aussie?

Yes, first I knew of that rule... My S.I.L. has been looking to put larger wheels and tyres on the back of his Nissan GTR R34

[Surferdude]

Some Mercedes and BMW cars have different tyre sizes front and rear. Does this ban them from Aussie?

If They're fitted as OE. then it's not a problem.
Fitted as after market options tends to breach the ADR's. All of which makes sense to me. By fitting different sized tyres front and rear you're upsetting the balnce of the car intended by the designers.
However, my VW Beetle looked good with reversed front rims (15") and 6" x 14' rears.  But the ADR's hadn't been heard of then.

Some Mercedes and BMW cars have different tyre sizes front and rear. Does this ban them from Aussie?

If the vehicles are factory released with different size tyres then that is allowed.

A way around this law legally is to be registered with one of the street car clubs as many modifications are then allowed but technically then you are only supposed to drive the vehicle on club/street event days, a lot of it depends on the attitude of the officer on the day and also your own attitude because if you get on their wrong side it'll be more expense for you.

[The Gonz]

 
I only went that wide on my 83 Commodore but gee they looked good.

[Just Rick]

To be fully correct fitting any tyre not on the tyre chart inside your door is illegal,but I have heard of a bloke up our way who had 235x45x17's on his,even though they fitted,not a either good look nor ride and as Craig pointed out Aqua planing.

[shwaaboy]

Thanks for your replies everyone. I'll just bite the bullet and put the same tyres on the rear.
Cheers!

[omegaspeedy]

A friend just bought a Ford Explorer (or similar) which he fitted different sized wheels ???? to and he got a fault light on the stability control system because the computer saw different RPMs. I guess the computer thought the wheels were slipping! 

Anyway, don't know if this would happen to a Hyundai stability control system.

[cruiserfied]

A friend just bought a Ford Explorer (or similar) which he fitted different sized wheels ???? to and he got a fault light on the stability control system because the computer saw different RPMs. I guess the computer thought the wheels were slipping! 

Anyway, don't know if this would happen to a Hyundai stability control system.

This will happen if there is a difference in diameter but if you can match diameters while going wider you should be fine.

[The Gonz]

Diameter versus width is what the profile figure gives. Use that to calculate your total diameter - at the tread.

[joshtammi]

When I put bigger tyres on my old rex, i bought something off ebay called a speedohealer. Its a device which is wired inline with your speedometer wire and has a control box that adjusts the signal coming through the wire by any percentage required.

e.g. if you bought wheels that were 5% larger, the speedo would under read by 5%, which is worse than it over reading when it comes to speed cameras etc, you just dial in 5% (can adjusts anywhere up to 999.9%)

The best way to do it though is drive on a long straight road with cruise control on at 40km/h 60km/h 80km/h 100km/h indicated with the new wheels. Use two GPS's to get your actual speed. (I used a proper GPS and then a mobile app which both read within 1km/h) then take the average percentage increase.

So if it was actually 43, 64, 85, 105, the total percentage increases are 7.5%, 6.67%, 6.25% and 5%, the average of those is roughly 6.4%, i'd then still add half a percent at least to that figure to be safe and have it under read. Problem I found is that speedo wasn't linear. at 40km/h indicated i was doing 43 but at 100, i should have been doing 107.5 but was only doing 105. That is because it comes off the gearbox not the wheels in the rex I think. So if your car is not very linear it might be better to use the highest percentage increase you got to be safe.

dial that in the speedohealer and your cluster will be accurate again.

As for the actual attachment to the car, its difficult cause you have to open up the dash, find the right wire to splice into, then put a control box elsewhere, but I did it in about an hour and had no major issues. It comes with a splicing hit so you don't have to solder anything.

[pug20g]

Some Mercedes and BMW cars have different tyre sizes front and rear. Does this ban them from Aussie?

Some Lexus have it too - I believe it is called staggered. I always wonder what size the spare is!

[cruiserfied]

I know the Chrysler 300c SRT8 has staggered front to rear but only width not rolling diameter. I assume that most factory setup would be the same.
Also the 300c SRT8 doesn't have a spare, just a can of goo and a compressor.

[The Gonz]

Here's an example of a wide tyre mod on an i30.
I never really thought it could affect visibility until I saw this.

(click to show/hide)