McGoo

Right guys, ive recently bought some 18's for my wrx hawkeye. When i bought the wheels i assumed they were all 225/40-18s which is what i need.

It was only when i had the tyres removed as i got them powdercoated that i realised that i had two different tyre sizes. I have two 215/40's and 225/40's rear..

My wheels are 18 x 8 et35. I knew this setup could be a close fitment type of deal so I wanted to fit the wheels to the car to check for rubbing before i paid for new tyres.

Now ive fitted the wheels and had them on for a few days im more than happy with how they fit, no rub or catching so now im questioning how important it is to have all 4 tyres the same on Subarus symmetrical AWD system?

On my old wheels which came with the car i had a little knock at slow speeds from the rear, im told this is normal as the diff is locking etc but since changing the wheels it seems a little more noticeable and its concerning me..

This is my first subaru so want answers from people who have had odd tyres on there car before, not from people who have never tried it.. Will i be ok for a few months while i get new tyres or should I put my standard wheels on to be safe?

Thanks matty

McGoo

ive taken the wheels of for now and put my standard wheels back on until i can get four new tyres.

The car seems alot happier now!

JGlanzaV

iTrader: (3)

I beleieve diffferent rolling radius' will wind up your diffs IIRC

McGoo

Yes that what ive herd.. Will it cause permanent damage or is it just a case that the diff allows slip when it shouldn't?

Im abit concerned as when i went to put my old wheels on i noticed they had odd tyres on too!

I had two part worns fitted about 4 weeks ago from a cheap tyre place and had nothing but trouble with them.. They used to vibrate a little when you hit 70-80... At first i thought they were out of balance so i had them rebalanced and still had the issue so i gave up getting them balanced and got my new 18s sorted..

When i came to refit my standard wheels yesterday i noticed the profiles weren't right so i checked the sizes and i had 3x 215/45-17 and 1x 215/40-17!
So some how they have managed to fit two different sizes?! I went back to the shop yesterday and they fitted the right size for me and its 10x better now!

I just hope its not cause any permeant damage!

Does anyone have any experience with this or anything similar?

alcazar

iTrader: (2)

Permanent damage. Don't drive it like that except to get the tyres changed.

McGoo

Can you elaborate? Do you have any evidence to back this up?

What exactly will be damaged?

The car is running all the same size tyres now, it seems back to normal and hasnt got any issues that i know of.. I dont think it will cause damage in such a short time. I think it will just take life of the diff worst case scenario as a-posse to killing it right out.

Im going to do an drive terrain oil change soon to make sure everything is ok.

alcazar

iTrader: (2)

1. No.
2. Seventeen years of Scoob ownership.
3. Centre diff.
4. Glad for you. Take care.

McGoo

well 17 years of ownership does not make your "opinion" valid, i say "opinion" as thats what it is, as stated in point 1 you have never ran odd tyres so dont know what permanent damage it will cause if any..

Lets do some calculations shall we using a tyre size calculator..
http://www.miata.net/

my wrx has 215/45/17 stock tyres, it has a 135/70/16 space saver..

Tire Size Comparison
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/45-17 3.8in 12.3in 24.6in 77.3in 819 0.0%
135/70-16 3.7in 11.7in 23.4in 73.6in 860 -4.8%

the spacer saver is -4.8% smaller than the stock wheel, do you think subaru would supply a space saver of that size if it was going to damage the transmission that drastically? I doubt it... they would just supply tyre foam..

Now lets look at the maths for the tyres ive been running for 3 weeks as i didnt realise the tyre shop fitted the wrong tyre..

Tire Size Comparison
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/45-17 3.8in 12.3in 24.6in 77.3in 819 0.0%
215/40-17 3.4in 11.9in 23.8in 74.7in 848 -3.4%

Only -3.4% difference... not as bad as running a space saver it seems?

Now lets look at my new alloy wheels which i bought without knowing they had odd tyres even though im getting new tyres fitted..

Tire Size Comparison
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
225/40-18 3.5in 12.5in 25.1in 78.8in 804 0.0%
215/40-18 3.4in 12.4in 24.8in 77.8in 814 -1.3%

so my new tyres which I started the thread about only have a difference of 1.3% in diameter from front to back which is 3.5% closer to equal size than it would be if you had to run a space saver...

Im still getting all new tyres fitted before i fit my new 18's so there all matching but I started this thread to get answers from people who may have had odd tyres on there subaru for whatever reason and wanted to find out if they had any issues down the line... It seems like the only answers I will get are from people who have NO experience in this issue so there just assumptions.

Im quite happy with my findings from the tyre calculator and the car has been driving perfectly fine since ive put the stock wheels back on and it drove fine with the 18's on (with the odd tyres) so i can comfortably say no damage has been caused to my center diff but i will still fit matching tyres to my 18's for safety..

I now consider this case closed.