Tyre diameter mismatch
 

I have a spare set of rims with four tyres, all the same brand and model. Two tyres are brand new, while the other two have about 80% tread left.

The older tyres are 193cm circumference and the new ones are 194.

Questions:

My car is an MY99 WRX - will the centre diff explode if I so much as fit these rims & tyres?

Would I do any damage to the car if I fitted these rims at the RAC driving centre, used them on the skidpan/track, then swapped back to my regular road tyres for the drive home?

Subaru's official line is that the centre diff can overheat and catch fire! Very dramatic

Isn't it only when u mismatch them that there becomes a problem?

How are you measuring diameter? Running a tape around the edge and dividing by pi or guesstimating where the diameter is?

I would just look on the sidewall of the tyre and make sure they are all the same. If they are all say 225/40R17's, then go nuts imo. If some are 16" others 17" different profiles etc, then no dice.

Put wheels/tyres on at home...go to RAC...drive on track...drive home...swap wheels/tyres back...no problem...and no mismatch issue...

Thanks for the comments.

They're all identical Accelera Alpha 205/50/16 with the same tread pattern etc. I ordered the two new ones using the info from the sidewall on the older pair.

Two are brand new and look like they have 6mm-8mm tread at the centre, while two are unknown age and look like they have 2-3mm less tread.

To start with the new ones were 2cm more in circumference (not diameter - I put a tape around the tyre) I pumped the old ones up to 40 and dropped the new ones to 30, and that's how I ended up with 1cm difference.

Dont stress it! WRX arent as bad as the old Calibra's! Those things would explode the centre diff if you ran over a honkey nut and it stuck to a tyre.

1. Put tyres with more tread on the rear of car.
2. Pump tyres up to desired track pressure.
3. Rip handbrake and apply liberal amounts of throttle on first corner.
???
4. Profit.

I would have said put the bald ones on the rear for more sliding and less understeering!

Edit: Woot 2000 posts!

I remember reading in a 4wd magazine that you could actually chuck the wheel on a machine that could trim the tread (for getting the tyre perfectly round again - for flat spotted tyres). Dont know if any tyre shops in Perth have anything like this.....