I need to take one of the rear wheels off the car for a few days, and I want to avoid going to the extent of changing all the wheels on the car back to the 16's if I can (simply because I can't be stuffed!), but I'm a little concerned about the difference in rolling radius between the spare and what's on the car.

The car has 235/40R18's running all round, and the spare is a 225/55R16, which equates to a 4.4mm difference in rolling Radius, or 1.3% speed difference. My query is will this be at all harmful for the drivetrain being AWD, or am I just worrying over nothing? I can quite easily just swap all four wheels if it's going to present negative repercussions driving with one wheel slightly out from the other 3 for a few days, just hoping I won't have to.

Thoughts? Cheers.

This is my input, i know i'm no professional, but when i had a popped tire on the front i didn't drive it other than when i was taking it to get the tire fixed. Only reason, if you have say an 18' tire and a 16' spare tire, the drive shafts are going to be spinning at different rates and this could (being my understanding) damage the diff, just like if you were doing a single wheel burnout in that one tire is moving faster than the other but this would be less of a difference, but always occurring.. if that makes sense.


As i said, im no professional, but i'd do the safe option and either get a 18" spare or if you pop it and your not near home, tow it.

I don't think its wheel size that really makes a difference, only the overall rolling diameter, and I suppose wheel weight to an extent, so overall, its a pretty small difference. I've thought about it in the respect that every time you turn the wheels are turning at different speeds, but this is only for a brief moment, where as this would be constant regardless of driving direction. On the Verada I just put it on the back and didn't worry, but I'm a little more hesitant on this car.

If not an awd I would say just put the on as the rear is not driven, so couldn't be any harm. If worry just take two wheels off and put two 16's on the rear. But I'm no expert just giving ideas. Ask Mad magna he might know :)

Presti: He's asking if it'll affect his AWD drivetrain, not driveshafts. A full sized spare wheel within a few mm of all the other wheels will have a negligible effect on anything in a FWD/RWD car, just being driven for a day or 2.

Lugo: I'd be trying to find a tyre/wheel combo a bit closer, just for peace of mind. You'd hate to cook your centre diff, just because you cbf changing all 4 wheels over. What about a 235/45/17? How close in OD is that?

If it's only a more worn tyre/very close it'l be fine BUT if its a totally different size DONT.. Stormie in his 380 literally bent his front left fender enough to foul the door when opening by driving around with 1 stock wheel on the front left.. not sure what his rolling radius difference was.

9mm total diameter change? No idea if thats too much or not.

Oh and it'l probably over-work your diff, his auto started getting quite upset as well.

There are no real other tires that are close in 16" rims or in the 17" rims too :(

Lugo: I'd be trying to find a tyre/wheel combo a bit closer, just for peace of mind. You'd hate to cook your centre diff, just because you cbf changing all 4 wheels over. What about a 235/45/17? How close in OD is that?
Well I already have that size spare which is the only reason I was considering doing it. I'm not going to go to the extent of buying another wheel just to use for a few days, just wanted to avoid changing all four wheels if I could. Looks like I'll be best off changing all four just to be safe. Better safe than sorry like they say I guess.

A 8.8mm difference in diameter shouldn't affect things too much. Don't forget your tyres have, what, 9-11mm of tread on them when new, and get down to 1.5mm before they have to be replaced. 8.8mm is no different to putting on an unused spare when your stockies are approaching the end of their life.

I did a whole heap of maths which if I was right says the tyres have a circumference difference of approx 4%.
When turning a U-Turn, the difference between inside and outside wheel is more like 14% distance traveled, so 4% isn't much.

Personally, I don't think it will be a problem.

I'd recommend putting it on the front as the front diff is open, but the back is LSD, so the back diff will get worked harder with different wheels.


Here's a good question about your tyres and their condition.

From memory, normal tread depth is 8mm.
How much tread is left on your spare compared to the tyres on the car? If the spare is new and the others worn, that 4mm difference in radius has just vanished :)

IMO - put it on the front, don't drive far or fast and get the other(s) fixed asap.

Just thinking about what you said Oggy, it all makes sense. Only problem I can factor in, is the spare is the wheel which is slightly larger in diameter, with 80-90% tread, while the wheels on the car only have 20% tread left and are smaller to begin with. So if you factor in tread depth, I'm going to have to nearly double the difference in diameter. I'm just going to change all four wheels and be done with, then there's no risk what-so-ever.

Thanks for the mathematics put in though, to the lot of ya's, goes appreciated ;)

Red Valdez is spot on. It won't make any real diiference, particularly over just a few days.

Glad I came across this thread - as I have buckled a mag wheel recently, and Dont like driving on it.
I wasnt sure if I would have to change all 4 mags back to the standard offerings if i need to change just one....

late last year - I drove around for a day on a single standard mag and 3 19s, to work in the moring then into town to get the tyre repaired.
It was a rear wheel and I didnt feel any difference other than that corner of the car. Trans didnt get upset and no odd noises anywhere else.

Why don't you just buy a 5th wheel/tyre to suit the other 4 and stick it into the spare wheel well? Thats what I did and now I have a complete peace of mind. It shouldn't cost you more then 500-600 bux but is sure better then replacing the whole diff. That plus labour will set you back more as well as make you buy that in the end.

As if you'd spend $600 on a spare wheel..

It was only a few days anyway, I put the 16's on. There's many cheap alternatives to spending hundreds on a spare wheel and tyre.

/thread.

As if you'd spend $600 on a spare wheel..

It was only a few days anyway, I put the 16's on. There's many cheap alternatives to spending hundreds on a spare wheel and tyre.

/thread.

sometimes you dont have a choice. :P need at least 18"s to go over the brakes... however if thats not an issue i agree... stockies!

Well thats exactly what I did when I purchased my new alloy wheels - I bought 5 rims. Now I have all wheels in same size. Full-sized spare will ALWAYS beat a space-saver. Also the difference in traction between space saver and a proper wheel is huge. In emergency situation if you brake and the space-saver is on the front, the chances are it will send you spearing off into a ditch thanks to the uneven traction.

Hope I am not too late here Lugo, you have a rear diff, one wheel will spin faster than the other and can do very serious damage to the LSD in the rear

If you are in need of same diam wheels, you are welcome to borrow a pair of 18's I have here if you need to, this way you will keep the rear end spinning correctly

If you do get stuck, best to keep the speed below 80ks at all times, treat it as if it was a space saver lol

It's what I did when I had my spare on for a few days, but kept the driving to a minimum and kept to a min of 60kph.