Howdy,

 

Does anyone know if it is bad in the short term (0-200 miles) to run a full size spare that might be a little different in size from the other tires?

 

I ask because after purchasing new tires last year I had a sidewall puncture and had to replace the whole tire. The repair shop mentioned that I might have to replace all 4 tires if the flatted tire was worn too much because the difference in diameter between the new and older tires could cause the all-wheel-drive mechanism (sorry, i don't know what it's called) to wear because it would be trying to put out more torque because the wheel with the new tire would be spinning slower than the rest and the computer would be trying to compensate for that. Sorry I don't know the names of all the gizmos.

 

Thanks!

 

gringorio

i've heard 5% -10% circumference between all the tires... any more than that and it can start to get pissed off. if you have to run a different size tire, get the same sized ones up front, and put the off sized one in the rear end, and put in your 'FWD" fuse, that'll disengage your rear end, so you wont' completely chew up your AWD.

No more then 1/4 inch diffenrce in circumfrence between tires (which is not a lot). You can have a new tire shaved to match the others, but with just one year on the tire i dont think there would be much differnce. If its a major size differnce you can burn up the 4wd. the automatics seem a little more tolerant of tire differnces (in the same tire faimily) then the manuals. The manuals once the damage is done its done. The automatics will give you lots of warning that things are not right.

 

nipper

The automatics will give you lots of warning that things are not right.

 

Really... Not to ask too stupid of a question, but what kind of warning should I be looking out for? /Thanks...

Really... Not to ask too stupid of a question, but what kind of warning should I be looking out for? /Thanks...

 

You look for slightly burnt fluid, and the typical signs of torque bind. Torque bind is when the car feels jerky on turns (the driveline is binding up). This will be most obvious in parking manuvers or full turns. When you feel it you know what it is.

 

nipper

Well, acutally, if you consider that both front and rear diffs are "open", there is no risk to the gearbox. The diff between the wheels will compensate for the difference. Maybe the oil inside it might run a little warmer, but I doubt it'll do any damage.

 

The danger with AWD is if you/someone decides that the front tyres look worn and replace BOTH of them, but leave old tyres on the rear.

Well, acutally, if you consider that both front and rear diffs are "open", there is no risk to the gearbox. The diff between the wheels will compensate for the difference. Maybe the oil inside it might run a little warmer, but I doubt it'll do any damage.

 

The danger with AWD is if you/someone decides that the front tyres look worn and replace BOTH of them, but leave old tyres on the rear.

 

Yes they are both open, but the front axle runs at a faster speed then the rear axle because the front wheels have the abaility to steer the car. This is what can overheat the viscous coupling (in a manual) and cause torque bind or total failure, if its a mismatch or a constantly differnt axle speed differnce. The fluid will never get to cool down. In an automatic since there is no viscous coupling, it can be more forgiving.

 

nipper

 

nipper