Had a tire fitter tell me fitting that because its a AWD fitting 305's on the back to replace the 295's could damage the diff and send the whole car out of sync. Is he right ?

 

Short answer - no.

 

Cheers Chris I have edited the post as I dont think I made clear the motor is AWD, know nothing about this sort of stuff, is your answer still the same, if so can you give me a bit more so I can give him a bit back. Cheers

 

Who's the fitter? Is he an independent or working for a national chain? National chains tend to be wary of doing "non-standard" stuff.

 

he works for a national

 

I go to a thinner setup in the rear (square setup) for winter with no adverse affects. Should be absolutely fine for the AWD.

 

Thanks Breaker

 

Tire width shouldn't have any bearing on AWD performance. And as you know the oem sizes are staggered setup and are wider on rears than fronts.

What AWD is sensitive to is rolling diameter. If you have a meaningful difference btw the diameter of the front and the rear wheels then the wheels at front and rear have to rotate at different speeds and that can mess up some AWD systems

I had national chain tell me they wouldn't replace just the one punctured tyre in my Land Rover as the others were down to 7mm and the new tire would be 11mm tread. They claimed that difference would damage the car. So they tried (unsuccessfully) to sell me all four!

LR disagreed (as did I) and I never had any issues

 

305 in back will be perfectly fine If you put 265 in front. That will keep the relative Tire diameters Fairly close.

 

I use to have a B4 and B8 Audi S4's and with quattro staggered fit is a no no, the car must have square fitment (same wheel and tires size on all 4) there must be the same amount of rubber (width) on the ground from all 4 corners and to avoid the chance of damaging the differential.

I'm not sure about F Type AWD but since they come staggered from the factory, going from 295 to 305 shouldnt be an issue.

 

I was going to fit the Mich 305's now and leave the fronts as there are newish 255 Pirellis on (3k miles) was going to put Mich 265's on when bit more run out of the Pirellis. So its a no can do and must bin Pirellis ! just as well the pubs are shut ! But excuse me if I am coming over thick I get the different diameters but how would changing the width effect the motor. simple terms please would be appreciated.

 

They won't effect the motor/engine but they just might effect the AWD system and cause it to throw a hissy fit.
Coz the overall tyre diameter difference between front and rear might be outside of the parameters the computer(s) are set for.
305/30s on the rear will be a tad taller (greater diameter) than the stock 295/30s and should be OK and within tolerance with stock 255/35s on the front but don't quote me on that!
I seem to recall that the Unhingd one has posted about this somewhere.
Note that the F-Type SVR (AWD) comes stock with 305/30 rear and 265/35 front.

 

A friend had a Vauxhall Calibra 4x4 which had a single rear tyre replaced after a puncture and every time he went above 50mph the rear drive would cut out (it defaulted to FWD). He replaced the other three tyres and all was well, so some systems may be more sensitive than others.

 

To clear some things up here.... as said before, width doesn't matter. The rolling circumference is what's important when it comes to AWD systems. Some systems are more sensitive than others, but for systems like ours with a clutch style center differential, you don't want to be messing with different diameters front to rear. For example, if you run a tire with a 650 revolution per mile diameter in the front, and a tire with a 640 rev per mile diameter in the rear, the center differential has to physically slip to account for the 10 revolutions difference between the front and rear every mile. This can excessively wear the center differential extremely quickly and make the AWD system basically useless when it fails.

The difference between 650 rev/mile and 640 rev/mile is only 13mm across what is a 31" tall tire, or just 6.5mm of sidewall height. That's the difference between a brand new tire and a worn tire of the exact same size.

Some AWD/4WD systems have mechanically locking center differentials which can result in the tires slipping as opposed to damage to the driveline. But the F-type center differential is a part time clutch style system, which only engages when the wheels are slipping, or the vehicle is under heavy load. With this, everything is okay until it isn't. The system can handle some difference front to rear, but how much it can tolerate is anyone's guess. Best not to mess with it just for a bit more aggressive look.

Important to keep in mind; changing width of a tire also effects its overall diameter. Some people don't realize that the sidewall height portion of the tire spec is a percentage of the width, not an actual measurement.

295/30 = 88.5 mm sidewall
305/30 = 91.5 mm sidewall

 

Precisely why pairing a 265 front tire with a 305 rear works well:
255/35 = 89.25 mm sidewall
265/35 = 92.75 mm sidewall

 

Stohlen, Unhingd and OzXFR, why is it when I read your posts I feel as though I should get the bus. Thanks a lot fellas it did sink in eventually. All the best.

 

has anyone fitted 21" wheels on an AWD F-Type? Are there any problems with doing that?

Jeff

 

Mine isn't awd anyway i paired as it was. did order (almost same set-up i used always on my C4S's):

Michelin Pilot sport 4S 255 30 21 93Y
Michelin Pilot sport 4S 305 25 21 98Y

They arrived and look great:

 

I have 305s in the rear of my AWD. Absolutely no issues at all