Just got car back from service and was told it would be a good idea to change one of the tyres so that all the tread heights are even otherwise it could affect the differential. Anyone heard of this?

 

Yeah I have heard about this and there have been several threads about it. Apparently the wear on all four tyres should be the same otherwise it can strain/wear the differential. Why I don't know!!

Cheers

 

The diff is to allow for the different speeds of the wheels when you are going round corners, and when one wheel might slip more than another. If you are going down the motorway it is assumed that all the wheels are going round at
the same speed. This is the case if there is the same make and size of tyre on all four corners, and with the same wear on each tyre.

If one tyre is unworn compared to the others it will cause the diff to be continually working to allow for the difference in speed. Likewise if tyres are the same size but a different make, since the rolling circumference is what matters and some tyres are stiffer or softer than others for the same size.

A small difference in wear isn't going to do much harm, but subaru.com has this to say:



On All-Wheel Drive (AWD) vehicles, it is extremely important that the rolling or outer circumferences of the tires be within 1/4 inch of each other. This means that you must physically measure the size of the tire. This is best done with the weight of the vehicle off the tire and at the tread centerline. Also be certain that the tires are properly inflated since this can affect your readings.

If you need to replace just one tire, the same holds true. The measured difference in circumference between the replacement tire and the other tires on the vehicle cannot exceed 1/4 inch.

Depending on the vehicle mileage, it might be better to replace all four tires.

If the vehicle mileage is low and the tires have been rotated and driven at the proper inflation pressures, then you may be within the allowable 1/4-inch difference in circumference.

If the difference between the new/replacement tire and the current tires is within this 1/4-nch maximum, then the tire should be compatible. If it is not, then other tires will need to be replaced to conform to the 1/4-inch rule.

So, in terms of tread depth, 1/4 inch in circumference is about 1 mm of tread. The difference between fairly-worn and unworn can be a lot more than that...

 

as my user name suggests i had a lot of vauxhalls and this was always a big issue with the cali turbos,why so many of them are now running 2wd due to transfer boxes blowing up.

 

Yup, had this explained to me by someone who does know about these things when i had a scoob. I had a tyre damaged and had to replace it with a new one, the old tyre was a month or so old (how chuffed was i at that!) and the garage could have stung me for two but checked the size differential between old and new and where happy to do just one.

Rolling radius of the tyres needs to be close or it can damage the diffs