Thread: Quick opinion about tire sizes

c=2(pi)r. One rotation of the 17" is 215.98", the 16" is 217.49" (about 1.5" more) making the circumference of the 17" 99.31% of the 16". I seriously doubt a less than 0.7% difference will really be noticeable.

As a temporary spare I wouldn't have worried. Uneven tyre wear is just as bad.

Quarter of an inch = approx 6mm which would be the difference between a new tyre and a part worn one.

Would have thought it would be ok.

Proviso:- Unless you're running auto lockers. Small diameter difference can affect them.

Just dont forget that real tyre rolling radius is very very very different to some calculated dimension.
I always laugh when people calculate out tyre differences to 7 decimal places from some online tyre calculator. The only thing that matters is how it sits on the vehicle.

I reckon you will need to chuck the tyre up on a rim and side by side it with your current set.

The numbers on the sidewall are a pretty loose indicator for actual rolling radius (the measurement that counts for driveline differential wear)

S

Originally Posted by roverrescue

Just dont forget that real tyre rolling radius is very very very different to some calculated dimension.
I always laugh when people calculate out tyre differences to 7 decimal places from some online tyre calculator. The only thing that matters is how it sits on the vehicle.

I reckon you will need to chuck the tyre up on a rim and side by side it with your current set.

The numbers on the sidewall are a pretty loose indicator for actual rolling radius (the measurement that counts for driveline differential wear)

S

What Steve said.

Tyre sizes vary batch to batch more than that for the same make/size tyre.

The UK made Dunlp race tyres I once had to use could vary be nearly 15mm in diameter, Dunlop Motorsport here had to run the stagger tape over each tyre and match them in pairs, and they all supposedly came out of the same moulds.

Another factor in the equation is the pressure in each tyre. The calculated diameter is only relevant when the tyre is unloaded at a given pressure. Once you load the tyre your effective rolling diameter gets smaller as the tyre compresses, this effective diameter can then be controlled by adding more or less pressure, so you just have to adjust the pressure in your 16" rim to get the the same effective rolling diameter as the other rims....

I do not believe that effective rolling radius changes significantly over the entire range of practical pressures, at least for radial tyres.

A key feature of radial tyres is that they have a belt behind the tread that does not stretch or shrink in normal use. As the wheel rotates, this belt is laid down on the road, with an effective length for one rotation that depends only on the length of this belt, unless the pressure is so low that the belt buckles and lifts off the ground within the footprint (which may possibly happen at "soft terrain" pressures but not otherwise. (or unless the tread slips on the road)

Since changes in pressure do not affect the length of the belt, they will not affect the effective rolling radius. But as notes by other posters - differences in tyre construction for tyres of the same nominal size will do this.

John