on a 5.5x15 wheel, does 195 or 185 give more grip?

[Tam Lin]

There's this historic Porsche 356. Its wheel size is pretty much limited by regs to 5.5x15 at the front. Does fitting oversize (195x55x15) tyres improve straight line and cornering grip, or does the stiffer sidewall of a 185x55x15 actually give better grip, despite the smaller width?

Any hints as to where to look up the answer appreciated...

Edited by Tam Lin on Monday 30th June 22:20

[RT106]

I'd suspect that with the same tyre construction and rubber compound the difference would be absolutely minimal.

You might find that the extra width of the 195 allows you (or the tyre manufacturer) to use a slightly softer compound of rubber for a given durability, which in turn will give a little more adhesion.

I'd expect the 185 to be more effective in the wet.

[JPBailey]

It's dependant on the weight of the car.
A heavy car will generate more peak grip on the 195 while a light car will get more peak grip out of the 185.
The tyre needs enough weight on it so that the tread and side walls can flex and do their job.
A too lightly loaded tyre will skitter across the road before it gets to the optium slip angle.

Also the 195 will likely have the stiffer side wall construction because it will be designed for a heavier car.

Edited by JPBailey on Monday 30th June 23:22

[Tam Lin]

JPBailey said:

It's dependant on the weight of the car.
A heavy car will generate more peak grip on the 195 while a light car will get more peak grip out of the 185.

Thanks both. Any idea what the relation of load index to optimum grip (i.e. the weight loading for the axle necessary) is?

[JPBailey]

I'm not an expert on this but one thing that I read a while back was that standard hot hatch tyre fitments used to be pretty much optional and people upgrading to 16" or 17" would get less peak grip.


I'm guessing that a 356 wieghs in at 800 to 900kg and this is definatly 185 rather than 195 terratory.
185 was the main tyre fitment for the 90's hot hatches, Golf GTi, Peugot 205, etc

Also try http://www.356registry.org/Tech/tires.html

Edited by JPBailey on Tuesday 1st July 18:22