A tyre question -bigger ones!

So I am thinking about swapping my rears for something with a bit more grip.

The stock Mustang tyre is a funny size at 275 40 19 and means that to keep the same rolling radius but on a wider wheel I really need to go to 305 35 19. Most tyres in this width are 30 profile not 35, but that is 5% smaller in radius meaning the speedo will be out by quite a lot.

Interested in what tyres people are running on 19 inch rims that are 305 or thereabouts?

Second question, do you think there are any issues staying with the stock width fronts (255 by memory) with 305 wide rears i.e. is the width difference too much?

The 275mm width tyre is already quite wide for a 9.5" rim and certainly anything wider will be compromised in fit on the OE rim. So if you want 305 width no matter what sidewall ratio, you'll need a wider rim.

I think the P zero fitted to our cars has been widely acknowledged as lacking grip especially in the wet and you'll note Ford has swapped to Michelin for the 2018 model.

Many owners have claimed improved grip going to a wider tyre but I wonder how much comes down to the change in tyre brand itself. I've not seen anyone run a wider P zero (if it exists).

A 275 is a wide tyre and if you're happy with the stock rims I think you'll find more grip with another manufacturer. You may want to fill those wide guards with a wider tyre - fair enough - but this will mean a rim change and so while you're at it you'll be changing the fronts too and so these can be wider as well. You'll have to play with a tyre calculator to maintain the overall diameter. With respect to the front tyre, whether you choose to maintain the 255/40 diameter or match the rears is another debate and it would be interesting to see what others think about this.

Oz, thanks for the response, i didnt mention rims in the initial post for a specific reason -that being that I have aftermarket rims already and intend to buy new rears to match which will be 11 inch.

It actually gives me the option of moving the rears, 275 on 9.5 inch rim, to the front or leaving the existing fronts as is, 255 on 9.5 inch rim. I think buying the new rears gives me two sets of fronts -but they will be PZeros, and my rears definitely wont be (other than the fact that there seem to be very few tyres that are the correct size, one happens to be Pzero's).

I have MRR 350 19x10 F & 19x11 R. I have MPSS 275/35 on the front which is a really good match to OEM & 295/35 on the rear. I would had preferred 305/35 on the rear as a closer match to OEM but couldn't source locally. Some have purchased them from the US with their wheels. I didn't want wider than 305 or lower than 35 so went with 295/35.

miro67 said:

Second question, do you think there are any issues staying with the stock width fronts (255 by memory) with 305 wide rears i.e. is the width difference too much?

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If you up the friction at the rear you might find it oversteering a bit on the limit trying to put power down out of corners particularly if you're running FI, but beyond that no.

I'm not sure you can move the rear rims to the front due to the different offsets of the stock wheels. So make sure you check out the clearances in that regard.

Also if you have new rear 11" wheels, they need to have at least 50mm offset otherwise 305's will rub the fender for certain as you have lowered your car. If you hadn't lowered the car they may only occasionally rub on big bumps or high speed undulations. Stock rear offset is 52mm, and from memory front stock offset is 27 or 30mm.

I went with Michelin Pilot Sport 4S, 305/30 rear and 275/35 front. Grip is absolutely sensational now. Difference over the PZero's is chalk and cheese. But getting the correct wheels is tricky. I know that the Shelby Venom 20" wheels work with my tyres and so do the 20" Ferrada FR8's Forged. I went the latter after originally choosing the Ferrada FR4's only to discover they didn't have enough offset to bring the tyre outer edge in from the fender, and they rubbed. My rears are 11.5 inch and fronts are 10 inch. This was Michelin recommended.

Regarding change of rolling circumference, at 110kpm on my speedo I'm actually traveling at 106kpm. I think this is probably a good thing. If you alter things so that you are actually travelling faster than your speedo is showing (i.e. the reverse of mine) that is illegal.

Good luck and do you homework would be my advice.

Happy for you to PM me if you want to chat
Cheers,