Biggest Tire for an S550

As you can see, the largest tire rolling out of the factory was a 275/40/R19 staggered setup, equipped on the 50 Year Limited Package. By all means, a 275 is a very meaty tire. Back in the Foxbody era a 275 tire in the back was all those Foxes could handle, and they rolled out of the Ford factory with 225s! If a 275 hanging out the back isn’t enough tire for you, well, you’re in luck. There is extra room back there.

It has been shown a S550 Mustang can comfortably accommodate a 315 tire in the back and a 305 wide tire in the front. Running a 315 tire requires no modifications for clearance, (only requires a rim wide enough to mount it: minimum 10 inches) and the same goes for the 305 width up front. Conversely, back in the Fox days, guys were flipping or removing quad shocks, rolling fenders, shaving control arms, etc. just to run 275s in the back and 245s in the front! The Mustang has come a long way from then.

As far as rim height is concerned, provided proper offset and backspacing, a 305 wide tire on a 20" wheel can be cleared. Certain other sizes can be utilized as well, however it can depend on suspension setups.

When upgrading to larger tires, it's important the new size be factored such your speedometer won't be thrown off to any great degree. The industry standard is a tire swap should create no more than a 3% error in the speedometer in either direction. Selecting a comparable tire size in relation to the stock size is paramount for speedometer accuracy. This goes the same for changing rim sizes. What you're looking to do is keep the new rim/tire combination the same height as the stock rim/tire height with a margin of 3%. Therefore, if you select a larger diameter rim, like going from 19 in to 20 in, then you will need a tire with a lower profile sidewall to correct for the additional rim size.

The best way to figure out the measurements of your new tire/rim size is to input all the details into a tire calculator. There are many of these available online. The information you need is the current size on the car and what size you would like to go to. The calculator will then spit out the necessary measurements to retain speedometer accuracy. In the event the exact size you want is not going to work, most calculators will generate an alternate list that will closely mirror the stock rim/tire ratio.

There are two answers to this. First off, they look darn good! Seeing a fat tire hanging out the rear of a Mustang is the epitome of the classic muscle car look. Secondly, there are some performance benefits as well. In the drag racing world, a wider tire is going to have a larger contact area with the asphalt, which will enhance traction and help plant power throughout the run (especially on launch). You may notice serious drag cars have immensely wide tires out back, but skinnies up front. The reason for this is simple: drag cars go in straight lines. They don’t need to turn, and thus don’t need a large amount of grip up front to counter the forces of turning. Running a skinny tire up front saves weight and is less accelerating mass to worry about, thereby shaving some more time off the run. In the autocross sector wider tires will not only help straight line traction, but will help with braking and cornering too. More grip lets you brake later, turn sharper, and accelerate earlier out of a turn.