Pwny

I'm pretty sure bigger rims/tires will not affect your speedo as long as the overall wheel diameter doesn't change.

Stndbck

I have 18" -- the optional polished aluminum rims.... I would not put anything bigger on the car. Just "looks wrong" in my opinion.........

YellowFlame

Going to wider tires often softens the road harshness, but getting up to 20sleaves so little sidewall that it often then gets harsher. Less than 45 series will generally start to harden up your ride."Rubber bands" (35 or 30 series for example) do indeed ride rough, there is just not much sidewall leftin them. Alsomuch easier to damage a rim with rubber bands. Tire choice makes a big difference too.

Try to keep the diameter of the tire the same as stock.

255/45R18 is a great compromise in my opinion. Looks plenty big, is same diam as the stock 235/55R17s, yet stillrides smooth.

Rubrignitz

Hmmm, that could be the case. My 275/35's were close to the same diameter as the 235/255(?), 18's. I think it was around 1/2' diff. Programmed the revs/mile back into the SCT and the speedo was back in order.

The difference in handling was obscene! MUCH better. I would say in order of .85 to .90 G. Huge difference.

ThumperMX113

I'll stick to a set of 18s!

O.C Stang

When we bought our mustang we had a chance to get one they also had with 20's on it but that car had alot of road noise compared to the one with the factory rims.Nice looking but I plan on upgrading the wheels on this one and have decided on 18's instead.

EricM

I have 20s and I'm lowered, and I don't think it's harsh. The roads suck here too. A couple things that probably help:

Avoid getting 30 series tires like a lot of people here have (unless you're going really, really wide). You definitely don't want something with a smaller overall diameter than stock, either for looks or ride. You can get something slightly bigger if you want. 275-40-20 (around 28.5" diameter) fit fine, have an inch more sidewall than 275-30, and look better filling the huge wheel wells.

Get some new dampers with your new springs. A lowered GT with D Specs should ride as good as or better than a stock GT.

MustangGT0405

18" wheels are best for performance. I would suggest 18 x 9.5 with 275/45/18's all around so that you can still rotate your tires on a DD.

That is my plan.

Another thing to think about is getting tires put on your 20" rims with low pro tires.

I had 18 x 9.5 powdercoated rims on my old stang and ran a 35 sidewall and everytime I replace the tires some newbe at the tire place would take out a big hunk of powdercoat trying to get the low pro tire on. This happend at 4 different shops in 2 states. Everytime I made them buy me a new rim, (2000 Cobra R's for the curious).

I would think on a 20" rim it would be worse. I hope that an 18 with a 45 sidewall I wont run into that problem.

Good luck

EDIT****

Oh and these were all at nationally recognized chains not po dunk places

zkiller

20s would suck for traction and add a ton of weight to the car if you are a drag racer.

ocorral20

well i had 20s and i dont really recommend them.. i lived in a part of town that had many rough areas on the road and since i had 20s and there is not much tire and cushin there. i kept breaking lug nuts and studs .. plus it throws all your suspension out of whack .. your car is not desined for this.. i would recomend a smaller wheel..