Alex.B

I got some 20x10.5 +45 for the back and 20x9 +35 for the front. I've been looking around for a good answer on how it'll fit into the rear of my car. If I put 285/35/20 on them will it be flush or close to flush with the fender?I've tried using those offset calculators and it's hard to tell where to measure on the wheel to see how far it'll stick out. It looks like it'll be 1"-1.5" depending on what size tires I get (285-305mm tires). any and all feedback, thanks!

houtex

You didn't say type of Mustang. V6 or GT or... So I'm going GT because that's what I got. The 2006 Mustang GT 17" wheels are 17x8 offset +45, and came with 235/55r17 wheels, so you can use that as your baselines for the following link:
https://tiresize.com/wheel-offset-calculator/

Go there and put in 235/55r17 for the original, and +45 as the offset. then put in your 285/35r20 offset +45 for the new rear wheel/tire combo. It'll show you the calculations for all the dimensions. Which in this case will be the top is about .35" closer to the fender, and both inside and outside go out just about an inch.

Just a suggestion, but using the tool, I think you need to make the fronts on your setup be 255/40r20 so the diameters will 'match'. They'll be just about the same height, although the fronts will be a whole .05" closer to that fender at the top than your rears. My goodness, whatever shall you do... As far as the inside, compared to the original, you'll actually be 'out' on the inside by .04". So about the same as original. The outside, though, will be out more by .74".

Ok, that's a lot to take in and figure out. Let's make it easy! As an example, I compared my setup, the 2010 GT500 convertible 18x9.5" rims, 45mm offset, with 275/45r18s all 'round: .25" closer at the top, and .75 both ways out/in. Awesome is also lowered 1 inch. She also has the 17" rack and pinion setup (no 18" stop in it, I just didn't bother, who cares, right?), and she does not rub anywhere in any capacity at anytime. So it looks to me that with those numbers for your potential setup as I see it, your rears and fronts easily come within a known acceptable situation side to side, so you're good to go there! Only issue might be the tops of the tires... .1" at the rear is probably not a problem. .15" at the front, though... that may or may not be an issue if you're lowered. Again, mine's an inch down from stock. And been that way a while, so I bet it's more like 1.2". She still doesn't rub, so... you might be good all 'round, I think.

And just because, I then did the 'real' tires in the back for the GT500, which are 285/40r18 on those wheels, compared to stock: .1" farther at the top, and about 1" either side in/out. The GT500 fronts are 255/45r18. .1" farther away, and .35" either side in/out. You're ok on the rears, but busting out of the GT500 specs on those fronts.

So... all that to say... Best guess and usin' maths and measure twice, mount tires once... I think you're good to go with your offsets and the 255/40r20s in front, 285/35r20s in the back.

Hope that helps, good luck! And welcome to the forums!

/I'd definitely double check me, it's your money. But I feel pretty good about this, please do let us know!
//If you're doing this to a V6... and maybe even the GT since 20s... then the steering rack should get stops installed to prevent issues: https://www.americanmuscle.com/roush...ack-stops.html