Tire and Wheel recommendations to maintain speedo

Greetings,

Looking for some help with regards to my tire setup. I have the 20" Foundry's on my ride with the stock Pirelli Summer tires. This is my first year with the stang and didn't even know summer tires were a thing. They need replacing at 15k. I live in the northeast so we see cold weather and it was a soul sucking experience not being able to drive the beast when the temp went below 40 degrees. My intention is to upgrade to all season tires but I wanted to maybe update the rims as well.

I know I can get the Continental DWS tires in my size but the trouble is finding a rim that will go with this set up. Looking on American Muscle, I can find the AMR wheels in 20's but they are only 8.5 wide and only accomodate a 255 35r20 which is smaller than my current rims. I am looking for plug and pray without any loss of speedo accuracy?

Does such a combo exist or am I beat? Also, I dread having to take my ride to a shop so buying the rims and tires already married up with TPMS is the ideal solution and I don't mind paying a premium for that convenience.

Thanks!

all you really have to do is match the tire radius of the stock 265/35/20 the car came with.
i had these same wheels before and now im on 305/35/19 and going to 325/30/19. even if the size is slightly different the speedo would only be off a couple mph.

Find tires within a 27" to 27.5" rolling diameter, and your speedo won't change much if at all. Most tire websites will list the diameter of the tire in their specifications, but it's also fairly easy to calculate as well.

Quick math lesson:

For example: 255/35/20 (Width in millimeters/aspect ratio/rim diameter in inches)

Aspect ratio is the ratio of tire width to one sidewall's height. So in this case, the sidewall is 35% of 255 millimeters. A diameter of a tire has two sidewalls. So to find the diameter of the sidewalls, we multiply 255mm by 0.35 and we get 89.25 mm. Again, we have two sidewalls, so double that: 178.5 mm.

And because we're mixing up metric and imperial, we have to convert that sidewall into inches. Divide 178.5 by 25.4 (25.4 mm in 1 inch) and you get ~7 inches. Add your rim diameter for a total of 27 inches.

So this is what I'd put in a calculator (the 0.7 is the aspect ratio, 0.35, doubled):

255*0.7/25.4+20

Thank you.

Definitely helpful.

Math is hard though...lol.

I am looking at going smaller actually now. From reading on other posts, I am thinking that 19's are the sweet spot for this car. I am hoping that a 255 40r19 on 19x8.5 rims will do the trick. AM has quite a few sets to choose from but all of the all-seasons are sold out currently.

Yeah, or you could just go to www.wheel-size.com.
Click on the Tire Calculator tab. You just enter parameters interactively and all your answers pop up, including some that you didn't ask for!
%speedo change is one of them.