Advice on upgrading to 20'' Aftermarket wheels from 18''

Hey, I'm shopping around for new Wheels on my 2017 GT and I have some questions because I'm kind of confused with all of the sizes, etc :doh: (My first Mustang, sorry!)

1) How does it work when upgrading from 18'' to 20'' wheels? Would I lose any performance or will it mess up my speedometer, etc? and are there workarounds for that if true?

2) Can I just buy the wheels and use my current tires or since they are bigger wheels would need new tires as well?

I was eyeing either of these :

https://www.americanmuscle.com/black-fr500-20x85-2015.html

https://www.americanmuscle.com/msbullitt-black-20x85-2015.html

Any other general advice would be great Thanks!

You can change wheel sizes without throwing off speed, etc. You just have to keep the same diameter. That's why some people like running a smaller wheel because it allows you to have more sidewall 45 or 40 which makes the car absorb the road better than say a 20" wheel with only a 35 or 30 sidewall.

Using a website like this can help you. http://www.willtheyfit.com/

You enter in your current set up and the new set up and it will let you know all the differences.

Contact @Need4SpeedMotors

They have set up many forum members here for years.

Tons of knowledge and great customer service.

Here is my setup in 20" x10" front and 20" x 11" rears

Lowered, no rubbing with massive 285/35/20 front tires and 315/30/20 rears.

Speedo is spot on and not recalibrated.

Overall Diameter is very similar to factory setup.

aajohnny said:

Hey, I'm shopping around for new Wheels on my 2017 GT and I have some questions because I'm kind of confused with all of the sizes, etc :doh: (My first Mustang, sorry!)

1) How does it work when upgrading from 18'' to 20'' wheels? Would I lose any performance or will it mess up my speedometer, etc? and are there workarounds for that if true?

Click to expand...

Yes you're going to lose straight line performance (both acceleration and braking) unless you buy very expensive and very light 20's. Even at the same weight, more of the mass is further away from the wheel center. Heavier wheels and tires is also bad for ride and handling. Just think about it. 19s tend to be the sweet spot for these cars.

You won't need to recalibrate your speedometer as long as the tire diameter is within ~0.2".

aajohnny said:

2) Can I just buy the wheels and use my current tires or since they are bigger wheels would need new tires as well?

I was eyeing either of these :

https://www.americanmuscle.com/black-fr500-20x85-2015.html

https://www.americanmuscle.com/msbullitt-black-20x85-2015.html

Any other general advice would be great Thanks!

Click to expand...

You will need new tires.

Tire outside diameter stays the same
From 18-20's, the wheels get bigger, the tires get thinner. Do it right and your speedo won't change much if at all.

So your old tires won't fit at all. They're designed to wrap around 18" wheels and won't stretch to 20.

The bad:
* Often times the 20" wheels and tires will be significantly heavier than a similar 18" setup, so your suspension will have more weight being thrown around going over bumps. Depends on which wheel you choose tho (some are pretty light still)
* smaller sidewalls on 20" wheels mean less tire cushion so those bumps will hurt more and you're more likely to bend a wheel if you hit a pothole.
* the weight being concentrated closer to the outside of the overall diameter means acceleration / decel will be a bit slower (although really it makes the biggest difference in 1st gear, and we have plenty of torque to overcome that).

The good:
* shorter sidewalls mean slightly more feedback and response to driver inputs, as there is less rubber between you and the road.
* they look goood.

Note you might also wind up with a slightly different final gear ratio - how that effects performance is an "it depends" answer (with several factors).

For example, I've got a Perf Pack car, came with 3.73 gears when the stock PP wheels/tires are used, but with my new 20", and a slightly taller setup, my new effective ratio is 3.63.

So first thing first

Since you have 18" wheels you cannot use those tires on the wheels for 20" you need proper tires.

2. For fitment what needs to be known is what wheel you are after to get the proper tire sizing and fitment combined with your vehicle in mind. Keep in mind if you lower the vehicle you need info towards that as well.

Let me know if I can help even if its a different wheel your after. Ill help guide you to the proper fitment.

MELLOWYELLOW06 said:

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Badass. :cheers:

Thank you all for the help! I appreciate it greatly. Those are the Wheels I really liked but I'll shop around for smaller ones.

What about my situation in 19''?

aajohnny said:

Thank you all for the help! I appreciate it greatly. Those are the Wheels I really liked but I'll shop around for smaller ones.

What about my situation in 19''?

Click to expand...

All the same 18->20 stuff applies to 18-19, its just less of a change. 19 is a good sweet spot for performance and appearance on these cars. Again, keep an eye on the weight of your chosen wheel. Rotating unsprung weight is the enemy.

Most 19" tires will have enough sidewall to keep the ride decent and protect the wheels from potholes too.

aajohnny said:

I was eyeing either of these :

https://www.americanmuscle.com/black-fr500-20x85-2015.html

https://www.americanmuscle.com/msbullitt-black-20x85-2015.html

Any other general advice would be great Thanks!

Click to expand...

Don't buy skinny 8.5" wheels for your 3700+ lb, front heavy car. I wish AM would get their heads out of their butts and spec some decent size front wheels.

Go 10" squared or 10 F / 11 R. LMR offers a few SVE options that are wider, but not much more expensive than the AM wheels.

Also note that the hardcore track rats prefer 18s. That opens up the option of keeping the stock tires for now. But you need at least a 255 tire for a 10" wheel. And IIRC, the standard GT tires are only 235.