30 vs 35 Sidewall

[GK292]

Hi everybody,

I am planning to put 20s on my mustang. I saw that a lot use 305/35-20 in the rear and 285/35-20 in front.
I calculated a bit, and saw that a 30 sidewall is quite close to the original diameter.
That said, I would still prefer the 35 sidewall for a beefier look and better ride/performance.

How did you guys deal with the Speedo? Has anyone used forscan to adjust for wheel diameter?
I read about it in the F150 forum but didn’t find information about the S650.

Thanks in advance!

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[Skye]

https://www.mustang7g.com/forums/th...ist-database-spreadsheet-by-livinitup.159997/

The thread above might offer some insight as to what can be adjusted and what cannot. The Powertrain Control Module and Body Control Module are, for the most part, locked. I have not been studying what's been found and attempted. But if I wanted to, I'd start at that thread.

To compliment the on-going discussion, I thought I'd ask two questions:

1. In percentages, what is the difference between the original wheel/tire combination and what you're planning to install?

2. How important is the accuracy of the speedo for you and where you live? For example, on my model, I changed the tire, but the differences are so slight (less than 3%), it was not worthwhile to adjust the speedo. But for someone in areas with speed cameras, or, a driver that simply wants the most accurate setting, they might want to get this exact.

 

[kinelisch]

I do agree that the 35 wall looks way better than the 30. I am assuming the quality of the ride improves as well. The speedo might be 3-4 mph off.

 

[Magic88]

Agree with Skye, I think the difference is so small, it's not worth it to try to adjust through forscan. I've got a radar detector with GPS, it matches up almost identical with my speedo.

275/35R20 F
305/35R20 B

I flip the detector from car to truck, I put 35's on my F150, and it took me 2 or 3 days to get it to line up the speedo. Even after, it feels like the detector lines up better with the Mustang. Obviuosly if you go bigger like 285's and 315's the difference would be greater

 

[Ken H.]

I have 305/35/20 rear. Speedo off 3 mph at 25-30 mph. Off 4-5 mph at 55-65. Speedo reads slower than GPS. Speedcams & “strictly observed” zones definitely an issue.

 

[Ken H.]

Wheels & tires forum
Rijndael post on 3/02/25 lays it out

 

[AJ2FAST]

Hi everybody,

I am planning to put 20s on my mustang. I saw that a lot use 305/35-20 in the rear and 285/35-20 in front.
I calculated a bit, and saw that a 30 sidewall is quite close to the original diameter.
That said, I would still prefer the 35 sidewall for a beefier look and better ride/performance.

How did you guys deal with the Speedo? Has anyone used forscan to adjust for wheel diameter?
I read about it in the F150 forum but didn’t find information about the S650.

Thanks in advance!

I would definitely run the 35 over 30 for the better ride, but when I switched mines my Speedo was off but for some reason it automatically fixed itself I have a radar in my car that shows my speed and it was off at first when I started running 305/35/20 tires but now my speedo back perfect again don’t ask me how idk

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