Forscan, tire circumference change, speedo reading

[TonyT930]

I have a 2017 Mustang gt convertible with 255/40r19 front tires and 285/35r19 rear tires. Using forscan, if you go into the BodyCM and change the tire circumstance to 2095mm, the speedometer will read true. I verified it using my GPS. If the cruise was set at 70mph, the speedo and GPS read 70mph. If the cruise was set at 80nph, the speedo and gps read 80mph. On flat ground of course...

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[MiamiGT350]

What happens at 2141mm to match the rear tires?

 

[TonyT930]

What happens at 2141mm to match the rear tires?

The speedo was still reading about 3mph faster than the actual speed.

 

[MiamiGT350]

I found one of my old threads. I believe it is because the tire compresses under load.

https://www.mustang6g.com/forums/threads/forscan-and-tire-size-changes.183407/post-3693183

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