20” wheels vs 19” speedo

Just brought my 430 home. Previous owner had 20” wheels on it, and it seems my speedo is reading about 5mph faster than actual speed against GPS. Is this something an SDX can solve, would this cause the mileage to climb faster?


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As a reference maybe someone with OE wheels and a GPS can tell you what theirs read.
Most speedos are 2 to 5% optimistic anyways. This way you don't repair anything.


"If it ain't broke fix it till it is"

 

They normally read about 5% faster than your true speed, even with stock wheels and OEM size tires. And btw, the wheel diameter is not significant as the key is the overall wheel/tire diameter. If the combo adds up to 26.5 inches or so, it doesn’t matter to the speedo if it’s 20” wheel plus 6.5” tire or 19” wheel plus 7.5” tire.


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https://www.discounttiredirect.com/learn/tire-size-calculator

Yes, the total diameter with the combo of wheel & tire.
I have 20's on my F430 w/30 ratio tires......at 65mph, my set up will be 65.2 mph at the rear with 295's and 65.5 mph at the front with 245. So......not off by much.