18 in to 20 in rim speedo difference?

[National Superbike]

I'm looking at a 25 XLT with the BAP but it only has 18 in rims and they look rather small in the wheel wells. I currently have 20 in wheels on my STX and like the way they look and handle. I can buy 20 in new take-offs from several places here in ATL for about 1200/set. Anyone have experience with how much it will affect the speedo? Any other concerns I should think about?

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[Graygoose2021]

The rims don't matter, its the tire circumference. If you went to like a 34" tall tire, in 18" or 20" wheels, your off by 2mph

 

[National Superbike]

The rims don't matter, its the tire circumference. If you went to like a 34" tall tire, in 18" or 20" wheels, your off by 2mph

I understand but I have no way of measuring the circumference of these wheel/tire combos. But if it is minor, like 2mph, that's not a problem. The 18's have a taller sidewall (65% vs 60 on the 20's) so they may be very close.

 

[Brooster54]

Try this calculator. https://tiresize.com/speedometer-calibration/

 

[Graygoose2021]

I understand but I have no way of measuring the circumference of these wheel/tire combos. But if it is minor, like 2mph, that's not a problem. The 18's have a taller sidewall (65% vs 60 on the 20's) so they may be very close.

yes the calculator will show you tire height. You'd have to step up tire size to change 2mph.

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