Incorrectly setting wheel size

[Yung.Wxlf]

Hi all, first post here
im really new to bikes and I've just got my first one recently.. The dude who delivered it told me that the wheel size was actually 24" despite actually being 20" after measuring it myself. He also claimed that it could easily get 20mph but at 20", I can only get about 16mph. Seems like he just lied to oversell the bike to me? If anyone could help clarify what differences would arise from the display thinking the wheel is 24", rather than 20" it would be awesome, thanks

Red Rocket 88s
250w 36V with Bafang C961 display

 

[Manbeer]

Really, the only issue you would see is an incorrect speedometer reading. from time to time, people will change the wheel size setting to get around a speed limiter though. If there is say, a 16mph limit, by decreasing the tire size the controller thinks you are going slower, this allowing a higher rpm to meet the speed it thinks the wheels should be rotating.

Edit: just saw your bike, i would say the actual diameter on those 20x4.25 tires is likely closer to 24. Cool looking bike btw

 

[amberwolf]

If this is your bike
https://www.redrocketlifestyle.com/product-page/rocket-88-black
then it comes with
Tyre: 20 x 4.25inch Fat tyre, KENDA K1032
which would be about 24" or so in diameter, fully inflated. (the rims would be about 16").

So you would set the bike computer (or controller display) to match whatever the actual inflated diameter (or circumference, if that's the input method it uses, which is typically more accurate), in order to get the correct wheel speed displayed. You can measure the actual circumference via any of several methods.
https://www.google.com/search?q=measure+bicycle+tire+circumference

If they really are 24" (like a 4"+ tire could be), rather than 20" (like a 2"-ish tire could be), then based on this chart
https://www.bikecalc.com/wheel_size_math
they're probably around 2100mm circumference, vs about 1600mm circumference. That's about 4/3 ratio, which is about the same as 20mph vs 16mph, so it's pretty likely that the bike really is going about 20mph with 24" wheelsize in the display/bike computer.

If you have a smartphone, there are also a number of free apps that use your GPS to tell you how fast you're actually going. I've used this one and it has matched known-correct speedo readings closely.
https://play.google.com/store/apps/details?id=com.morkim.thunder.gps
didn't really like it but it did work. There's plenty of others.