Speedo correction

[7and7is]

Just a quick question about mechanical speedo correction, is there a formula when changing wheel size, my Eglide comes stock with a 16" 130/90, and the PO installed a 21" 90/90 Avon, so trying to figure out how off my speedo is.

[tzienlee]

what year ElectraGlide ?....

[Tattooo]

Avon, so trying to figure out how off my speedo is.

There is an easy answer......... NO difference that will make a difference.............

[7and7is]

what year ElectraGlide ?....

1988

[tzienlee]

sorry thought it was on a Harley not an evo,...
pre 84 fl's had the speedo off of the transmission, so no front wheel drive to worry about..

[confab]

I downloaded a spedo app for my smart phone and taped it to the tank for a few minutes.

They're pretty accurate and it is quick and simple.

[TriNortchopz]

a 130/90 x 16 tire (typical - varies with mfg) is 25.21" tall

a 90/90 x 21" tire (typical) is 27.38" tall, using this RB Racing Tire Calculator:

To Calculate Speed, Gearing, or Trail: How tall is my tire?
Formula: Diameter in Inches = Rim Diam Inches + [ (2 x Aspect Ratio x Section Width) / 25.4]
https://rbracing-rsr.com/tirediamcalc.html

and we know how to calculate circumference: C=2πr

130/90 x 16 has a circ. of ~79.2"
90/90 x 21 has a circ. of ~ 86.02"

then a calculator on this page says:
A 130/90 R16 tire has a Diameter of 640 and Circumference of 2012 (640mm = 25.197", 2012mm= 79.21")
https://www.blocklayer.com/rpm-gear.aspx

now you got the difference in circumference to calculate the ratio.

[7and7is]

Not to sound ungrateful Tri, but I smoked a lot of dope when I was younger and that stuff is above my pay grade, I'll just go with Tattooo's answer.

[TriNortchopz]

okay. circumference ratio is 1.0861 (86.02" / 79.2")

21" tire is larger diameter, and a larger circumference, so is spinning slower than the 16" that the speedo was set up for, what the speedo was calibrated at...

so, at 60 MPH on speedo, calculated speed is 65.17 MPH (60 x 1.0861).

[7and7is]

Thanks Tri