OK, I am basing this solely on the cheap Taiwanese speedometer that I got from Spooky Tooth. If you check the picture below you will see that 200 rpm equals 21 mph so I'll start from there: 200 rev/min = (21 mi/hr) / (60 min/hr) converting hours to minutes gives: 200 rev/min = 0.35 mi/min Thus: 200 rev = 0.35 mi Doing some unit conversions: 200 rev = (0.35 mi) * (5280 ft/mi) = (1848 ft) * (12 in/ft) = 22,176 in Now divide both sides by 200 : 1 rev = 110.88 in One revolution is equal to the circumference of the wheel and the circumference of a circle is equal to pi times the diameter so the diameter is equal to the circumference divided by pi: diameter = (110.88 in) / (3.14159) = 35.29 inches So unless you have a mighty large front wheel these speedometers are useless! Likely they are a kids toy designed to make them think they are going a lot faster than they actually are.

the speed in mph works fine on those speedometers,its the rpms that wont be accurate unless youre just peddaling because the rpms on our motors will be alot higher than those numbers.

The rpm would have to be for the wheel since that is the only thing it can measure. If you can't trust one scale you can't trust the other. It definitely is not a "one size fits all" device since there is no way to calibrate it to the bike. I stand by my conclusion, it's just a kids toy! Humor me, I'm currently unemployed, stuck at home, and bored silly!

I also got one of those speedometers from another source (see EBAY) and that meter seemed to work until I got past thirty M.P.H. then it pegged. After that it stopped working. Before that I tried cheep digital speedometers but they never worked (I think the vibration killed them.) Does anybody out there know a speedometer that might work? Mike Frye

if the circumference is pi X wheel diameter, then that's 26" X 3.14= 81.64 inches, or 6.8 (approx) feet per wheel revolution 6.8 X 200 wheel revolutions is 13,600 feet, per minute since there are 5, 280 feet in a mile, that equals just over 2.5 miles in a minute which of course is over 30 miles an hour- so no, it appears that the revs scale at least does not match the speed scale the accuracy of the speed scale could be determined on a pre-measured course at a constant rate of speed, 60 mph for one minute, so say, 20 mph which would take 3 minutes to cover a mile.

Ok Fin, Humor...I can manage that! Wang chung say...faster you go, needle go up...Go slow, needle go down! We no calibrate...pedal faster...see what happen!!! We have correct product! Ahh...you install motor...this make difference...front wheel faster with motor! Instrument for pedal use only! You miss use product...we are not talk to you. Jim

I just measured out my wheel travel, and it came out to just about 81 inches exatly per erevolution My diameter is just slightly less than 26 inches, with the smooth 26 X1.75 tires I put on- the 2.25 cruiser tires would be a little greater diameter and slightly more than 81 inches of travel these speedos always claim BOTH 26 and 27 inch speed, with no apparent change of calibration, so I'd be curious as to which wheelsize they are more acurate on- I suspect 26"

why not install a digital speedo on the bike too and see if the speed on both matches. and make sure the analog speedo you buy is for a 26" bike.

Kat, I'm not sure...I haven't tested any of these speedo's against a GPS. It would be good to know if anyone offers a fairly accurate speedo/odometer for a 26" bike. I actually prefer a mechanical instrument, but I have yet to actually source a good one...if they exist for bikes. Jim

The digital ones are pretty much dead on calibrated with a GPS. Problem is copper plug wire causes reset and most think (incorrectly) that resistance wire causes loss so miss out on the best speedo solution.

Say, aren't you the guy who writes all those instructions for assembling lawn furniture, setting digital watches and putting motors on bicycles. Now we all know where those things come from: Clearwater Florida. Tom

rubber insulated solid core works with my schwinn digital speedo perfectly. only 1 place I know it's available tho, and it's $26 for an entire 6cyl wire set.

I took mine apart and shortened the hair spring up until the speedo was accurate between 20 and 30 mph.

from what I hear the mechanical speedos were designed for 27in wheels with 27/1.25in tires, aka old schwinns like my '73 'Burban because they were so common

I would be supprised if they are accurate for any wheel size over the full scale. However it can be calibrated for reasonable +- 3mph over 25 to 30% of the scale. It probably is diffrent with each unit. What do you get for accuracy with the 27" tire?

My brief experience with a couple mechanical speedometers is they are garbage. Way off compared to GPS. Explains a lot of the ridiculous WOT claims. And only good for a few hundred miles. If you need accuracy and go thousands of miles digital is the solution.