2" mini speedometer , what ratio?

[XSLeo]

I was reading this and as Pamcopete on his 81 CB750, I checked the speedo drive on my 81 XS650.
I unhooked the cable from the speedo, put a small tape flag on the cable so I could see the cable spin easier.
I lifted the front wheel. I spun the wheel and watched the flag. It spun just a bit less than three turns to one turn of the wheel.
Watching the valve stem on the wheel three turns of the flag and the valve stem was about 1 inch past one turn.
I think that's close enough to 3-1 ratio. So a 2-1 ratio speedo will read wrong. How much wrong I can't guess.
The 2-1 is 33 percent change from the 3-1. Will that make a speedo 33 percent fast or slow?
The 3-1 is 50 percent change from 2-1, is that a 50 percent speedo error?
I can't answer these questions.
If i had a 2-1 speedo I could try it and find out. It's not worth it to me, I like the stock speedo. Easy to see and read. Those small ones only look good when stopped. I think trying to read them at speed will take to much attention away from the road.
Leo

 

[pamcopete]

XSLeo,

I think that you were looking at what I was looking at. My calculations above yielded a ratio of 1:2.828 ratio so that is pretty close to 1:3. That's probably what you observed as well as I did when I spun the speedo gear box.

I think that the terms 1:1 and 1:2 are Harley terms, so they are repeated when talking about the speedo drive ratio. The real number for Japanese bikes is 1:2.828 ratio.

 

[XSLeo]

Harley also had a 2240:60 speedo. Some years the speedo drives off the front wheel, some the rear wheel and some the tranny.
Leo

 

[Phail]

I finally did another test on my 2" MikesXS speedo today at 65MPH. It's right on the dot.

 

[dahondaboy]

I finally did another test on my 2" MikesXS speedo today at 65MPH. It's right on the dot.

thank you!

 

[abyssmaltailgate]

Not sure if anyone in this particular thread is interested, but I looked on Avon's website at the diameter of their 21"x3.00 MKII tires because a lot of guys, including myself, run or plan on running this tire. Avon notes the OD of the inflated tire is 27.2" (http://www.avonmoto.com/products/classic/speedmaster-mkii), which makes circumference/rolling distance 85.45" (27.2∏ = 85.45) or 85.45/12 = 7.121ft. At 60mph. or 1 mile per minute, the wheel will rotate 5280/7.121 = 741.4688rpm (741.5). So the speedo drive needs to be 2240/741.5 = 3.0209 or 3:1 ratio... not that speedometers on choppers are a thing lol

 

[fredintoon]

Yeah, my son allowed style to overtake substance and fitted a set of Mikes 2" gauges.
Good:- they are about as accurate as the stock gauges.
Bad:- they are hard to read and (his set anyway) died real quick.

 

[59Tebo]

Yeah, I just wanted the 1.9" gauges for "The Basketcase" because they look cool, are unobtrusive, and I kinda really plan to ignore them anyways. I need them (primarily the speedometer) so the bike will pass NYS inspection. I can have someone pace me to verify the accuracy (or lack there of) of the speedometer, thus satisfying NYS's requirement of an "accurate" speedometer. Yup. That's actually a law here. Choppers are tough. Basically, if it had it when it left the factory, it needs it for inspection. Speedometers, front brakes, turn signals, horn, reflectors, the whole enchilada. If it has one, and it works, you're good. If people are honking at you, too slow. Is they're standing still when you go by, too fast. You'll figure it out.