Gear Box Ratio and final drive ratio vs MPH and MPG

vooodooou · #1 (**permalink**) 07-28-2009, 06:11 PM

HI
I have been search this forum and i just cant find much on gearbox ratio and final drive gear ratio.
So i will start one.
To all those in advance thanks for your help and maybe we can all learn something here.

where to start. i got a 49cc 4 stroke 142f-01B bike kit off ebay. it is using the pocket bike clutch and transmission.
clutch is a 3 shoe pocket bike type with 14mm shaft.
transmission is a chain drive 3:1 ratio.
output gear is a 12 tooth 415 pitch freewheel.
with 44 tooth sprocket.
26" wheel with a roll out of 83".
after doing the math it comes out to 11 to 1 gear ratio, or 50.01 MPH at 7000 rpm.
which would be cool if i had 10 horse power to push it.
Top speed. is 29 mph but even peddling the clutch lasted a week, all this telling me GEAR DOWN dummy.

What i need is to find out what final gear ratio everyone is running (26" rim) and top speed and gas mileage and and ect . thanks

roll out is the distance the tire travels in 1 revolution.

HoughMade · #2 (**permalink**) 07-29-2009, 09:57 AM

Mileage, I can't tell you, but I get a top speed over 35 mph (approaching 40 mph-probably over-revving, no governor). My clutch is original and over 2 years old:

Honda GXH50
Grubee Skyhawk Stage II gearbox (3.7:1)
11 tooth output freewheel to 48 tooth rear sprocket.
Overall ratio of 16.1:1.
The bike is a 26" with 26x2.125" tires, but I don't know the measured rollout- ideally, it's supposed to have a 26" diameter as bike tires are measured, not at the rim, but to the tread, but I would have to measure to confirm.

I am thinking of bumping up to a 50 tooth rear sprocket which would cost me a couple MPH on top (OK with me), but with give me a bit more torque. Overall gear ratio with that would be 16.8:1.

tyrslider · #3 (**permalink**) 07-29-2009, 10:16 PM

Hough is right on the $ as usual! 16:1 to around 20:1 depending on what you want are around Ideal.

vooodooou · #4 (**permalink**) 08-10-2009, 03:08 PM

update: i replaced the output free wheel from 12 tooth #415 to 9 tooth #35 non free wheel gear and a 55 tooth #35 sprocket on rear. 18.3:1 ratio

it rides well, can start from a standing start no peddling. so i went and find some hills for the next challenge, 3 miles from my house i a good testing hill.
this hill is at a 19+ deg angle, the clime is about 600 feet to about 200' foot grade improvement.
it slowly climbed right up the hill at 10mph

by removing the free wheel we have no free wheel now, so coasting with engine at idle is out.
this will hurt mpg because we have no free wheel.

on the plus side, we now have engine braking this is a good thing.
if you have older side pulls ect, extra braking is good.

good luck all

tyrslider · #5 (**permalink**) 08-10-2009, 08:52 PM

That's great news. I hope it lasts! The engine braking is the thing I miss after switching to the shifter version of my transmission. Anyway, good news, keep us posted!

vooodooou · #6 (**permalink**) 08-19-2009, 12:08 PM

hi all
update: fuel mileage, getting 119 mpg in town start and stop traffic.
this will improve after i change the clutch back to steel 4 stroke clutch it has a aluminum 2 stroke clutch now all i could ge in a pinch.
motor is just starting to break in good.

i am planning to take a short trip this fall. i will keep you all posted.

ZnsaneRyder · #7 (**permalink**) 08-25-2009, 01:41 PM

I tend to find that a faster gearing gets more MPG, but it depends on how much you pedal on takeoff and if your engine has enough torque to push that gear ratio without bogging down. Most of the time, high RPM and low gearing consumes more fuel.