I have a star fire 58 cc motor , and I am running a 24" rear tire with a 40 tooth sprocket and I top out at about 24 m.p.h. Is this because of the 24" rear tire or is it more to do with the sprocket size? If I were to switch to a 26" rear tire How much faster might I go? If I put a smaller sprocket on how much would that help?

It shouldn't go that slow! Did you put that 3" wide rear tire back on it? Did you get all the drive chain issues worked out? Once you get it going, I'll help you tune it.

It could be a little of both- and the wider tires. I have a little formula when thinking about wheel size- just a quick approximation of percentages- If you think about a wheel, say a 26" - then there are four factored to 100- so any 1 inch change in size is roughly equal to a 4 % change, the dif between 24" and 26" is 2 inches, or roughly 8% then- so then you can expect an 8% gain then in changing from 24 to 26, (or 12% to 27"), it's like putting on a sprocket 8% larger For sprockets: Then, since there is (even more roughly) a three factor for the sprocket (3 x 33 teeth) a change of a tooth is a roughly 3% difference in either direction- 3 teeth makes about a 9% dif so if you went to 26" wheel, you get 8% gain and if you went to 36 instead of 40 teeth (4 x 3%) you get 12% gain- and so with both these things you get approx. 20% gain in top end gearing your 24 mph now (aprrox 4 to 100) is increased 4% with every mph- 20% would be 5 mph- roughly, on paper, at least Wheel width and total weight are not so quantifiable, but I've found great dif in wheel width especially and then there is engine performance.

Ha ha! (this is a joke okay) I observed your post and i decided that with every inch you gain 1 mph, so with 24" tire you go 24mph, and if you put on a 26" tire than you will go 26mph and ect.... lol try a 50 inch and then see how fast you go lol

in my experience, it's sprocket size, not the tire size. my 24" with a 28t sprocket out accelerates any 26 i've come up against, and has a higher top speed. a stock 26" i built with a 40t sprocket does 31 according to the school speed traps. in fact, my 24's faster than all the 26's i've built, too (so far.) the way to make these bikes faster (top speed) is to tune the motor to get it's peak performance, then gear it as low as you can where it'll still pull you around. my theory is, you could put a 16t on the rear engine side and a 60t on the pedal side. once you pedaled the bike up to speed the engine will take over until it hit's it's max rpm. you become the transmission, and the engine becomes the final drive. that's my bonneville speed theory anyway...

I just thought it was a lucky fluke of hp engine performance. tuning gearing. My 24 inch builds all accelerate faster than a 26 inch bike and go just as fast at top speed After 35 mph the bad roads here are too scary. The gas company is frantically replacing all of the street side gas mains. I have already fallen in an unmarked trench. 35 plus to zero in one second. I was lucky and only broke the left crank arm.

I have to imagine 0 stop from thirty had to knock the wind out of you too! lol. Glad to hear it was just a crank arm!

Wheel size is directly related to gearing, top speed has some other factors involved that are not so linear. Right now with a 26" I would agree with Nash's formula you would get about 26 MPH. How many miles on the build? The chinese 2 smokes tend to take some time to break in, and will get smoother, more powerfull, and reach higher RPM with some use.

I have a basicly stock 48cc with 40T and 26" wheels. My bike does about 27-28 mph, and depending on the wind and incline sometimes over 30.

well on my first bike a 700c wheel i averaged 27 cruising speed next to my pops car and medium acceleration i later put the same kit on a 26er and now as of today my top speed is 25.3 with a gps for speedometer i have alot better acceleration and i dont mind the 26inche tire over the 700c cause i have yet to have more than one flat and the 700 i had too many to count and went thru a new rim in a few days

Was just doing some quick calculations for a project i am working on... so here it is...assuming i have enough HP at any given rpm i should expect these speeds... tire diameter is about 24" so about 6.28 feet per revolution... X 2500rpm = 15700 feet per minute x 60 = 942000 feet per hour divided by 5200 = 181 mph... a little to fast for this paticular project... so a 4 to 1 primary reduction gives me 625 rpm...then secondary reduction (from jack shaft) of 2 to one gives me 312 rpm x 6.28 feet per minute = 1959 fpm x 60 =117561 fph divided by 5200 = 22.61 mph ... pretty close to what i want and 6.5 hp with about 400 pounds should be ok.... does my math look ok?

This is what mine works out to. 20....crank gear 82....clutch gear 10....counter shaft sprocket 40....rear drive sprocket rear wheel circumference 81.8" RPM 1000 4.7 1200 5.7 1400 6.6 1600 7.6 1800 8.5 2000 9.4 2200 10.4 2400 11.3 2600 12.3 2800 13.2 3000 14.2 3200 15.1 3400 16.1 3600 17.0 3800 17.9 4000 18.9 4200 19.8 4400 20.8 4600 21.7 4800 22.7 5000 23.6 5200 24.6 5400 25.5 5600 26.5 5800 27.4 6000 28.3 6200 29.3 6400 30.2 6600 31.2 6800 32.1 7000 33.1

That seems pretty close to my calculation... you are using aprox. 2 x 4 to 1 reductions and coming pretty close to half my speed.... seems right, thanks for that...

The nice part about700 c tho is that they come in all kinds of widths- i've heard conflicting stories about fit tho- I'll go looking for some more answers- I've heard from one track racer that not more than 28mm (1 1/8") will go on a narrow road rim, but I think there are three separate widths of rim, or more, out there. just last week I asked a vendor on ebay if a 35mm would fit on my rim- his reply was "if it's a 700 rim, it will fit"- I'm not sure- vendors say stuff they don't know about some times to make a sale. Anyway heres how the conversion works- if you don't know already: 1 inch is 25.4 mm and so the 700tires in mm sizes are 25mm- 1inch 28mm- 1 1/8' 32mm- 1 1/4 35mm- 1 3/8" 38mm- 1 1/2" (1.5) 42mm- 1 3/8" you get the picture- roughly every 3mm is 1/8' , and 25 is the 1 inch starting point I've got a 27 x 1 1/4 on the front already and I'm probably gonna put a 27 on the rear- I've just discovered the long lost 27 x 1 3/8 tire, that I will probably use there, for a slightly wider stability- a cruiser that wil roll about as well as a ten speed. Right now it's got a 1.75 26 on the back- I was gonna go to 1.5, but I have an alloy 27 wheel already, and a hub grooved rear hub of the right diameter - so I'm going even lighter With a 36 sprocket, I'm sure I'm already topping 30 when I want to, but I never really open it up. The other build slowly coming has 700C wheels- http://motorbicycling.com/f15/sprocket-bolted-drilled-flip-flop-24770.html I'm thinking I might put a 38mm on the rear (1.5") if it will fit on the rim- and probably a 32 on the front (1 1/8) because it's only a 50 motor and you can get by with a smaller width on the front- I ride on flat urban streets for transportation-

i have a mongoose with 700c wheels a 66cc grubee and a 36 th sprocket. if you tuck down into the handlebars it will hit 40 mph but it's wound as tight as a cheap watch. it's all stock no mods with very few miles, checked by gps. i've found that almost any 66cc kit with a 44 th sprocket and 26" wheels wil do between 30-35 mph top speed.

so you guys runninmg 700c arent having any problems with bending rims and flats i was having huge problems