a 25 tooth sprocket will fit on a 9 tooth driver
9 tooth driver
a 9 will do but an 8 would be better
calculating sprocket RPM is very easy if you know what the RPM of the drive sprocket is. If you know that then it is a simple fraction problem. If you make the drive gear the numerator or the top part of the fraction and the driven gear the denominator or the bottom part of the fraction (DRIVE/DRIVEN) I.E. if the drive sprocket has 25 teeth and your driven sprocket has 100 teeth (25/100) that will reduce to (1/4) or a 4:1 ratio. So if your drive sprocket is spinning at 1000 RPM then your driven sprocket will be spinning at 250 RPM. You can plug your specific sprocket tooth count into that equation and come up with the right answer.
You can get a 9 tooth sprocket, you will need a conversion kit to do it. Haro makes a decent one just do a search for Haro 25/9
you never need a new rim its only the hub you change
You must have replaced a part. Either the Driver, Sprocket, or backrim. Your ratio isn't set right. with a 16 tooth driver you have to have a 32 tooth sprocket (i think. it might be 36) with a 9 tooth driver, you have to have a 25 tooth sprocket. i don't know how many spokes you need in your rim for either one though. go to a local bike shop and ask about it. sorry i couldn't help much. i havent messed with bikes in a while.
Yes...this is the most comon gear ratio on bmx's. this is because it allows a good balance of accelration and top speed while still giving you the clearance to be able to roll into most quarter pipes.
25t means that there is 25 of the little spikey teeth on the outside of the sprocket :]
Divide your rear sprocket into your front sprocket. Like 25x9. 25 divided by 9 = 2.77. so 2.77 to 1 is the drive ratio. If you go with a smaller sprocket like a 24 in the front but still a 9 in the back, you are lowering the gear ratio to 2.66 to 1. So your bike will be slower but easier to pedal. If you go with a 25 in the front and a 10 in the back, you end up with a 2.5 to 1 ratio. Don't confuse gear ratio with gear inches, that's a calculation including the wheel and sometimes the circumference of the tire.
Most single-speed bikes(particularly BMXes) use a chain that is 1/8" wide. Chains are always bought a bit too long, and then cut to size. Use a chain breaker tool. No one can tell you how many links you need, it depends on the chainwheel/sprocket combo and the size of the frame.
Reccomend you use Google Earth for that one.