The diameter of your sprockets will give you a rough idea of the ratio, but if you want to be more accurate, count the number of teeth on each sprocket to get your actual ratio. Say you have 8 teeth on your one inch and 56 on your ten inch sprocket. Your ratio is 56 to 8, or 7:1 (56/8 = 7, 8/8 = 1). Remember, I'm pulling these numbers out of my...err, out of thin air. You need to count yours if you want to be accurate.
Pretending for a moment that your measurements are accurate. You are going from 1 inch on your motor to a ten inch on your jack shaft (from what I read). So your first ratio is 10:1. Your jack shaft spin's at 1/10th the speed of the engine. On the other end of your jack shaft, you have another 1 inch sprocket, this time going to a 5 inch sprocket on your wheel, so you have a 5:1 ratio this time. Your rear wheel will spin at 1/5th the speed the jack shaft is spinning. And if your jack shaft is spinning at 1/10th the speed of your engine, then the rear wheel is spinning at
1/50th of your engine's speed.
To make it simple, when using a jackshaft, you're
multiplying the ratios. 10 x 5 = 50 and 1 x 1 = 1. so you have 50:1. You're
not adding ratios, as in 10 + 5 = 15, or what I think you did to get 16, which was probably 10 + 1 + 5 = 16. Remember, multiply.
If you're using a little weed-eater engine, you probably want 25 to 1, not 50 to 1, though you might want to check with the guys who've actually done it. You can get that by having the small sprocket on your jack shaft be a 2 inch sprocket (now the ratio from the jack shaft to the rear wheel is 5:2, aka 2.5:1), or you can leave the 1 inch on the jack shaft and use a 5" as the large sprocket on the jack shaft(now you have a 5:1 ratio between the jackshaft and the engine, the jack shaft will spin twice as fast as at 10:1).
You can use
this link to fiddle around with your gear ratios to see how fast what ratio will get you. Use the "transmission gear ratio" as your engine-to-jack-shaft ratio, and the "differential gear ratio" as your jack shaft to wheel ratio, and remember your tire radius is
half of your tire diameter, so a 26" tire's radius is 13". Oh and estimate your engine RPM to be at least 7,000 at top speed. These little weed whacker engines can easily do 10,000rpm under
no load, but pushing a bike is a big load for such a little engine.
Phew. Good luck!