If you lost top end speed you will need a smaller back sprocket not a larger one. Or better yet, install a larger front sprocket. I have the same bike and I´m trying to get a larger front sprocket to make highway driving a bit better. I want to lower my rpm at highway speeds. I have checked with all the after-market suppliers and none of them list optional sprocket sizes for your bike. Presenty you have a 15 tooth front sprocket and a 45 tooth rear sprocket. A 3 to 1 ratio. Front sprocket turns 3 times, back sproket (wheel) turns once. As you can see, if you had a 30 tooth rear sprocket then you would have a 2 to 1 ratio. Front sprocket only has to turn twice now. So now if the front sprocket did turn 3 times (representing same engine rpm) the back wheel will now turn 1 and a half turns. This would be a HUGE increase in speed. In fact the bike would be poor off road because first gear would be like second! What you want is maybe a 16 tooth front sprocket or maybe a 42 back sprocket. You will probably have to bring the bike into a shop so they can see the type of sprockets to know what fits. I´m sure different ones will fit, they just don´t list them in the books because people don´t usually change sizes on a bike like that. Be aware that the front sprocket is inside a casing and if you go too big the chain could rub. And if your back sprocket is too small you might not be able to adjust the chain tension properly. Knowing the gear ratios in the transmision is of no help because you can´t change anything there. Be aware that it doesn´t take much of a size change to get noticable results. You can also take your old back tire, put a mark on it and roll it exactly one turn on the ground and then measure the distance. Then do the same for your new back tire. The new smaller tire will have a shorter distance. The difference in the two measurements is all you have to make up. If your good at math you can figure out what sproket sizes will solve the problem. Good luck.