Stock gearing is 42/14 a final reduction ratio of 3.0 42/15 would be 2.8, or 7% higher final drive ratio The stock 5th gear is a 1.050 ratio, and that would be considered a final gear, a one to one ratio, for practical purposes. 6th gear is .904 ratio, an overdrive gear ratio. Which is good, because it allows a lower rpm for higher speed, but the 250 doesn't have the torque to pull it, the acceleration at highway speeds with that ratio is nearly non-existent. By going to the 15 t, that changes the ratios for all the speed in gears. So, that means 1st will be taller, 2nd, all the way through to that overdrive 6th. The 351 engine revs so fast, I find that I'm out of 1st gear in a second or two. The stock gearing is fine for trail riding. Many riders go to a 13t, to get the 250 to a higher performance level (at the expense of a lower top speed in 6th) But I do quite a bit of street riding on mine also, and for that I think it needs taller gearing. When you get the bike into the overdrive 6th gear, and have an overall ratio also taller, the 351 will have the torque to handle it. All that comes down to a bike that will cruise in that overdrive 6th at a higher speed at a lower RPM, and the engine will provide enough torque to have decent acceleration at those speeds, for passing, etc. If the 250 runs 7k rpm to achieve 70 mph, then the 351 running the 15t may run 70 mph at 6500...I'm not an expert by any means with all these numbers. I just think the 15t will help the 351 be an overall better Dual Sport performer.