I've been contemplating for several months about which TE to purchase, the 610 or 510. After thinking about the actual use of the bike I've concluded the following would be my normal use: pavement (under 65 mph) 10 % fire roads (under 75 mph) 35 % double track ATV trails (25 to 75 mph) 40 % single track (1st, 2nd gear stuff) 15 % The 610 has allot of attributes that make the decision difficult, namely the gear box ratios and fuel capacity. However since I weigh 185lb with full gear I'm reluctant to purchase a 310 lb bike and still ride it fast in rough terrain with the expectation that it will perform like a 260 lbs woods bike. Maintenance is not an issue and I fully expect to cope with any mechanical issues including valve checks and oil changes with ease. I've had no problem splitting the cases and doing entire rebuilds on modern day 4 stroke dirt bike engines. Gearing seems to be the stumbling block of the TE510.. but is it really? To address the gearing issue I put together an analysis in Excel looking at various gearing combos and comparing them to the TE610 and also the stock 13/47 TE 510 set-up. Here are the conclusions I was able to draw: The TE510 with a 15/47 combo yeilds the following and % difference from the TE610 1st - 17.16 6.7% taller 2nd - 13.82 8.5 % lower 3rd - 11.44 20.5% lower 4th - 9.32 21.5% lower 5th - 7.89 22.5% lower 6th - 6.985 29.1 % lower with a 15/42 combo on the TE510 I noted the following in comparison to the TE 610: 1st - 15.338 16.6 taller 2nd - 12.35 3.1 % taller 3rd - 10.223 7.7% lower 4th - 8.329 8.6% lower 5th - 7.056 9.5% lower 6th - 6.243 15.41 % lower Conclusions: With a 15/42 you essentially loose first gear and add a taller 5th and 6th With the 15/47 you change your ratio by 13.3%, 15/45 by 17% and 15/42 by 22.6% from stock TE 510 13/37. So one could summize that your speeds would be affected from stock as follows if RPM remained constant with the 15/47 and 15/42 combo. 1st gear @ 10 mph stock to 11.3 mph (15/47) or 12.26 mph (15/42) 6th gear @ 65 mph stock to 73.6 mph (15/47) or 79.7 mph (15/42) 1st gear speed changes are not impacted as significatly as 6th gear speeds. As speeds increase, the ratios have a more significant impact. My questions to those of you that own TE 510's 1- Who does 250 mile day rides on fire roads/ATV trails on their 510 and what gearing do you run? 2- Has anyone tried a 15/42 gearing combo? 3- When riding fire roads with stock gearing, do you even use first gear? Are 2nd gear starts the norm with stock gearing? 4- What is the rpm range of the engine (lowest rpm for crawl speed and rev imiter RPM)? I know some of my gearing stats may be confusing but if somone can post an excel spreadsheet for me the ratios would be much simpler to understand and draw conclusions from.