OS engines. Any of them(Speed, v-spec and JX Ninja) will do 10+ mins. I regularly pit at 10 minutes with the truggy and have with the buggy. As other have said, there are a ton of factors that contribute to mileage, not the least of which is the driver. I'm not a very throttle happy driver, so I tend to get better mileage than most do. I can go 14+ minutes with my OS speed in truggy. Plenty to comfortably pit at 10 minutes. Buggies, having a smaller tank, usually get slightly less run time. I usually pit at the 7:30 mark with the buggy, but on some tracks I can comfortably pit at 10. To pit at 10 minutes, IMO you have to be able to run 12 minutes on a tank. You never know when you will have to run an extra lap or two before pitting. The extra 2 minutes will insure you don't run out of gas in those situations.
