About 52 mph is what I've had mine up to. It like 49 a bit better. I believe I get about 6-7 miles per gallon.
I love my M818, but it definitely does rattle your teeth out. It gets a bit better with a trailer, but not much. I destroyed my spring seat shock, broke a convoy light bulb, and knocked the shifter out of gear by hitting too big a bump in the industrial part of town at something like 14 mph.