I have a 512 in my Duster. It is a fairly mild 600/600 type of build with a hyd roller cam and pump gas compression. I'm not sure on the mileage. It calculates out to about 12 mpg on the freeway but I rarely drive it far enough on the freeway to notice the mileage. I might drive it 10 miles to the track and then destroy the mileage by racing it then drive a few miles home. The car isn't quite comfortable enough to cruise cross country. Street manners are pretty good. The trick is to keep the cam small and to use a good converter. A little bit of gear out back helps if the cam gets too big but then you're starting to lose the whole streetable battle. If you aim to drive it on the street then keep the compression down around 10:1 or less and the cam on the small side. No reason to get greedy when you have 500 cubes.