The best peak efficiency I've seen for a SI engine (Prius) is 37% BTE, and for a CI engine is ~41% IIRC in the 3L Lupo, and given that cars still need to warm up, aren't operated at peak BTE all the time, and so on, average BTE tends to be much lower. The Prius for instance is at the high 20s over the ftp cycles IIRC, and I imagine the 3L Lupo is similar. Electric setups otoh, ike the AC150 from AC Propulsion, are in the 80s in terms of road load efficiency, and with charger efficiency in the high eighties low nineties, we're looking at ~60-70% road load efficiency from the plug. Even super big low speed diesel are only at ~50% peak BTE. If we were to look at the average efficiency of most vehicles compared to production EVs, the difference would be greater since throttling losses for conventional vehicles still kill efficiency, although I think manufacturers are gearing vehicles better and all that given the high gas prices we saw.
Anyway, the familiar metric is probably the best, although in that case it should be mpg for FF powered cars, wh/mile for EVs, and maybe even MPGge if we ever get a natural gas powered vehicle in the garage, w/ maybe a provision for the super eco geeks among us. A direct comparison is a total pain.
