Originally from www.dervman.com - the ultimate nerd site for the pursuit of utter pointlessness, now brought into generic use for any car driver whose quest for fuel economy (that's Gas Mileage for you
seppos) overcomes any other requirement of praticality or enjoyment for a car.
Tends to either own a diesel driven car (DERV = DIESEL ENGINED ROAD VEHICLE) or spend all his time justifying the use of small petrol engines, silly air intake modifications, lean burn devices, ECU monitors etc.