Ultralights I'd agree with Jabawocky about the efficiency of injected/electronic engines in an RV. You can expect to run the FP version very efficiently by climbing out @ 75% and about 110-120 KIAS and cruising at a pre-determined TAS. I use the Johansen proven number of 140-145 KTAS and fly as high as possible to achieve WOT and find that it really does allow you to lean out to somewhere in the 23-28 LPH range. I'd say an RV7 with an IO-360 with EI and a FP Sensenich plus LRT would be a pretty good choice
happy days,