Yall are right, almost no forklifts use gas nowadays. It is either diesel or propane (or electrics for freezer/cold storage use) Larger lifts are almost exclusively diesel and warehouse lifts are propane mostly, with the diesel option.
An interesting thing I recently heard is the new Tier III emission (after January 1 2007) lifts are now safer to run with the diesel (with ULSD) inside a warehouse than propane. Particulate emissions have been eliminated (same as the new 6. 7L Cummins) and hydrocarbon/carbon monoxide emissions are not a factor with diesels like they are with gas/lpg engines. I am going to research this issue further because I have a big customer who wants new lifts but runs them inside a large, high ceiling warehouse. They really want diesel because they have dozens of trailers with reefer units and have diesel (and LPG) on site. The big advantage of clean diesel here: safe storage- no explosive fumes, easy refueling and higher fuel efficiency. In addition, a diesel forklift engine will run circles around a propane or gas engine for obvious reasons us diesel heads already know... Low-end torque. We have one of our rental Daewoo lifts there now and they love it. It is not the tier III, but the new tier III Daewoo diesel powered lifts 3000-6000# capacity have a Japanese built CUMMINS engine (natural aspirated, 3. 3L 4-cylinder).
The Japanese manufacturers (Toyota, Mitsubishi, Nissan, Komatsu, TCM) also have tier III compliant diesels which are computer controlled, exhaust aftertreatment, ultra-low emission engines. Since they are computer controlled, the possibilities are nearly endless when it comes to power and governed RPM's with some tinkering. I would assume these engines would go right into a late model ricer truck or SUV with very little modification. They would sure run cleaner than the gasser they replace!
Sorry to "hijack" the thread, but trying to offer some input.