From: The War Room
There are many types of engines beyond diesel/gasoline, and adding these would definitely be a big step towards making this game a bit more realistic, particularly as worlds without life lack oil deposits. In my last playthrough, I discovered two GR devices that collect energy from the atmosphere, meaning that I was swimming in energy relatively early in my game, and yet had constant problems with fuel - not very realistic.
Here are some alternatives:
1) Electric engine - very popular in the early 1900s for luxury cars, it suffered from short range. It should, however, be a tier 1 tech, with a severely restricted range. It would probably require its own truck depots, with fewer points, before it gets upgraded similar to other technology boosts - the main problem is not to create a proper engine, but how to store the energy - frequent depots would solve that. With proper upgrades, these could have a longer range than diesel. The only thing required would be to make units use energy rather than fuel, but perhaps an energy-to-fuel conversion facility to simulate recharging would do the trick instead?
2) Steam - coal, wood, perhaps alien plant forms. More important for trains than for trucks, but can certainly provide a viable alternative. Nazi Germany used wood-powered trucks as an emergency measure in WWII. Mechanics-wise, it would be nice to see a facility that converts these to fuel - it's relatively easy to convert a conventional truck to use this as propulsion so it makes sense.
3) Stirling engines - not for trucks though, probably, but it could be a great upgrade for your existing power plants on cold planets - they're generating a lot of heat anyway, so you can make good use of that temperature difference to produce even more power!
I need ten females for each male...