You pretty much experimentally determined the correct answer, but you got some of the cause and effects wrong.
Each engine in the game produces some amount of motive power, and consumes some amount of thermal energy to do so. Bigger engines produce and consume more power. The ratio of motive power to thermal energy is the engine’s efficiency. Electric motors are very efficient, steam engines about half that, diesels a little less efficient than steam, gasoline combustion somewhat less than diesels, and gas turbines are terrible.
Each fuel type produces some amount of thermal energy when burned: diesel is a little more energetic than gas, both are better than methanol which is better than lamp oil, and charcoal is the worst by a long shot.
Each second when a vehicle moves, it is slowed down by rolling resistance and air drag (or air drag and water drag for boats). It then accelerates back to cruise speed. The ratio of the amount it accelerates to the amount it can accelerate at the speed is strain, and strain is multiplied by the engine’s thermal consumption to calculate how much fuel needs to be burnt this second. Also, a vehicle’s top speed occurs when its acceleration and slowdown are equal to each other - the vehicle can’t accelerate to any faster than its top speed.
Air drag is proportional to the square of the vehicle’s speed, but each vehicle has it’s own coefficient of drag (CoD) calculated from the design. Generally, long, thin vehicles have low CoD and things like turrets, exposed passengers, and aisles increase CoD.
Water drag is also proportional to the square of speed, but water is 1000 times as dense as air, so the effects of water drag are much greater than for air drag. The CoD for water is determined mostly by the percentage of your vehicle’s tiles that have boat hulls in them and the vehicles draft. Your vehicle’s draft (how much of it is underwater, basically) increases with vehicle weight and decreases as the length and width of your vehicle increases, as well as when more of your vehicle has boat hulls.
Rolling resistance is proportional the vehicle’s speed and weight, with some factors for wheel design. Generally, harder and narrower tires produce lower rolling resistance - the ultimate example being the railroad wheel, which has a really low rolling resistance - but the better a tire’s rolling resistance on pavement or wheels, the worse it is off-road (or off-rail, for railroad wheels).
A vehicle’s off-road performance depends on the wheel surface area and wheel design relative to the vehicle’s weight and the off-road terrain. The more wheels you have, and the wider those wheels are, the more wheel surface area you have and the better your off-road performance. The heavier your vehicle is, the worst the performance. The off-road performance reduces vehicle acceleration and speed to some percentage between 0% (terrible off-road performance) and 100% (amazing off-road performance).
So, from your observations:
- Light vehicles have lower rolling resistance, so they slow down less at any given speed than heavier vehicles. That means there’s less strain on the engines, so less fuel consumption.
- Engine size is really complicated, as you note. Smaller engines consume less fuel for the same amount of strain, but a larger engine can have more acceleration and less strain for a given vehicle design. But large engines also weigh more. On a tank, the difference between the weight of a V6 and V8 isn’t very much compared to the overall weight. On a roadster, it’s a bit more noticeable.
- Aerodynamics is generally a huge factor, especially when you’re going fast on roads. At low speed, rolling resistance matters more.
- Wheels affect rolling resistance and off-road performance, but off-road performance is relative to top speed. If you spend a lot of time off-road or have a very heavy vehicle, get treads or multiple off-road tires. Bicycles are often light enough to get good off-road performance even with standard tires.
- Speed matters, but it’s speed relative to the vehicle’s top speed. A pimped out roadster that can do 300 kph is probably feeling very little strain, cruising at 130 kph. A mobile fortress that has a hard time reaching 90 kph is going to be under a lot of strain at 80 kph.
The current CDDA vehicle model uses a decent but imperfect approximation of real world physics. Doing things that would reduce fuel consumption in the real world should generally be effective in CDDA: stay at a constant speed that isn’t too fast, use an appropriately sized engine for your needs, don’t go off-road unless you’re designed to, be aerodynamic. Sadly, the requirements of running a mobile fortress in the post-Cataclysm threat environment makes doing that fairly hard, and your typical deathmobile consumes gas like a M1 Abrahms does - 1L per mile or so.