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A completely different proof of concept may be a crazy vehicle class we have here in Sweden, the "A-tractors". An A-tractor is originally a normal passenger car, usually a larger RWD. It can also be a 4x4 pickup or even a big truck. The A-tractors are modified for a maximum speed of 30 km/h (around 20 mph) by adding an extra gearbox. The body must be modified so it can't carry lugguage or more than one passenger. The purpose of an A-tractor is to be a cheap "engine" for towing trailers or agricultural tools. As tractor it's allowed to drive by a 15 year with only a quite simple license. (minimum age for car licence in Sweden is 18 years). -Guess you can see now who the average owner of an A-tractor is? Yes, most A-tractor drivers are 15+ teenage boys who don't like to freeze their ass off on a two-wheel moped in the winters. The standard A-tractor is a Volvo 740/940, modified to look like a pickup.
There's A LOT of cheeting with these A-tractors, not only to make them go faster, but also to save fuel. Maximum speed is limitid by gear ratio with the engine running on MAXIMUM speed! For a 2.3 liter Volvo petrol engine this means 5000 rpm at 30 km/h (aprox 20 mph)! Exept for a crazy fuel consumption you have to replace the engine every month or so... Here we talk about 20-40 litres per 100 km!
You simply can't use a street legal A-tractor unless it's a low rpm diesel.
Most A-tractors are therefor illegal with unrestricted gearbox. Some boys drive way to fast but most of them stay below 50 km/h (30 mph). Still these vehicles consume serious ammounts of fuel, often MORE than they did as normal passenger cars. The reason is that there the low engine load at low speed makes combustion seriously inefficient no matter what gear you use. Going 30-50 km/h instead of 70-100 km/h will make almost any car consume a lot more fuel. Obviously it's not pumping losses and friction that is the main reason, it's entirely about engine load. To go slow, you should have a smaller engine!
Diesels are a lot better for slow vehicles and my guess the reason they perform better at low rpm's is the fact that they inject fuel multiple times during each power stroke. In a petrol engine, the entire charge is set on fire at once, increasing heat transfer losses at low rpm's.
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