If you put unstable fuel in a non-rail vehicle like a car or tank it goes insanely fast when its speed should be reduced by 60% (I think the 1400% acceleration may have something to do with it, not sure though).
I do like the idea that unstable fuel is, well, unstable- therefore making things go fast, but it feels too inexpensive to work like that. You could make its fuel value extremely short, so if you do use it to make cars go super-speed there's the catch of running out of fuel very quickly.

I think it would be more balanced if Hgl fuel has less fuel value (so it gets used up faster). This way it can still be used to make cars go brrrrr, and with mods like Bruner Fuel Bouse (makes burner things like furnaces faster depending on the acceleration of the fuel) to achieve fast-working machines while being balanced. :)
I probably should have marked this as a suggestion instead of a bug, my bad.

EDIT: Making the fuel types' speed, acceleration and fuel value configurable would be really cool.

Hi, sorry for taking so long to notice the suggestions. I like the idea of making everything configurable and I think that Hgl would not mind a little lower energy values as you mentioned. I'll work on it! Stay tuned for an update.

After some testing, I have found that the vehicle speed modifier only affects trains, whereas the top speed of other vehicles is affected only by the acceleration modifier. Trains are affected by both, while vehicles are affected by just one. Perhaps this is why the top speed modifier barely changes on vanilla fuels?