The tick to tick speed changes are a complex combination of energy output of the engine, vehicle weight, wind resistance, rolling resistance, friction of the surface you are on, and some other factors.
Unfortunately there is some not modifiable speed below which vehicles just come to a complete stop instantly, at higher UPS, that value becomes a relatively higher speed, and thus at a certain point the vehicle cannot gain enough speed to be greater than this minimum speed in one tick, and won't go anywhere, like the car in reverse at 240.
I try not to do too much manual tweaking of values, but if I have some time I will go over the variables for vehicle speeds and see what can be done.