The mock-up looks pretty sweet. Some suggestions, even though I know you mentioned it was done in a hurry:
I wouldn't bother with a speed indicator. Is knowing your speed really going to matter that much? Unless you're going to model the car physics realistically it doesn't seem like it's needed in a 2Dish car game.
It might be cool to tie the primary/secondary ammo to a gauge like that, though. Similar to a gas gauge on a car except it tells you when you're out of bullets, and it'd tie into the car theme. It might also prove beneficial because you wouldn't have to draw up weapon logos and ammo types for the interface, could just use the gauge and put the name of the weapon at the top or something.
Also, keep important information like health, armor, and ammo in one area so the player doesn't have to look at two different areas of the screen.