Formula Dé blends itself nicely to large scale driving, and the game Dark Future by Games Workshop seems to cry out to be used, with only a slight modification.

Be warned however, that this scale requires a lot of space and may even require the building of some parts. Still, if you're into the 3D aspect, then its a good step forward, especially as there are plenty of Matchbox scale cars around!
If you're just going to be using the parts from one Dark Future box, then you can either make a small racing circuit an go round it for the double the amount of normal laps, or you can pick up the pieces as they are used and transfer them from the back of the map tot the front when they are needed. The only problem with this is when the cars get seperated and start to spread out.
However this makes for a good long distance sprawling race, from point A to point B.

The Board and movement:

The Dark Future board is already marked out with a similiar sort of grid layout to the one used in the normal game. It is 8 lanes wide however.
This shouldn't represent much of a problem though as cars take up two lanes when they move, so this is okay. When maneouvring in the straight though, you can now elect to only move one lane, which can have serious blocking repercussions! The only real benefit is that there is more space on the track to show the cars and the car counters can show their true locations instead of just almost close positions.

When starting the game, start the cars one lane in from the either side and not on the edge.

Corners are a little more of a problem though, but not much. When the circuit is layed out initially, it will have to be decided which corners are going to be 1, 2 or 3 stop corners. Then you'll have to make on the corners what sort of movement is allowed.
Some ideas to get you started:

Bends:	Inner 3 tracks - Straight ahead only.
	Middle 2 tracks - Straight ahead and in only
	Outer 3 tracks, First two sections - Straight ahead and in only
	Outer 3 tracks, Middle section - Straight ahead only
	Outer 3 tracks, Final two sections - Straight ahead and in only
Corners:	Inner 3 tracks - Straight ahead only.
	Middle 2 tracks - Straight ahead and in only
	Outer 3 tracks - Straight ahead and in only

When handling corners, use the innermost lane that the car occupies to determine whether the car is in the inner, middle or outer tracks.

Thats about it really, the rest of play proceeds as normal!

Combat Racing:

Again, not too many changes are needed here, but the following might be useful.
When shooting, you'll obviously need a larger scale ruler. The one provided in Dark Future can be used, or else a scale of 1" in Formula Dé is three inches in this large scale.

Ramming poses a slight alteration to the rules. Because the larger grid position allows for more accurate car positioning, the normal ramming rules should apply when a car isn't exactly behind or to the side of another car. If the car is exactly behind or to the side of a target car, then the ram should be automatically successful.

The top diagram shows a ram that follows the normal rules for ramming as the two cars are not directly in line with each other The diagram below shows a ram that is an automatics success as the two cars are exactly in line with each other

And again, thats about it, have fun!