tuscon race track (PDF)

Tucson, AZ Race Track
This is actually a really nice thing: a race track! Yep, you didn't read it wrong,
there's a hidden race track behind the mountain on the west side of Tucson,
and you can get an achievement if you visit it!
First you need to go to the Bitumen unloading point on the left of Tucson:

Now you can see that there's a road not marked on the map and blocked with
some big cones. Go slowly againts them and you can pass through without
damage to your truck.

After that, just continue driving along the road...

...and you will arrive at the "Truck Racing circuit", where you'll be able to test
your driving skills!
Don't worry if you damage your vehicle or if you run out of gas, there are a
service station and a gas station near the track.
Right after entering the track you will get the achievement "Start your engine!"

Here's another screenshot of the race track (nice disposition of those cones at
the entry!).