It took 32 years to find the answer to this question, I was trying to hunt down a film. Here is the question on this blog:
I asked that question on:
After a few ideas it turns out the film was:
Spacehunter: Adventures in the Forbidden Zone
And the truck didn't have six wheels after-all, guess my child mind embellished it somewhat!
Actually looked like this:
This link is a great review of the film too.