My family used to rent a lot of VHS tapes over the school holidays when I was young, and this movie was amongst them, which would make it pre-mid 90's, (possibly 70's or 80's from what I remember of the film work). It was a B-grade horror film, and possibly a foreign film. I base this assumption on the fact I only remember renting bad horror films from this place once, and I saw Rats: Night of Terror around the same time. I live in Australia, so 'foreign' in this case means anything that isn't from the UK or USA. (Rats was an Italian/French film.)
The movie started with a small group (a dozen or so) scientists rushing to complete construction of their time machine. The nuclear war was only a few hours away and they planned to escape to a point where the fallout had decayed and rebuild humanity.
When they got to the future, the world was basically one big desert. They struggled to survive and some died. The lead scientist then admitted they were all sterile from the time machine and so couldn't repopulate anyway. One or more of the female characters became deranged and killed most of the rest of the crew. To escape, the lead female (and possibly surviving male) went back through the time machine to just moments before the war. The killer followed them and died as she was torn apart by the time machine (possibly after she killed someone else and was attempting to escape).
The heroine returned to the future as the sole survivor, but was dazed and confused (she possibly had a head injury and was concussed). Along the way there had been some dialogue that suggested humans had survived the war; the mutants on the surface had lost their motor skills and basically had become cavemen/animals, while those that hid in bunkers had retained their understanding of technology.
The heroine walked to a road and a car pulled up, ostensibly to give her a lift. The driver placed her in the trunk and there was a loud scream. What followed was some dialogue with the passengers in the car (possibly this was where the explanation of the survivors came from) and it was indicated the survivors used the mutants for fuel since they weren't useful for anything else: the boy in the back seat asked his parents if they would ever use survivors for fuel once the mutants were all gone, so they'd mistaken the dazed heroine as a mutant and used her as fuel for their car. The end.
I've been searching for the name of this film for almost 2 decades so hopefully someone can identify it for me; I've probably made it sound a lot better and more coherent as it actually was; I remember it being very poor.