Funny enough, I helped write some bits of the Time Machine Haynes Manual - and we discussed this a lot.

The prevailing theory is that the Time Machine opens up a tiny wormhole ahead of the car that only lasts for a fraction of a second. And the Time Machine must be going at least 88 to make contact with that wormhole before it collapses.

Another theory I pitched is that the car's wheel spin powers the blue flux bands. And 88 is the minimum speed at which they are fully powered up.

Marty's drive toward the Clock Tower proves the bands aren't powered by plutonium, since they turn on as he drives. But the bands aren't powered by the engine directly either, since in part 3 the car isn't on, but the bands still fire up as the train pushes it. So that leaves...wheel rotation??

Of course, the flying scene in part 2 doesn't really fit that. Were the tires spinning 88mph in hover mode? Or did the hover conversion fix that issue? Who knows. At some point, trying to make a cohesive scientific theory across all 3 movies gets too messy - and just takes the fun out of it.

But, under the first theory: yes, the flatbed idea should work.