That's actually a joke by the producers of the film.
The idea is that faster than light travel is "impossible" and the top speed of a DeLorean was 85, so to go 88 miles per hour was "impossible".

/bored blog, and I've still never watched those films all the way through, they're shit.
( Master Of Turnips. Only gays and morons believe in ghosts., Mon 8 Jun 2009, 15:39, archived)

Are you sure it wasnt just 88mph
so that it lit up all 7 segs of the LED 7 seg display ?
( turb0t, Mon 8 Jun 2009, 15:43, archived)

Wikipedia is my source.
Implicit in all time travel is the ability to exceed the speed of light, c. 88mph (142 km/h) is a trifling fraction of this value, though of course the Flux Capacitor plays a significant part in this achievement. However, 88mph is not a spurious figure - it is in fact a subtle joke on the part of the movie making team. The DMC-12's speedometer is calibrated to a speed of 85 mph, thus rendering the speed of 88mph, impossible. This is supported by the view of the instrument panel viewers have when Marty first breaks the 'time barrier' at the opening of the first film.
( Master Of Turnips. Only gays and morons believe in ghosts., Mon 8 Jun 2009, 15:44, archived)

It had a GM speedo that only went to 85, yep
well, the very early ones did. But that's irrelevant as the car was significantly faster than that. If it's a joke, it's a fucking rubbish one. I suggest that wikipedia has been merked, there.
( the mighty badger Aphrodite, on a bar stool, by your side, Mon 8 Jun 2009, 15:53, archived)

my bike's speedo goes up to 120.
That's fucking hopeful.
( spangolin - the odds are good but the goods are odd, Mon 8 Jun 2009, 15:55, archived)

That's what's commonly known as "urban bullshit"
but I'm surprised you've fallen for it.

Top speed of a standard European DeLorean was 130. Even the emissions-reg US one with the detuned engine would do about 110.
( the mighty badger Aphrodite, on a bar stool, by your side, Mon 8 Jun 2009, 15:43, archived)