The
CAFE foundation found ~1% increase in aircraft speed through waxing. Your guess is as good as mine in translating that to vehicle drag, but I would suspect a 0.05-0.2% decrease in drag depending on the initial surface roughness.
The bed liner material will create more drag, but I doubt it would be measurable. If you wanted, the sand paper texture could probably be smoothed with a layer of wax.
- LostCause