Obtain a piece of fuel line or surgical tubing or similar twice as long as is needed. Zip-tie off a loop of maybe 12-18" in your line of sight. Introduce a slug of liquid that fills the tube for a [small] length. Mark of above and below the liquid's level in inches. Feed one end to one test location and the other to another. Duct tape everything down.
At speed if there is a pressure differential, the liquid will move up one side or the other. You can do math to figure out at the ambient air pressure what the absolute values are, or you can just do comparisons.
Else you can get a Dwyer Magnehelic on eBay for cheap.
Aerodynamic wheelwells are skirted and spatted and air curtained. With minimal volume around the tire. All this works against a tire that needs to clear debris and climb tree trunks.
I'd accept that it's an open wheel vehicle and work with that. Mainly a three-piece visor/windscreen and a full boat tail with a hinged floor. An example on a Beetle:
Clear plastic with Rain-X, else perforated metal with sightlines cut out/holes enlarged. Here's the shape I'd work toward with a rock crawler:
A curved bottom cage and reverse tumble-home on the upper roll cage for more interior space.
—— took me that long to write it out.