@John Williams great scenario to think through — the 50km figure is actually the drone-to-drone spec, which by definition has both endpoints moving, so that's already the "both mobile" case. The geometry that gets you to 50km is specifically about eliminating ground reflections and multipath interference, which happens when both nodes are elevated, not because one is stationary.
For your lead/follow vehicle setup, you're closer to the ground-to-ground case, which is rated at 5km. That's the number to plan around — not because both ends are moving, but because surface-level deployments deal with terrain, vegetation, and ground multipath that elevated nodes don't have to fight. Whether the vehicles are stationary or rolling at highway speed doesn't meaningfully change the RF math; the limiting factor is the propagation environment, not the motion itself.
So the short answer: motion isn't your problem, elevation is. If you can get the antennas up above roofline height on both vehicles you'll recover a lot of that range headroom.
