well, in the man v car example the size of his base does distort it a lot. Base the light vehicle the same way and problem solved! A man standing next to a car should still be taller than it, unless it’s a 4*4 ‘cos his kids go to school on Mount Anapurna.
I generally feel you can get away with one scale down anyway when it comes to buildings – 20mm buildings do not look awful with 25mm figures. The vast majority of wargames houses are very very small though.