I agree with JX. 1/4 wave stays verticle, while a 5/8th , waves and the antenna to body relationship varies, thus the impedance and swr and radiated power.
But, rule of thumb is,,,, the greatest signal strength is in line with the most car body.
Antenna on left rear fender, radiation is greatest across body to right front fender.
so slap a mag mount in the middle of the roof, or take out most interior dome lights,
punch a hole from the inside for NMO mount. run cable thru the headliner to the door post etc.
for mag mount, i run the cable down between the doors,drill a hole at bottom of the door post
and get cable inside. Clear silicone to tack the cable to the body to keep the wind from moving. But then I have a car for my 'use' , not to look at or have someone admire .
I never wash cars.
I glued a mag mount 1/4 wave to a plow truck of mine with silicone glue to keep moisture from under magnet too. I also used glue to tack cable as poster did above. A car/truck is a tool and I and not worried about a fashion statement but rather best possible performance in a lower profile.