Dave,
Building a tuner, depending on the type, can be fairly easy, sort
of. The difficulty level jumps by an order of magnitude if it's going
to be an 'automatic' type thing. Not impossible, but certainly much
more complicated.

Mobile antennas are more efficient as they get longer in relation
to wavelength. Typically that means a big ugly thing for the lower HF
bands. An itty-bitty thingy at VHF/UHF. A typical HF mobile antenna
is very seldom very efficient once you get below something like 10 -
20 meters or so. Any mobile antenna on 80 meters is very, very seldom
more than about 5% efficient at best (actually, more like 1 or 2%, if
you're lucky). So, depending on the bands you plan to work, and the
length of that unloaded whip in relation to the frequency's
wavelength, don't expect very much. Efficiency is directly
proportional to 'ugly' and impractically tall on a vehicle.

Screw driver vs. bugcatcher antennas. Below something like 20 meters,
the bugcatcher, hands down, except in convenience (changing coil
taps). Above 20 meters, or so, almost anything that's close to a 1/4
wave length works pretty good except for those street lights and tree
limbs, sort of.
- 'Doc

(It isn't efficiency, as such, that you should worry about, but the
radiation pattern, which is definitely controlled by antenna length
and height.)