predator 10k + cobra 29 short range...

Just because an antenna can be tuned to an SWR of 1.0:1 doesn't mean it's an efficient radiator (or receptor) or signals. You could put a dummy load on the end of your coax and see a perfect match (which is why they're made) but still not get out or receive more than a few hundred feet.
The Predators are generally good antennas, but remember that the part you see is only half of the antenna; the coax shield is connected to the "phantom half" of the antenna.
In a perfect world with no overhead obstructions, wind resistance, or "da gummint" looking to write equipment citations, a 102" whip dead center in a roof of no less than about 110" radius from the mount would be the antenna system to have. It would approximate the performance of a vertical dipole, which would be 102" in opposite directions from the mount, and aligned vertically. (FWIW, I posted a thread about a solution for making a vertical dipole out of a mirror mount and a spare whip.)
Typically it isn't practical to put a full size whip on the highest point, and on lotsa trucks, the roof isn't even metal. So there will be compromises.
Trading antenna element length for a coil is a significant one, but it's usually tolerable if you don't give up too much on other factors.
I've been told by an RF engineer friend that the Predator 10K twin-coil version is somewhat lossier than their other models because one of the two coils actually shunts much of the signal to ground. I'm only a technician and not in a position to judge that point.
If you've got a really sub-par ground plane, though, you'll have a very poor "radiation angle"; that is, your signal will be focused more towards the sky (as will be it's maximum receive direction), and no amount of changing the length will do you any more good than at least making sure the transmitter doesn't burn up, or have all your signal wasted as heat in the coax.
FWIW, I run a stock Cobra 25 NW ST in my Chrysler minivan. I've got the luxury of lots of "roof real estate" for experimenting with antennas. I routinely get 8-10 miles from a cheapy Barjan 24" mounted in the center of the roof to my stock base (Royce model 619, 102" whip mounted to the first-story gutter on my house). When I change to the Barjan 48" version, it's slightly better; I can measure it on the antenna range, but I don't notice a difference in real-world performance. When I change to my rear quarter panel mounted 102" w/spring, range goes up reliably to around 12 miles.
When skip is rolling, I frequently have S7 to S9 background noise on my base, which is a good reference for me because the S-meter is calibrated for S9 and the antenna doesn't move around. Even with that much signal "noise" coming in, my mobile can still be heard with the rooftop antenna(s) out to 4-5 miles before it gets buried. Having the noise blanker tuned correctly helps the mobile receive a bit, but other radio signals aren't what it's made to filter.
If you want any kind of "range symmetry", i.e., being able to hear as far as you can talk, put a little time into optimizing your antenna system. Power amps only help on transmit, and only the very best ones are clean enough to keep all your signal on the frequency where you want it to be, anyway.
HTH,
-- Handlebar --
p.s. Geez, if this were a repeater, I'd have timed it out about 10 minutes ago.....