Don't buy a Nagoya UT-108 dual band mini magmount antenna. They're available on Ebay from a few places. There are a few other in their range of mini magmount antennas, which are smaller or have different connectors.
On my car roof, it was a lot worse on 70cms than the proper mobile antenna (Watson W-770 copy) which was mounted quite close to the ground on the tailgate. Probably the RG-174 coax being so lossy but it didn't seem to be resonant on any amateur band at all. Even when the car body was in the way, the W-770 was much better.
The mini magmount was OK around 165 MHz, compared to other antennas, but none of the others were meant to work on that frequency anyway.
Other mini magmount antennas are probably the same, but the last one I had (I think it was from Moonraker) fell apart soon after I bought it. Other stuff from Nagoya looks like it's a direct copy of the Diamond/Watson antennas.