A 2 Meter 5/8 Wave antenna is too long for a magnet mount.

Not if the magmount is one of those which is around 7" diameter. Also, with
the one I have, the coil at the base is something like 4 turns of 1" internal
diameter spring steel so, if the whip does hit something, there's a lot of
'twang' in the system. There's almost no way that the magnet can be
dislodged.

I stand corrected. Thanks.

http://www.hamcity.com/detail.aspx?ID=786

If you go to page http://www.kathrein.de/de/car/montag...ad/936127a.pdf
(the English description follows the German description) you will see the
144-MHz Kathrein antenna model K 51 17 2, which is a 5/8 wavelength
magnetic-mount antenna with a swivel. The whip is 1.38 m long (about 4.6 feet)
and the magnet diameter is 95mm (about 3.7 inches). The tuning coil is located
inside the mount..

That antenna is very common in Europe; they sell it since more than 30 years.
Please note that 57% of the German highways have no speed limit (see
http://en.wikipedia.org/wiki/Speed_limit) and it is not uncommon to see the big
Mercedes or BMW running at more than 120 mph).

Another 144-MHz 5/8 wavelength antenna I have has a magnet of comparable
diameter.

73

Tony I0JX