decide on the best arrangement for your circumstances there are better antennas but the dv27 is a good one and would work with a magmount however there are issues regarding how rigid it is eg might topple over at speed as others have said Now I say "what most folk call a "DX27", because really it's proper designation was a "DX25 5/8 wave" (you knew that didn't you ) a good stealth cb aerial that just replaces the one on the car not brilliant range I know but good enough for talking around a town/city/local area the very best arrangement in my opinion would be to drill a hole in the exact centre of the cars roof ,use a stud mount and hide the cable inside the head lining if the door pillars are hollow and there is access you can even use these as conduit which will shield the cable from any engine related interference just a bit of food for thought rab if your not concerned by range and you are using this locally take a look at this

Worth mentioning that I used to own & run (and do EVERYTHING) the local electronics shop in Chester, and we had a big selection of new and used CB radio gear. The shop is now long gone, but I kept some bits, including 2 of these aerials (amongst some others) because I like them for a variety of reasons, (and I'm now using one again). Can't comment on this particular Sirio version, but if it's anywhere near as good as my original DV27 (can't remember what make it is) which I bought in 1981 you won't be disapointed. Then there was what most folk call a "DX27". A much heavier affair, very obviously helically wound. A good performer but needed a strong mount. dv27 Im using one at the moment as my base antenna this is due to not having room for a cb antenna atm but i hope this is only short term ......works very well indeed I swear by mine. Easy to get perfect SWR & consistently strong signals up to 20 mile+ using static mobile with 4 watt.