Nice chat on the repeater this morning about working mobile on HF.
Although I only use the FT817, I find I can make some reasonable QSOs on HF, particularly on 10m. I've made contacts on CW, SSB and FM from the car - including via some of the 10m repeaters. Not bad for 3 or 4 watts.
The antenna is a Comet UHV6, which allows 50/144/432 MHz and HF coverage by screwing in a coil to the top of the antenna. I have the 7/21/28MHz options and have been using the 28MHz one for most of the summer. Both the 21 and 28MHz extensions are quite short and don't add much to the wind loading of the antenna - so I'm quite happy to use these on the motorway. I'd be a bit more reluctant with the 7MHz extension which is a bit bigger!
Phil, M0PSH talked about using the Moonraker mobile antenna with a Firestick Magmount base which he was pleased with and Wolf, G4SSX used G-whips.
If you're in range of GB3WH (145.650/J) in the morning between 7am and 7.30 local, do drop by and say hello!