Running a stock HT in a vehicle sucks. You start with an inefficient rubber duck antenna, and add the attenuation from transmitting inside a vehicle. I knew a few hams back in the day who would run a 25-50 watt amplifier and external antenna when operating mobile. That setup will give you the equivalent of a regular mobile rig, although the ergonomics won't be as nice.
Running a mobile antenna is probably the easiest way to increase the performance of your HT when operating from a vehicle. Since most amateur radio mobile antennas use a PL-259 connector, you will need an adapter to connect it to the BNC or SMA antenna connector on your HT.
Here is a Weierwei V1000 2m amateur radio HT, one of the higher-tier radios coming out of China. Opening it up, we were pretty convinced its design was "borrowed" from an EX600 or Visar.
Looking at the antenna connector on the radio, we discovered it was an SMA male, which is pretty typical for commercial LMR and Chinese ham radio HTs.
The HT with a generic dual-band (2m/70cm) cellular look-alike magnet mount antenna. It's a quarter-wave on VHF and 5/8th wave (or so) on UHF. Cheap hamfest find. It uses a PL-259 connector, so we need an adapter.
SMA female to SO-239 adapter cable. I prefer using cable adapters as they place less strain on the HT's antenna connector.
Everything all put together and ready to go.
You can expect a noticeable increase in communication range with this setup because you are eliminating attenuation from transmitting through a vehicle body, and running a higher-gain antenna than the stock rubber duck on the HT.