Bigger isn't better. Specific antennas work best when used at specific frequencies. The Beofengs are 'dual band' radios, they work in the 136-174MHz (VHF) and 400-520MHz (UHF) bands which is a huge range. The stock antenna is therefore a compromise because it can't be most efficient over that entire frequency range. Get an antenna that will be most efficient (i.e., highest gain) at the frequency that you are going to be using. Antennas are most efficient when their lengths (or their electrical length if they include a loading coil) are certain fractions of the wavelength (1/4, 1/2 and 5/8 are most common).
Agree that an external antenna for the car will greatly improve your range. The metal car acts as a Faraday cage and will attenuate both transmit and receive signals and limit range. There are lots of choices for external antennas. If you are using UHF (GMRS) then here are some examples: https://www.theantennafarm.com/catalog/ … ennas-214/ You can get magnetic mount models or permanent bolt-ins that require drilling a hole in the roof of the car.
Also, make sure that both antennas are oriented the same way. That is, if the car radio antenna is vertical, make sure the antenna on the radio in the pits is oriented vertically. Doesn't seem like much, but mixed polarization can kill your reception.
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'85 Audi Coupe G(in &) T(onic)