I want to elaborate on what Sonic said, I think he hit the nail on the head with this one. The LENGTH of the antenna is very important for any radio device. Just search the internet for HAM radio sites and see how much time (and money) these guys put into precisely tuning the length of their antennas. One thing that I remember form my HAM days was that the antenna needed to be an evenly divided fraction of the full wavelength for that frequency. In other words it needs to be 1/2, 1/4th, 1/8th, 1/16th, 1/32nd, 1/64th, etc. the length of the wavelength.
So being the nice guy that I am, I've calculated the length for the various RC frequencies used in our hobby. I've attached a TXT file with the lengths in it, that'll keep the columns in line easier than just pasting them here. Pay close attention to the 1/64th Wavelength lengths, as this is probably what most will use.
This length is the TOTAL length of the antenna, from the solder point on the PCB to the end of it. One trick that you can do is the "pig-tail" trick you've seen a few times here, just wrap the antenna around a small straw or something, creating a spiral if you will. The theory behind this is similar to the bottom-loaded CB antennas where you have a coil of wire at the bottom of the antenna, and a straight vertical wire the rest of the way up. The reason that is done is because all you are worried about is the total length of the wire, not the length of just the straight part. So you could get a 1/2 wavelength antenna int he space of a 1/4 wavelength antenna in this manner. However, as with any antenna, the higher it is the better reception it has.
Based on that, I'd suggest using the 1/64th or 1/32nd Wavelength length of antenna (depending on the frequency you have), then bottom loading the wire by wrapping a few coils around a small straw just as the wire comes out of the car body, then leave 2 inches or so sticking up straight. Note: when wrapping the wire, make sure that it is an evenly spaced coil, no wire overlapping the previous coil. Not so important on insulated wire, but extremly important if you use bare copper wire. If one coil of bare wire touches another, you've shorted the antenna out and created a length from the PCM solder point to where the coils are touching. The rest of the wire is useless.
This would account for the differences everyone is seeing in their antennas, as the different between a 27MHz car and a 57MHz car is quite a bit in terms of antenna length.
I know that posting this is a bit hypocritical, since I have not personally tried this, but I'm an very confident that it'll increase the range of your car. I will be trying it myself later this week/weekend, and I will post my results. Currently I only have an OEM Bullet (RallyRacer), but I have two Bit Char-G's (Nissan Gloria and Black Celica) coming within a week. Yea, I know, I'm hooked already.
Hope this helps,
KFEdaddy2k