I haven't got the faintest idea, but....
The effect of an electric motor is given by the voltage and the current.
When running on batteries the voltage is given by the battery (regulated
by the speed control). The current depends on how many ampere the motor can consume. You could measure the current using an ampere-meter (multimeter).
The problem with a generator is that the voltage it generates depends on the load so when the electric motor isn't working hard and drawing lots of ampere the volts will go up and this could damage the speed controller and the radio receiver and/or other electronics so you MUST have a voltage control that will throttle down when the electric motor et.c isn't consuming all the effect from the generator. Basically you need some kind of device to keep the CO2-motor running at a constant speed.
You also need to convert the AC from the generator to DC.
The rpm-controller could be built around an ordinary servo where the generated voltage (converted to DC and smoothed) is used to control the length of the pulses controlling the servo.
A buffer battery can assist with the fine tuning of the voltage. If you should happen to select a too weak CO2-motor the only negative effect will be a loss of power when driving uphill or accelerating.

Conclusion: If you need to ask how to find out the effect that a RC Tiger consumes I honestly think that the electronics required for this project is too difficult for you. I don't mean to be rude but I don't want you to destroy all that expensive electronics from Tamiya.
med vänliga hälsningar / Robin

I guess I can help with this.

A 380 motor can run on 4.8- 6 volt. Depending on the number of turns (winds of copper wire around the core) can be mild or powerful.
However we are talking about one mild motor for that application. Something around 20-30 Watt (as a comparison a 500 motor for competition cars delivers around 200W)

For use in a tank it has to use some sort of reduction gearbox. Even a mild motor will do 8-10.000 rpm and of course with low torque. You'll need a reduction gearbox to move a tank.
If you want to know a bit more about motors look here http://www.pirespreto.com/newversion/motors.html

The motors on these pages are competition grade motors and they are called 500 or 540 motors. They have a bigger diameter than 380 motors
Here's a pic of a 500 motor


and a 380 motor


They are quite cheap (not the one above because it's a competition version), but for a few Euros you can get pretty decent motors from Graupner for example.

I'd like to ask why you want it?
I would agree if idea would be to run it from engine etc. But now... You will still have to have huge capacitor, wich would efectively acumulate excess energy, for bursts when you engage more functions/motors etc. It is posible to do, but personaly i find fegular battery smaller, easier to use, and cheaper than your setup. you wont get any nicer sound with all the engine, and i doubt about economy of running it. Becouse now, it sounds similar to puting another electric motor, to turn generator, which powers rest of system, again, why?

I misunderstood you completely.
I thought you wanted to put a 380 motor to power the tank...

then theres simple way of using LiPo packs, and voltage regulator for use in circuit. Oposite to NiMh or NiCd cells, LiPo cells can be recharged very rapidly, and as normal RC guys do, we usualy have 3packs par vehichle, one is in use, one is being charged, 3rd is cooling down.
Downside of any kind of energy generation utilizing anything moving (motor and generator for example) will result in wearing those. Small internal combustion engines have lifetime off 10 hours or so of peek performance, then it goes down. CO2 engines usualy are small or very small. Theres no oil suply for them, only oil used to oil parts during asembly. Wear of these engines is quite quick, and whole setup wont last realy long.
And finaly, as having quite some time spent on rc planes, and vehichles. I try NOT to spend much more than 30mins airtime, or ground time at any time. You get tired quicly, instead i prefer few runs of 10mins or so. If you intend to run your tank forever, like for all day long, im afraid all the rest components will wear till then. You will have several track breakedowns , then if you will switch to metal ones, you will quickly kill gearbox, as every single stone that gets into tracks will be nail in its coffin. Basicly, this kit is not supposed to last forever, its tuff, but in no case heavy duty.

The downside to Lipos, is that they have the unfortunate ability to catch fire or even explode if not used properly. I guess that's kind of realistic in terms of a tank...

in this case its imposible to srew up with lipos, currents are way too small.. Charging if proper charger used is foolproof aswell.
In either case, having few battery packs would save you from trouble of all the modifications etc.