I see 3:1's at RCCommand, RCtankwars, and I bet matomart probably has them as well.

Not sure if anyone sells *HL* 3:!'s, if you've checked those sites already, apologies.

While I suspect you could put resistors to reduce the voltage to the motors, 3:1's are a better option.

At least on most of the HL tanks, the maximum speed they attain with the 1:1 is pretty darn close to the real deal tank speeds (under ideal conditions), but the acceleration/deceleration is what is lacking for realism. That's where the momentum settings in some of the aftermarket electronics come in.