The 3.8 works in my car. I did the no rev. mod with only 1 battery, and it runs the 3.8. It is not a trail blaser but it go's. Just waiting to get a nother MiMh bat to install.... Oh ya battery live in very low. It runs about 1/4 of the time then my other ZZ with no mods....
I did not play with the gearing.. But with the same setup (gearing) that I used with the NOS motor (Red gears/stock tires). The topend was a bit faster but it had to have a long strait to get up to that speed. The NOS motor has more torque. better for getting out of the turns. Like I said I will soon do the 2 battery mod soon (2.4v). It is the voltage that gives you more RPM. I found this out by hooking it up to a varable power supply. One that I could control the voltage and current. I got greater increase from uping the voltage rather than the current. One final note this test was tested with no load on the motor. Load will make a differance.