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With a brushed (or any) set up, I know being overgeared can lead to bad things.
But what if you throw a motor with more tourque/less speed into the equation?

I've been running a traxxas XL-5 esc with a titan 12T 550 motor in my SC10.
Tranny has a 2.6:1 ratio, 87T spur, 18T pinion, stock size tires.
Battery size is nimh 7-cell 8.4v 3000mAh.

I overgeared it for street runs (75/26) just too see how fast it would go (pretty good IMO).
Foolishly I ran it in the dirt for a while (to run out the battery), and I cooked the motor (I know, I know)

Well, I had an old (as in lots of runs on it) 21T 550 (what the e-maxx uses) that was my "emergency back-up spare".

I decided, since this motor was slow, to just leave the 75/26 gears in there.
It ran good, REALLY good! Faster/better than the 12T did with 87/18 gearing.

I still need to get another motor though, but now I'm not sure what I want.

Opinions?

I am far from an expert and maybe someone else will chime in. From what I understand is that higher kv or lower windings have more top rpm but less torque and vice-versa. So swithcing to a higher winding gave you more torque and the taller gearing worked better.

Yeah, that's the way I understand it too.

I should have mentioned, when I said it ran better with the 21T, I meant in the dirt.
The 12T was much faster when run with that gearing on the blacktop.

I have a slash and a rustler as well (and a clod too, btw).
The slash has the velenion brushless system and is my "fast" car and is set up for on-road use.
I usually run a 76T spur and 31T pinion (tranny is 2.72)(tires are 2.8's).
I have 7.4 lipo's for it and also have tried an 11.1 (I really want one now).

I have access to a really long parking lot on the weekends and I alternate between cars giving them a chance to cool off.
Last weekend I tried something interesting with the rustler.

My rustler has the xl-5 and titan 12T combo. It had a 90T spur and 23T pinion in it (tranny 2.72). I put an 86T spur and 28T pinion and ran it. I was surprised....the motor wasn't hot. Sure it was warm, but not hot. The speed control was fine too.

This is why I did what I did to my SC10.

I suppose it would make more sense to gear it back down and put another 12T in it.
But I still can't get over how good it ran (in the dirt) with the high gears and 21T.
I run in the dirt a lot too. We have a really good spot out back for that.

Interesting post. I'm rebuiding my RC10T with a brushless system and expect to find the right gearing through trial and error (i.e. a handful of pinions and a temp guage).

I was planning to take my son with me to the track and let him run his Slash 4x4 (has the stock 3500kv set up). He has only run it on blacktop, some grass, but all flat. It gets warm, but not hot. Should I go prepared to gear his down to avoid trouble?

If the car can run in grass it should be fine in dirt as you have more wheel spin in dirt and it does not bog down near as much and your rolling resistance is less also.

Yeah I agree about running in grass, that's probably as tough on a motor as it gets.
My slash is the 2WD version, but I'm running the same motor. It gets pretty hot geared up the way I have it, but these veleneon motors are pretty tough (it's my brushed motors I'm worried about).

My father in law has a 4x4 slash and he runs it HARD. He also runs 11.1v lipo's too. He's broken a lot of parts (and stripped some gears), but never had trouble with his motor. I'll have to ask him what gearing he's running, but I'm pretty sure it's stock.