Based on batteries the 5200 should run longer. Amp hours are amp hours no matter what I think the main difference is the motors and tall gearing. I would think the 1518 would draw a fair amount more than the 1515 especialy with tall gearing. Like stated above as well driving style is everything. I tried this one time at the track by timing my practice run to lvc and then handing it over to a fellow racer and doing the same. He's an ex nitro guy and his "on-off" driving style knocked almost two minutes off run time.

Ran the numbers and the revo is geared for close to 70MPH! Try dropping the pinion to a 15 or 16 tooth. That will still put you above 40MPH and should give you a lot longer runtime

A 21t is the smallest 5mm Mod1 I have, but I will give it a shot. I was really hoping to be able to have my top speed around 60 on 6s.

My friend just told me he is running 20/49 on his emaxx. I thought that since I have a lower KV motor, not to mention longer, I would be able to gear my truck higher. But since bigger motors draw more amps, this is not true?

If your temps are fine you can keep running where you are. However I thought the question was about efficiency and runtime. High speeds are not efficient! Your friend is geared for about 20 in first most likely and therefor his setup is working less than 1/3 as hard

According to BrianG's Scryptasylum, my friend is geared for about 50mph in 1st gear and 80mph in 2nd (Original E-Maxx trans geared 20/49 using Monster GT tires (6.75" diameter according to Tower)). We are both geared to the moon, I think (trying to impress the few girls that go to Missouri S&T).

My temps are usually pretty good. The motor makes a nice hand warmer in the winter, I would guess about 100F depending on how I drive. It can get pretty toasty in the summer, so I'll definitely gear down then. I'll bring my temp gun on my next run too.