Pinion and heatsink ?'s
 

I have an E-maxx and am installing a 1515/2.5d with the 72 spur and need suggestion for what pinion. I used the speed calc. but dont know my diff ratio. What size heatsink for the 2.5d. I don't understand the "L", "XL" portion of my motor. Thanks ahead of time for your help. Everyone on this site has been awesome!!

The diff ratio for the emaxx is in the drop down menu near the top of the calculator.
S, L and XL refer to the length of the motor, but that is for feigaos, not the neus. What voltage are you going to be running- here are a few guesses for your ideal setup:

Differential Ratio: 2.8461538461538462
Transmission Ratio: 1.7222222222222223
Other Ratio: 1
Spur Tooth Count: 72
Pinion Tooth Count: 22
Total Voltage: 22.2
Motor KV: 1700
Tire Diameter (inches): 5.75
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0.01
Spur/Pinion Ratio: 3.27 : 1
Total Ratio: 16.04196 : 1
Tire Circumference (inches): 18.06 inches (458.83mm)
Total Motor Speed: 37740 RPM
Vehicle Speed: 40.24 MPH (64.65km/h)
Effective Speed Rate: 1.81mph/V (2.91kmh/V)
Effective KV Value: 1700
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...

6s seems like the best voltage for that motor. You really want a smaller spur than that though, other wise a huge pinion is required:

Differential Ratio: 2.8461538461538462
Transmission Ratio: 1.7222222222222223
Other Ratio: 1
Spur Tooth Count: 66
Pinion Tooth Count: 20
Total Voltage: 22.2
Motor KV: 1700
Tire Diameter (inches): 5.75
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor coil Ω: 0.01
Spur/Pinion Ratio: 3.3 : 1
Total Ratio: 16.17564 : 1
Tire Circumference (inches): 18.06 inches (458.83mm)
Total Motor Speed: 37740 RPM
Vehicle Speed: 39.91 MPH (64.11km/h)
Effective Speed Rate: 1.8mph/V (2.89kmh/V)
Effective KV Value: 1700
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...

I think MOD1 gears would be better, failing that get some integy delrin machined spurs- they come in smaller sizes than the stock traxxas selection.

If you get a finned neu, a heatsink cant be fitted/isnt needed, but if you get a smooth can neu... they still arent needed, but any normal 540-550 motor type heatsink will work perfectly.

5s works really well on the 1515 2.5d (1700kv) and 6s is pretty powerful, 4s will work ok, but it just won't have the power that it does on more voltage.
What pinion you use depends on what sort of driving you want to do and what voltage your going to run. I race mine on 5s with a final drive ratio of 15.2:1 and it works well for that, but have had it geared as high as 12.5:1 (51/20) on 6s for high speed driving without the motor reaching 120f.

Smooth can neu's are 39mm in diameter and I don't know of any heatsinks that will fit because most motors are 36mm in diameter, so the heatsink will be too small.

Oh, I didnt realize the neus were abit fatter than normal motors, thats a pain. Still, with sensible gearing they shouldnt overheat like a feigao does.

No, the Neus heat up FAR less with the same voltage/gearing. Yeah, the Neus are larger in diameter, so you can't use the RCM heatsink on even the smooth can ones. But, they really don't need any heatsinks (and this coming from someone who hates any kind of heat on their component!)