[lincpimp]

I have to disagree.

The 1518 will make more power, to get more power you use more amps, that decreasing the runtime. Lower gearing with your current setup will increase runtime, or going up in voltage, and then lowering gearing. Batteries are cheaper than motors, so go to 5s lipo, and gear down.

Plus the 1518 will be a bit slow on 4s, IMO. More of a 5s motor.

If you are happy with the performance of the 1515 stick with it. Higher voltage and lower gearing will equal less load.

Higher voltage setups are more efficient than lower voltage setups. Even if you keep the same overall battery weight (meaning your 4s pack will have more capacity than the 5s you switch to) you will notice an increase in run time. This figures that your driving style and top speed are the same, which if you retain the same kv motor will require you to run a smaller pinion as the motor will spin faster.