Q:

XL motor and M motor on IR receiver = same output?

I am really puzzled by my small car build with balloon tires (55976).
I tried swapping between an XL and an M (PF) motor, both geared down (up?) to about the same torque (I tested by how sleep a slope it can climb), both motors virtually run the car at the same speed.
I would have thought with the same torque output (just about 3-4 cogs of different sizes in both cases till the differential) the XL would give me much higher speed, at least 1.5 times if not twice, but to the contrary it looked to me like the M motor runs a hair bit faster.
Any ideas guys? I know this is a bit generic, but what could it be?

there are frictions but in both of my cases they should be roughly the same
the XL motor is quite a bit heavier
the IR receiver (v1) has limited output or my batteries don't? (6 AA kind), there is only one more M motor (for steering, so it is off during my test) on the IR receiver, IR receiver attached to the battery box directly
I am using 2 u-joints for the drive axle instead of a cv joint - could that make so much of a difference?

A:

It seems like you're mixing up gear ratio with torque with power. It sounds like you have the following scenario:
 - same gear ratio
 - same overall vehicle
 - two different motors
 - (almost) same final speed  
And you're asking why the different motors don't change the speed much. Here's what I know:  

The vehicle's mass and internal friction place a load on the motor. This is approximately constant for your vehicle, because practically the difference in mass of the M and XL motors is negligible compared to the total load anyway.  
The M motor has a higher unladen speed but a lower overall torque than the XL motor. This means that for the same load, the M motor's speed will be more greatly affected than the XL motor.  
So in this case: your vehicle might drop the XL motor from 200 to 150 RPM, but drops the M motor from 600 to 175 RPM, because the M motor is affected more. Coincidentally this particular load drops both motors to almost the same final RPM.  
If your vehicle were even more massive, the XL motor's speed would drop, but the M would drop even further, quickly becoming slower than the XL. Add enough weight and eventually that M will drop to 0 RPM while the XL will still be chugging along.

