Inclined planes, mass' effect on final velocity

[Clef]

Okay, so I've been working on an extended practical investigation. And I am a little stuck. Heres a brief overview:
Using a ramp(always at the same angle to the horizontal), and a truck and a stopwatch, I've timed the amount of time it takes for the truck to reach the bottom of the ramp, with different masses. Whilst I predicted that the mass would have no effect on the final velocity because it would not effect the acceleration, my results show that the larger the mass, the final velocity would increase very minorly. Why is this? I am guessing it has to do with friction, or perhaps my hypothesis was incorrect?

 

[PhanthomJay]

Okay, so I've been working on an extended practical investigation. And I am a little stuck. Heres a brief overview:
Using a ramp(always at the same angle to the horizontal), and a truck and a stopwatch, I've timed the amount of time it takes for the truck to reach the bottom of the ramp, with different masses. Whilst I predicted that the mass would have no effect on the final velocity because it would not effect the acceleration, my results show that the larger the mass, the final velocity would increase very minorly. Why is this? I am guessing it has to do with friction, or perhaps my hypothesis was incorrect?

Theoretically, with rolling friction/axle friction coefficients assumed equal for all trucks, their speed would be the same at the bottom, in a vacuum . Your experiment is taking place in air. Thoughts?

 

[Clef]

air resistance, of course!:)