A few things about your method are not ideal. It is probably too late to change this now, but at least you can make amends in your evaluation.I’m not sure why you are using a wheeled truck and an air track. You usually use a “v-shaped” unwheeled metal “vehicle” which is then pretty frictionless. Or use a wheeled truck on a long(wooden or metal) runway which you friction-compensate by tilting the runway until the vehicle when given a gentle momentary push then moves down the runway at a steady speed(Newton’s first law).
The way you are measuring the KE gain and GPE lost is not ideal as you are only measuring the average KE gain for a given GPE lost. This should give you a lower figure for the KE rather than the higher one quoted.
It may have been better to have measured the GPE lost from the fallen height to striking the floor(75cm say) and then to have measured the speed gained over a distance of 10-20 cm electronically AFTER the falling mass has hit the floor.( electronic timing should mean that you can measure over a shorter distance and still retain accuracy) The truck is then moving at a steady speed(no forces on it) and will be the final KE reached.
Your KE gained should ALWAYS be less than the GPE lost in such experiments due to energy losses — air resistance with moving objects, friction in the pulley and possibly some friction due to poor friction-compensation being the most obviouse areas.
I hope this long answer is of some use to you.

Phil