6 wheels more efficient than 4 wheels? Which would be easier to push in a straight line, a flat cart with 4 wheels or the same cart (and same load) with the same size wheels, but has six wheels?  and why? Thanks
 A: Where more wheels becomes an advantage that can make mobility easier is in reducing ground pressure.  On non-deformable surfaces like paved roads or hard packed soil this doesn't matter for mobility; but in softer soil or mud the lower ground pressure reduces the likelihood or severity of sinking into the soil reducing the risk of getting stuck and the amount of energy lost pushing soil around even if not.  
A: It would be easier to pull the car with 4 wheels because you would need less energy to make only 4 wheels move (less angular momentum).
What I'm saying is that its harder to accelerate and stop the cart with more wheels, but to make it keep his velocity, the force is actually the same because the attrition is the same with both 4 or 6 wheels. It only depends on the weight of the cart and the materials used.
A: Throwing in my two cents.
The reason large trucks which travel on hard surfaces have more than four wheels is so that the axles don't break.
The weight is distributed over 3+ axles rather than just two.
