Re: [Phys-l] truck mileage

[John Denker <jsd@av8n.com>]

On 05/07/2008 02:02 PM, Kathy Daniel wrote:

> one 5000# truck gets about 18 miles per gallon
> same 5000# truck pulling a 4000# trailer gets 8 miles per gallon
> same 5000# truck pulling a 4000# trailer + 10000# of cows ALSO  gets 8  
> miles per gallon.
>
> Any ideas why the empty and the full trailer both get the same  
> mileage?  This same pattern is also present in a different trailer and  
> truck combination.

0) We have to assume the speed profile is the same in all cases,
 otherwise there is no hope of making sense of the problem.

1) This is obviously fake data.  I'm not going to say anything
 very nice about fake data.

2) However, one can come up with a scenario where this data is 
 not completely wacky.

 The principal losses for a vehicle are:
 a) Rolling resistance (tires and bearings).
 b) Aerodynamic drag.
 c) Braking (and starting).
 d) Internal losses in the engine.

 In steady low-speed cruise, (a) and (d) dominate.
 In steady high-speed cruise, (b) dominates.
 In stop-and-go traffic, (c) dominates.

So if we assume steady high-speed cruise, it is plausible that 
added mass might not make much of a difference.

If you want to grasp at straws, you could cobble up scenarios
where a stake-truck full of cows would be /more/ streamlined
i.e. more aerodynamic than an empty stake truck.