Why is S car fuel mileage so poor?

[Eric Sanborn]

Brett Dikeman wrote:

> That doesn't say "we start the mileage measurement from a 
> standstill".  It implies, if anything, that the vehicle is started and 
> immediately accelerated to speed.
>
> A 340hp V8, accelerating a 4,000lb car to 50mph- is going to use a bit 
> more fuel than a 300hp V8, and if it does so faster, the car will 
> spend more time at a higher speed.  Lower gearing makes a difference 
> too, it takes a little more energy to throw everything around faster, 
> etc.  Over a 10 mile test, these things all make a difference.  I know 
> if I accelerated very lightly and drove 10mi, I'd get a drastically 
> different average mileage result than if I accelerated hard and drove 
> 10mi.  Probably close to several mpg...

It does not work that way.  The EPA driving cycles are fully specified.  
A 1.3l econobox car would be asked to accelerate at the same rate as a 
7.0l muscle car.  So acceleration rate will not be a factor.

According to Gillespie in Fundamentals of Vehicle Dynamics, 1992.  The 
EPA driving cycles have the following breakdowns (these are approximate 
since I eyeballed them off a graph).

Type of Load                        City                           Highway
Aero drag                              18%                              42%
Rolling resistance                    22%                             30%
Inertia                                    60%                           
   28%


The inertia value takes in to account among other things changing speed 
of the vehicle.  As you can see it is fixed.  I think the difference in 
econ values lies in a number of factors.  Part could be different 
aerodynamic features of the car.  Things that help down force usually 
hurt drag.   It is also might have to do with more pumping losses with a 
higher compression (?) engine.


-- 
Eric Sanborn
'85 4ktq

http://sofadog.net/4ktq/