## Sunday, September 14, 2014

### Least fuel-efficient versus most fuel-efficient small and standard SUVS

The data for this problem came from two EPA web sites.   The first lists the least fuel- efficient vehicle for each type of vehicle.

The second lists the most fuel-efficient vehicle for each type of vehicle.

Question:   Below are the EPA measures of the least and most-fuel efficient small SUV and standard SUV.

How much gas is saved by a driver who drives 10,000 miles per year switching from the least fuel-efficient small SUV to the most fuel efficient small SUV?

How much gas is saved by a driver who drives 10,000 miles per year switching from the least fuel efficient standard SUV to the most fuel efficient standard SUV?

 Lowest EPA Fuel Efficiency Highest EPA Fuel Efficiency Small SUV 16 31 Standard SUV 13 26
In some classes the EPA listed two vehicles an EV and a conventional vehicle as the highest fuel-efficient vehicle.   Analysis is restricted to vehicles that are not electric vehicles.

Answer:   The gas consumed is miles/mpg.   Take 10,000 and divide by mpg in the above chart to get:

 Lowest EPA Fuel Efficiency Highest EPA Fuel Efficiency Gas Saved Small SUV 625.0 322.6 302.4 Standard SUV 769.2 384.6 384.6

The amount of gas saved by converting from the low fuel-efficient to the high fuel-efficient vehicle is around 82 gallons greater when the conversion is done in the standard SUV category rather than the small SUV category.

Question:  How high would the small SUV fuel efficiency have to rise so that the net savings in gas from the switch in vehicle choice would be identical to the improvement that occurred in the standard SUV category?

Answer:   The gas savings in the standard SUV category was 384.6 gallons.   For this to happen in the small SUV category the high fuel-efficiency small SUV would need to run 10,000 miles on 240.4 gallons  (625.0 – 384.6).   The required fuel efficiency is 10000/240.4 or 41.6 mpg.

Note that for gas consumption reductions to be identical in the two vehicle classes the percentage improvement in fuel efficiency must be larger for the fuel efficiency class with the higher least fuel-efficient vehicle.  It is logical that there should be diminishing returns from improvements in fuel efficiency.   For a 100-mile trip, the reduction in gas consumed when mpg goes from 5 mpg to 10 mpg is 10 gallons.  When mpg goes from 10 to 20 the reduction in gas consumed is 5 gallons.