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Why a 40 mpg car doesn’t get 40 mpg (updated)

by David Zatz on

Numerous articles, including at least one at itself, have noted that at least one Dodge Dart will achieve 40 mpg, as per the agreement between Chrysler and Fiat. Industry observer J. White wrote that this does not mean it will be rated at 40 mpg highway when it comes out, because the test involved is for combined, unadjusted highway/city gas mileage (“unadjusted” means “using the original, generous testing standards.”)

In the agreement, White pointed out that…

“Fuel Economy Test” means the final, verified results of a combined (i.e., City/Highway) unadjusted two-cycle fuel economy test conducted by the U.S. EPA, or the Company if the U.S. EPA does not perform the test within a reasonable period of time… pursuant to the definitions and test procedures set forth in 40 CFR 600 et seq., and 40 CFR 600.512-86 in particular as in effect on the date hereof… [test regulations in PDF form]

While there are no formulas to convert from one test regimen to another, the combined gas mileage ratings currently in use on window stickers are weighted 43% city, 57% highway [ratios corrected]. 40 mpg “old style” translates as around 29 mpg city and 32 mpg highway using the tougher and (for most drivers) more realistic new standards.

That could translate to a current EPA sticker of something like 25 city, 34 highway; many combinations would be possible. Marchionne has already implied the new car would beat the 40 mpg rule, though this would only require it to have the same gas mileage as, say, the 1995 Neon (rated under the contemporary standards at 29 city, 38 highway, equivalent to roughly 26 city, 35 highway by 2008 standards).

Gas mileage is expected to rise when the 9 speed automatic arrives; ZF estimates a 10% or higher increase, which could bring highway mileage to a “window-sticker” 40 mpg. To compete for “green mind-share,” a 40 mpg highway version seems to be the new standard; automakers have increasingly been positioning themselves to be on customers’ minds if gas prices should spike. During the last sudden rise in fuel costs, customers seemed to go to brands with a reputation for gas mileage, rather than researching actual figures and buying accordingly.

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