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Both CUMMINS and KENWORTH note that comparisons are made in the following order:
1] Vehicle Spec
2] Climate
3] Terrain
4] Vehicle Use (driver skill falls into this)
Just dividing things between city & highway is a subset of the least important factor.
And then other details creep in. I live in the city where, for it's size (and of all cities larger) the ease of moving around is greatest in North America. That's the icing on the cake where climate ("humid desert" accoriding to USGS) and terrain (flat, at 35' above sea level) are the higher considerations.
For comparative purposes I use "South Central US" against very close vehicle spec to come up with a few folks to compare notes with.
And it is possible for "city" to be as high or higher than "highway", remember. The only advantage to "highway" is steady state. I've seen 23-mpg "city" for over 1k miles against tens of thousands "highway" minimum of 24-mpg [average].
I think the best we can do is a general range, and in that, the EPA numbers provide a solid basleine for percentage improbement . . until all the confusiion arrives in a multi-line spreadsheet where how to factor just the BIG diferences is difficult.
But, wish you liuck, nonetheless.
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