The only modern diesel sold (or was until recently) in the US is the 1.9L Volkswagen, and even that - a tiny 2800lb hatchback - barely broke 30mpg by the EPA test and as such does not comply with Congress’s pipe dream.
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I don’t know why in the EPA tests the VW TDI got that rating but I know people that use TDI New Beetles and Jettas for courier service and keep methodical records ,, 45 mpg is the norm for their fleet.
P.S. 2800 lbs isn’t “tiny” I’ve had a 2100# 1982 Civic and a 2300# 1986 VW GTI (that I put on a diet down to approx 2150#)..
Because Jimmy Carter's EPA asked a bunch of brilliant nerds who had never actually seen a car before how they'd test one, so they came up with an idiocy to be played with treadmills and computers instead of actually driving the thing.
45mpg that they may get, the EPA calls it 32 combined and that's the requirement manufacturers are required to meet.