I wouldn't go that far. The newer diesels aren't as efficient as the ones from a few years ago, but they are still better than a gas motor especially when their output is taken into account. Only the Ford V10 comes close in power, but it will die trying to pull like a diesel over the long haul. The discontinued GM 8.1 with the Allison was close but still short of a strong diesel.
Of the post 2007 diesels with DPF systems, the Fords are the worst MPG wise. An in-law has a new F350 SRW and he gets 13-15 highway empty. My bro-in-law gets 17-18 highway empty in his 6.7L Cummins, and my 2008 GMC Duramax gets around 16-17 empty. In town is a good 3-4 MPGs less. My friend has an LBZ equipped GMC (basically the same motor I have without the DPF), and he gets about 1-2 MPGs higher than I do.
I will admit that the older diesels were more efficient. Plus they are easier to chip and tune than the new ones. Far less electronic interference from the computers.
I love my GMC Duramax, but if I was looking for a used diesel, I would look for this in order:
2007 GM's with the LBZ motor. Some of the older GMs are good, but the LBZ was the best of the pre-2007 motors (IMO). They also come with the excellent Allison tranny.
Ford 7.3's. I forget the years that these were made, but there are plenty of Super Duties and Excursions with this motor. Not as powerful, but responds well to tuning and is a good solid motor. Avoid the 6.0, as they are problem prone.
Dodge Cummins of just about any vintage as long as it has a manual tranny. The Cummins is an awesome motor. Powerful, robust, and a torque monster. It also turns automatic transmissions into dust. It wasn't until the lastest generation 6 spd that Dodge could put an auto behind it that would survive. The six speed manual trannies they used were pretty robust and lasted a lot longer than the autos.
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