I find it very comparable to my HJ60. I got 19MPG with my HJ, and 18MPG with my 6.2 powered 60, but I have more power with the 6.2. So in my opinion the 6.2 is more efficient.i think the 6.2 was one of the worst diesel motors ever made as far as fuel efficiency goes
I don't think anyone mentioned another plus for the 6.2/6.5 is they were designed to replace SBC, so have the same bellhousing, flywheel pattern, engine mount locations, water pump hose orientation, etc.
So a 6.2 will bolt in using the commonly available SBC swap parts.
I did one back in July 2006 and have 11,000 miles on it sense the install, couldn't ask for more, I am getting 21 to 23 miles to the gallon average driving it hard town & country made all the mounts myself did it with a Ranger O/D unit used the 6.2 alt., p/s, a/c, & vac pump for the brakes, It has gear noise in the O/D unit, which is my only complaint, but I am getting ready to do a FJ62 with 6.2 and a 700R4 setup. I think that will be nice setup O/D with lockup convertor. Will see. Oh the GM sticker on the breather says 155HP@3600RPM . Yes the inj. pump is problem after app. 200,000 miles and a rebuilt one is app. $365.00 exchange.