Right before I got my diesel suburban in 2006 I was away from home doing a work thing, we rented a 2005 or 2006 duango with the V6.
It got 13mpg driving it around town.
Its going to take some work to get it near 20mpg for anything but pure highway driving.
1984 chevy suburban, custom made 6.5L diesel turbocharged with a Garrett T76 and Holset HE351VE, 22:1 compression 13psi of intercooled boost.
1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 4th Gen disc brake fbody rear end.