My best tank to date, no passengers but several hundred pounds of photography gear in my 06 W211 320CDI was 43.7 actual, calculated miles per gallon (US Gallons}/and 908 miles on a single tank in one sitting. Wasn’t really trying too hard, just driving my typical 5mph over posted limit across the southern Rockies driving between Austin and Reno Nevada for the Air Races in 2013. Average speed for that tank was 54 mph but almost zero stops, traffic, AC use...etc. normally 34-38mpg Highway.

Mine is a smaller/lighter sedan with 215k miles on it now and fully modified and reprogrammed to get most out out of minimal revs with zero remaining emissions controls. I have a similarly prepared 06’ Jeep Liberty CRD and 26-27 calculated mpg under similar highway driving conditions is the best it will do, closer aerodynamics to a GL at least at say 65-70mph average speed. Aerodynamics at highway speed and air density actually make a massive difference in drag and therefore fuel economy.

My record tank on the W211 was summertime temps, averaged around 6k feet AGL base altitude for that leg getting as high as 9400 feet on a fairly warm and humid day in early September. No AC going as it felt great compared to Central Texas where I’d left from. Reality is that air density was perhaps only 15-20% of my normal value, no traffic etc. Had a borrowed Golf TDI manual for a bit, got consistent 44-48 mpg full tank averages out of it in summer driving. My daily commute is close to a hundred miles round trip and I don’t turn on AC till around 105F outside air temp in the Benz unless I have passengers which is rare.

I dont practice the hypermiling bible style of driving, I try to stay very near speed limit or slightly over in most road conditions traffic allowing, I do coast a bit protracted to stops (getting ready to replace original rotors at 215k miles lol and only 2nd set of pads), I typically do not drive with MB cruise control active as it brakes down hills to maintain preset speeds and I prefer to coast those and build up a little surplus energy to get me started on the next uphill section. Typically accelerating I do not exceed 2500 rpm and am usually in C not S mode. I accelerate as traffic allows but bump shifter for lower rpm gear changes when possible as my car spools the turbo early and quick with current mapping and mods.

When i I have loaned car to realtor buddy to show clients around for week I logged my worst tank ever, 22.5mpg! When the wife drives it for a week typically 27-29 mpg, about the same as she usually gets in my Kia Optima SXT, slightly better than Kia but not much. When I drive Kia I typically log AC use full tanks of 28-29 mpg and milder weather pure highway 32-36mpg, it’s a high compression, turbocharged gasoline direct injection motor and auto trans. Driving style and ability to stay moving and tolerate life without AC here in Texas reaps big fuel economy rewards in all my cars except F-250 Ford truck which is 16.5-18 mpg AC or not in pure highway use, taller tires overdrive 5.5% but it’s crappy either way but old 7.3 turbodiesel is reliable and very capable off road.

I lost a ton of fuel economy data going back many years in an iOS upgrade crash last year sadly, it had my old 750iL e38 BMW V12 which typically did 18-21 mpg pure highway (and as bad as 11 in real estate duty)and 92 NSX which typically was 26-28 mpg in commuting or highway trip driving. I’ve also got a V8 Audi D3 A8L 4.2 gas that gets 20-22.7 pure highway and commuting and often drive wife’s Genesis Coupe and get 25-27.5 mpg from it. I’ve tracked fuel economy on pretty much every car I’ve owned since about 1990 when I started driving...bit OCD perhaps but it’s often an early warning sign of trouble to come.

Strong tail wind AND headed down hill? Quite oddly I got a top three to five tank last fillup? 39.6 US MPG.

You like in a high desert area right? Check local airports ATIS for true or absolute barometer for any given day, lower air density equals less drag, requiring less power, the turbo more than makes up for this on power side, but driving in say 27” mercury for an atmosphere vs say 30.2 the way it often is here is significantly different. Drag goes up as the square of speed so 10% less air pressure is like 20-30 % less aero drag.