My 1979 Euro K-Jet 280SE M110
gives me, on open-road work (cruising up to 120 km/h) and drinking 98 RON, 11-12 litres per 100 km
(approx. 23.5 - 25.5 mpg). If I extrapolate that to distance per tank full it comes to 800 km
/ 500 miles or better. This car has 300,000 km on the clock and is serviced properly every 8,000 km / 5,000 miles. It has a dry-element air cleaner. Although not a hoon, I am not a slow driver and tend to fang it up hills, passing all those who can't / won't maintain at least the posted speed limit. The handbook suggests about 11-12 litres / 100 km.
Around town it is quite thirsty, but as I live in a smallish rural city and walk to most of the shops (a block away from home), that's not really an issue.
OTOH, a poorly maintained 1978 280SE M110 (Oz spec)
, with oil-bath air cleaner, that I bought for parts returned, on the only two trips I made in it before taking it off the road, about 15 litres per 100 km
From my 1970 W108 280SE M130
(oil bath air cleaner) I also get 11-12 litres per 100 km
(approx. 23.5 - 25.5 mpg) -- though this car is less thirsty around town, probably because it has the fluid coupling rather than a slushy torque convertor. This car has almost 837,000 km on the clock.
My 1965 W111 220Sb M180
(oil bath air cleaner) is a very thirsty beastie: 15-16 litres per 100 km
(17.75 - 18.8 mpg), but it does have two dual-throat Zeniths. (Approx. 233,000 km on clock.)
My 1965 W112 300SE M189
coupe (oil bath air cleaner) is also thirsty: 14-15 litres per 100 km
(18.8 - 20 mpg) -- but is well within the factory-claimed range for the six-plunger-injected M189 engine of 11-18 km per 100 km. (Approx. 250,000 km on clock.)
Wherever I go, I always carry a large toolbox, gallons of spare vital fluids, and sometimes a 2.5 tonne capacity trolley jack, so a reasonable load in the boot. (Of course, I only ever need those tools and supplies when they aren't on board! Carrying them seems to prevent problems.
I have no complaints about fuel consumption. I wonder if there is usually a big difference in thirst between Euro, US, and Oz-specced versions.