I have to start out by mentioning. The 240ds will do on average worse if they have an automatic transmission. How much worse I do not know. It is reasonable to still see this five mile per gallon difference issue with them as well though.

Your car possibly falls into the maximum of 25 miles per highway gallon. I have never found a reason for the discrepancy between various 240ds.
One poster claimed he purchased his new and he never ever saw better than the 25 miles per gallon. On highway trips.

It may be driving style but I do not buy that. We all drive I suspect enough the same on highway trips we should not see a five miles per gallon difference. That said a good tune up may improve things somewhat.

It never hurts to check for dragging calipers. I noticed the other day that a rebuilt caliper I replaced a year and a half ago is dragging a little more than the others. I periodically wet my finger and check the wheel temperatures.

I coast to a stop and expect the wheels to be cold or all about the same temperature. For the test. A common problem when changing pads is to not clean under the old pad sliders and lubricate before installing the new sliders. The rust expands and should be filed ground or sandblasted off. Wire brushing just polishes it. I never bother to lubricate the end of the pads as the sliders are stainless steel. I guess you could use a trace. The high temperature caliper grease is expensive but a container lasts a very long time for a guy like me. Many if not most places do not do brakes really properly. At least in our rustbelt area. For the conditions they develop.

On a lot of short trips in a town or city. You might improve millage by using synthetic oil. From a cold start it takes some time to get that oil warmed up if it is conventional. The oil drag is really heavy on these engines when cold. Synthetic oil does not appear to have the heavy viscosity when cold.