The power requirement to maintain constant speed on a level road with no wind increases with the cube of speed, so fuel consumption normally increases noticeably with increasing cruise speed, and typical cars achieve lowest steady state (top gear) fuel consumption at about 35-40 MPH, however, modern cars that have very tall gears that are not useable until higher road speed may have a slightly higher lowest fuel consumption cruise speed.

Various anecdotal evidence such as yours is sometimes presented, and I can only take it with a grain of salt - most drivers don't keep accurate fuel consumption records over a long period of time with fairly constant driving habits, which is what is necessary to draw general conclusions.

Spark ignition engine thermal efficiency varies widely with revs and load from around 30 percent at high load near the torque peak (but less than WOT to avoid power enrichment) to zero at idle. A lot of spark ignition engine loss is the pumping loss associated with manifold vacuum, and the internal friction losses increase with the cube of revs, which means that running tall gears so steady state cruise is achieved at low revs and high load is best, however, offsetting this is the fact that most spark ignition engines have less than ideal spark advance for peak thermal efficiency at low revs and high load, either due to emission requirements or detonation considerations.

Diesel engines have little pumping loss because they are not throttled, and since detonation is not an issue, injection timing can be optimized for peak thermal efficiency at all speeds and loads if the control system is sophisticated enough to provide the required granularity, and their higher compression ratios generate inherently higher thermal efficiency.

Duke