As the vehicle moves faster, it must push the air out of the way faster, which causes more drag and takes more engine power. This is not linear, with the equation being 1/2 x air density x drag coefficient x cross sectional area x speed^2. So a small increase in speed can cause a large increase in drag. Tires will also cause more drag with higher speed, but this is linear and quite small compared to air drag.
Inside the engine and transmission, parasitic load from the water pump, oil pump, fan, and internal friction consume more energy at higher RPM. Additionally, an engine has to work harder to overcome its own air pumping losses at higher speeds. But an engine operated too slow and out of its power band will need to work harder and require frequent changing of gears, which reduces economy.
Every engine and vehicle will be different, so often it is best to experiment with different speeds. In general, there will be a sweet spot where the engine has enough power to cruise without bogging down but RPM is not high enough to cause excessive drag. For example, my automatic transmission car has a fuel economy peak between 55 and 65 mph (1980 to 2340 RPM). This is a sweet spot for boost, torque, and efficiency in my case, where the torque converter can remain locked 99% of the time. Below those speeds economy tapers off slowly because of unlocking converter and more frequent gear changes; above those speeds economy drops sharply as both engine drag and air drag increase.
A manual transmission gives much better control and has no torque converter or pumping losses like an automatic, which is why manual transmission versions of the same vehicle will often get significantly higher fuel economy when handled by a skilled driver.