at different engine rpms you get different peaks extracted speeds to the wheels through the drivetrain
so the basic idea is not to push that throttle too hard that it goes guzzling fuel and you getting lower bang for you fuel and buck...
i usually do up to 2k rpm and the speed at most i get is around 90kms max with my car it wont go up in a jiffy but slowly after shifting through all the gears it will reach there and i get around 500kms to 550kms per tank of fuel with a/c and without a/c respectively and the tank full is setting me back 85dhs...
so different engine run efficiently at different rpms you just have to figure out yours by experimenting or by looking up the specs sheet of that engine...then last but not least there are other factors like weight ,pressure,aerodynamics,age and condition of engine and drivetrain...
most of the time you waste fuel at idle and in stop and go traffic as you are at lower gears so for more rpms you get less distance travelled as opposed to in higher gear...
P.S do these things while in the second or last track or else you will get in trouble with others who are in hurry....