Every engine is the most efficient at max torque, which for a typical car’s gasoline engine would be around 4500 rpm.
But that “efficiency” means fuel burnt per unit of power. At max torque, the engine makes much more power than you need for normal driving, burning more fuel than necessary.
As a rule of thumb, you get the best real-world fuel economy at full throttle just above the low rpm limit where the engine would run “jerky”.
That’s at 1000-1500rpm for a passenger car’s gasoline engine.
At that rev range, you may only get 40 horsepower out of an engine rated for 100 at max torque, but that’s enough. You only need around 10 to maintain your speed against wind resistance, and you don’t actually lose any time accelerating slowly cause you’re gonna be at the next red light soon, anyway.
For reference, when I’m accelerating from a stop to highway speeds, I’ll shift to 2nd gear as soon as I’ve moved one car length, 3rd at 30km/h, 4th at 40, 5th at 50, flooring the throttle the entire time I’m not shifting. Then I’ll stay in 5th unless I’m forced to brake below ~45 again. Up or down a hill I’ll go one gear lower.
In my Diesel van, I regularly drive 40 in 5th gear.
I can’t make you take my word for it, but this is what I learned in a work-sponsored course for fuel efficient driving, and it got me much better fuel economy than the manufacturer’s claim for any car I drove in the past 20 years.
