Well a gasoline engine uses 14.7:1 air:fuel mix ratio.
A 5 liter engine pulls in 1 more liter of air than a 4 liter engine every 2 RPMs, so it would have to use more fuel to stay at the 14.7:1 ratio or engine would meltdown, but it would also have more power BECAUSE it can use more fuel.
That the physics of it.
But in practical use there is also power to weight ratios to be considered, a 4.0l engine will be working harder, using more air, than a 5.0l engine to move the 3,400lbs Ranger down the road at the same speed.
And then there is the human element.
People don't usually swap in a V8 unless they want the extra power, lol, so foot to the floor burn outs are not great for MPG.
Bottom line, assuming both engines are fuel injected and with electronic ignitions.
The 4.0l SOHC is not easy on gas unless you run premium all the time, at 9.7:1 compression ratio it would ping like crazy on regular gas so it has the knock sensor and changes spark timing so it can use regular gas, and that takes away power so it uses more fuel to go the same speed.
5.0l is 9.0:1 compression ratio so has no problem running on regular gas, and will have more power, than 4.0l SOHC does on premium.
If driven "normally" you might lose 1 or 2mpg based in engine size.
Last edited by RonD; 04-05-2016 at 11:15 AM.