What was your older Mustang spinning(rpm) at, at 75 mph? What about your Mustang now with 3.31?
Most efi’s are going to cut all fuel on closed throttle coast... better for emissions and mileage. For manual trans cars and automatics with lock up converters, the engine is turned by the drivetrain so you get some engine braking as well. Sitting idling obviously takes some gas, though.
Technically, yeah, but you don't notice it happening except when you're watching a mileage computer. When you see the instantaneous mileage go up to 999 mpg, that's what is going on. As soon as you touch the gas, the injectors kick back in. Carburetors could never get the mileage of EFI for just that reason.
It's true there are many variables but as a general rule, the driver is one of the most influential factors in fuel economy. Some of the best gas mileage I've ever gotten has been up in the mountains but in that case, altitude can be your friend, especially if you're driving a NA engine, and I've read some folks claim they actually get better fuel economy after their engine mods and tune. I'm not saying that's true or not but if you improve the engines ability to breath and still drive conservatively, I can see where it might be possible.
Another gauge you can look at is the A/F ratio. During just about all driving, it stays very close to 14:1 which is optimal from what I understand. Driving down the road and letting fully off the gas peddle, it pegs as lean as it will go. Only way that's happening is if the fuel is all but shut off. It will maintain that until the engine rpms drop to just above idle, then the A/F ratio gauge drops back to 14:1. It's probably not as obvious with an automatic but with a manual transmission, you can feel it. I let off the gas, the car starts to decelerate, then the engine braking really kicks in and the A/F ratio gauge pegs out. Once the engine nears idle speeds, the rate of deceleration eases off and the A/F ratio gauge comes back down to that 14:1 as the cylinders are firing again.
