Hey there Ray! You said "Neglecting the friction a larger engine makes, it takes the same amount of fuel to make the same amount of power so with the right tune and driving style you should get pretty close to as good mileage as any import beater."
Unfortunately, it's not so simple. There are many other variables besides engine friction that impact steady-state (Interstate cruising) fuel economy. One area that directly impacts engine efficiency is compression ratio. All things being equal, higher compression ratio makes for a more efficient package - that is, the same power level can be generated with less fuel used. Many newer vehicles are running much higher compression ratios as we've gotten better at managing resulting higher NOx output and detonation. State of tune is also a big issue - all things equal, a car running with a richer a/f will consume more fuel under the same conditions than a vehicle running a leaner a/f. Rolling resistance is another small contributor - most of the smaller imports have much smaller tires which create less rolling resistance than the tires on our cars. Of course, if you're going net-uphill (from the beach to the mountains), the car's weight is contributing factor. But the biggest potential difference between older Stangs (and my car) and newer "import beaters" is aerodynamics. Those are a function of drag coefficient and frontal area. Any newer import is gonna have a lower drag coefficient, and most of them are physically smaller and have much less frontal area than our cars. Just a few differences off the top of my head - much more involved than engine friction.
93teal - great one-time measurement; however, keep measuring, I think you'll find you got an anomoly. First, a/f on the dyno is measured at wide open throttle where the computer is in open loop. That is, it's not using the feedback from the O2 sensors; it's using tabular data to determine how much fuel to inject, and the computer is designed to run a bit rich then because it's a safer condition for the engine. At a steady state cruise, before and after the afpr install, the car's in closed loop and it's using the O2 sensor feedback to run much leaner than at wide open throttle. So installing the new regulator and then concluding that you're leaner at cruise is like comparing apples and oranges. You were leaner at cruise than at wide open throttle before too - you just don't have any cruise a/f data to look at. Second, when you make a fuel pressure change, you can impact a/f ratio for some period of time. But the ecu in our cars has learning adaptive strategies. If a component is malfunctioning, the computer will "learn" based on data from the sensors (O2's in particular) and it will make and store changes that will allow it to adapt to new conditions. So, if your stock regulator was allowing too high a pressure, over time (relatively short period of time) the computer would trim the pulse-times of the injectors to make the system inject the correct amount of fuel even at the higher pressure so it's getting O2 readings within the realm of what it expects. And if it couldn't make a big enough adjustment to account for that, it would set off error codes and (usually) a check engine light to let the driver know something had malfunctioned in a way that it couldn't adjust for any longer. So, your new afpr might've leaned things out for a bit, but as soon as the computer adjusted for the new pressures, it would be controlling mixture to the same levels it did before.
I do believe a real 34 mpg is possible for a short run in car like yours - but only if EXTREME driving measures were taken; drafting when possible (heavy traffic running at speed makes a 3-4 mpg difference), coasting down hills, limiting speed, etc. I'd bet there were multiple things at work that stacked in your favor in terms of the calculation. A pump that allowed complete filling of the tank when you started out, combined with a pump that wouldn't let you fill completely when you topped off can make a huge difference in the calculation. Most cars can be anywhere from 1 to 2 gallons shy of full, and the fuel gauge still reads full. If the pump cut off early leaving you say 1 gallon shy of full, but you thought you were full based on your gauge, your mileage would've been about 26 mpg. If you've had a tire size change - to a shorter tire - then your odometer is telling you you're going further than you actually are. And then there is potential speedo error. However, those last two items were at work when you were calculating your mileage before too. Keep checking and let us know how the mileage looks over the long term. I'd guess that over the long run, you'll find your mileage calculations won't be that 'exciting'.
I was doing 65-80 the whole way with 2:73's.
