Quote:
Originally Posted by MetroMPG
I'd just hate to see someone draw a conclusion about a mod and decide it's "good" when the effect might have been the result of some other uncontrolled factor.
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And that's the exact point, right there. Eco-tuning a gasoline-powered vehicle is tricky, to say the least. I've seen in my experimentation things that just don't make sense - for instance, highway commutes with A/C using the same amount of fuel as without A/C (the Karen-mobile), or slightly less (the Fiat Dakota).
I'm pretty convinced it has a lot to do with the fact, that gasoline engines in general have to use fuel solely for the purpose of creating and maintaining intake manifold vacuum. This is why, for instance, it's possible to get awful gas mileage while driving steadily at either 25 MPH or 75 MPH. It's possible to make a vehicle more aerodynamic, for instance, only to see the expected fuel savings go away because the engine now has less of a load to push forward -> intake manifold vacuum goes up in order to compensate for the lesser amount of demand -> engine now has to work at maintaining this higher vacuum -> total fuel burned paradoxically goes up.
The converse is true, and I've seen this, too. Load a vehicle with more weight (like I did with the Fiat Dakota) -> engine loading goes up -> engine vacuum goes down to compensate -> paradoxically, slightly less fuel is used. The same could be said for inducing drag via removing the tailgate.
The worst part about all of that is that all of this is so wildly dependent on speed and engine loading and the power output of said engine, that it's next to impossible to exactly quantify the benefits of a given eco-mod for a gasoline-powered vehicle.
Now, for highway commutes, adding eco-mods generally gives the desired result. However, the old saying applies - Your Mileage May Vary. So, good luck and have fun with experimenting.