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Originally Posted by nick325xit 5spd
I'm basing this off of reports from a couple folks that did it.
And too tall gearing can absolutely increase fuel usage, especially in a somewhat underpowered vehicle. Especially in a manual that doesn't get the benefit of a sloppy torque converter like the automatics have. Sure, you can slip the clutch, but there's only so much of that that you can do.
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That would be valid if you slipped the clutch a lot (like A LOT) -- which you don't have to do with the 4.1 final drive and still wouldn't with the 3.64 (that is still a pretty short final drive in the grand scheme of things). If you are slipping the clutch enough to affect your fuel economy, you would also be replacing the clutch on a yearly basis.
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Modifying the stock ratio to a taller ratio will result in a decrease in acceleration. If you decrease acceleration by applying a taller ratio, then that lost acceleration will need to be gained by some other means...
...The only place on a stock E53 to gain more acceleration would be, the throttle. To compensate for the lost acceleration more fuel will need to be applied to make up for the loss.
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Do you think that when you are at full throttle at 2,000 rpm it uses the same amount of gas as full throttle at 3,000 rpm? It doesn't -- you are accelerating slower because the motor is using gasoline at a slower rate (because it's at lower RPM and is pumping air and fuel at a slower rate). It's not like using higher RPMs magically gives you extra acceleration without spending any gasoline to do it.
If you think of a piston engine as an air pump, which it first and foremost is, then it's easier to visualize that the engine produces more power depending on how much air is pumped through it. Full throttle allows the engine to pump the maximum amount of air at that RPM (which is 3 liters every two revolutions, for the M54). At 1500 RPM that would be 2,250 liters of air per minute. At 3,000 RPM that would be 4,500 liters of air per minute. Partly closing the throttle restricts the flow of air (since the throttle is just a restrictor plate). The amount of power it produces is roughly proportional to the volumetric flow rate of air through the engine. (This is not 100% exactly true in practice due to the volumetric efficiency, or VE, not being quite constant, but it's pretty close.)
Likewise, pumping the air into the engine using a supercharger increases the volume of air that you can get into it per revolution, because it pumps it in at a higher than atmospheric pressure and thus a higher density. When you supercharge your mileage suffers because you have to inject more fuel to keep charge temperatures lower, to protect from detonating, but that is a different thing from what we're talking about by swapping final drives.
The end result is that you pay in gasoline for all the acceleration that you ask for with your right foot.