Speed these days is cheap. There are a hundred different ways to go faster, from subtle computer modifications to monstrous quad-turbo systems, hot-rodders are spoiled for choice when it comes to squeezing more performance out of their rides. Steep gear ratios like the classic GM 4.10 series were once the hot ticket for cash-strapped racers; however, with gas a four bucks a gallon it's only fair to wonder if extreme gearing is really worth the cost in fuel and engine longevity.
Horsepower and BSFC
Drivers have long known that high rpm isn't good for fuel economy, but few ever really stop to think of why. Every time your cylinder fires it burns a certain amount of fuel; the more times it fires per second, the faster it uses that fuel. Since horsepower is a measure of how much work an engine does over a certain period of time, it's then fair to say that every horsepower your engine makes takes that much more fuel. Given constant torque, horsepower will always rise with rpm because the pistons fire more times per second. The amount of fuel that the engine uses -- measured in pounds per horsepower produced -- is its "base specific fuel consumption." The lower the BSFC, the less fuel the engine uses per horsepower, and the more efficiently it runs.
RPM, Throttle Position and BSFC
An engine is like the string on a guitar: It's tuned in every way to work perfectly when cycling at a certain rate, but doesn't do so well at other frequencies. In terms of efficiency, your engine's "perfect pitch" occurs just before or at peak torque -- when it's converting fuel to power most efficiently -- and under full throttle. That's right: creating an engine to operate under less-than-ideal conditions forces manufacturers to put a cork in the intake known as the "throttle plate." The further closed the throttle plate, the harder the engine pulls against it and the more power it wastes creating vacuum in the intake. So, ideal conditions will occur when the engine cruises under full throttle at peak torque rpm -- the further it gets from that, the more fuel it's wasting.
The Balancing Act
Given that, you may now have some idea as to why economy cars are often brutally slow. In order for a car to cruise close to full throttle at peak torque, the drivetrain can't produce any more power than is needed at that cruise speed. Generally, that means either using a very small engine or "de-tuning" the engine by opening the exhaust-gas-recirculation valve, retarding timing, leaning out the air-fuel ratio or deactivating cylinders to increase load on the engine. If you don't do any of those things, you must drop engine rpm to increase load on the engine, but then you risk dropping rpm far below peak torque and losing any advantage you gained from opening the throttle. It's a delicate balancing act under the best circumstances -- one made all the more difficult when you step up to steeper gears.
Steeper Gearing Effects
Steeper gears give your engine a mechanical advantage at the tires, which is good for acceleration, but very bad for fuel economy because they force the engine to run at higher rpm with the throttle closed further. Even if your steeper gears do put the engine right at peak torque, you're still using about 80 percent more fuel with the throttle at 25 percent open than you would at full throttle. Nearer to idle, the difference is much more pronounced: at part-throttle to 25-percent-throttle, you could end up using three times as much fuel than at peak-torque/full-throttle. Rpm, though, doesn't usually significantly change BSFC except at the extremes of the range. So, under the best circumstances, if you go from garden-variety 2.23-to-1 gears to 4.11 gears, don't be surprised to see a 30 to 50 percent drop in fuel economy at the same cruise speed.
Offsetting with Gearing
Of course, a certain amount of that goes out the window if you have an overdrive transmission, particularly a six-speed or more with two, very high overdrive gears. In this case, you might not see a great drop in fuel economy. You might even see an increase if the original gearing put the rpm far below the torque band in the first place. That's fairly unlikely though; the only time you're likely to see an increase in fuel economy this way is if you've installed very large off-road tires or racing slicks. In this case, steeper 4.11 or higher gears may serve to offset a drop in effective gear ratio from larger tire circumference. Even so, there's a better chance that the drop in rpm and increase in load via larger tires is actually helping fuel economy, and that steeper gears are going to cost what little you've gained.
The other detuning technologies mentioned above -- particularly cylinder deactivation -- will help if you have them, and many newer, V-8 performance cars do. However, you shouldn't count on them to compensate: Some computers detune according to engine load via the MAP sensor, simpler computers use an open-loop system that detunes according to throttle position and road speed. The only way to know for sure is to contact the manufacturer or research your car and find out what triggers the detuning or cylinder deactivation. All things considered, if you don't know how the computer is going to react to gears as steep as 4.11-to-1, then you're best off choosing from within the gear ranges offered by the manufacturer with that chassis and engine. Even then, you'll still need to reprogram the computer to correct the speedometer -- if you're using a stock gear range, it should have some provision for that range in the programming profile.