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My 2013 Subaru WRX has a 5-speed manual, and I long for it to have a 6th gear when cruising on the highway.
A friend has a 2012 Subaru STI with a 6-speed manual, and I noticed his car runs through the gears more quickly to achieve the same speeds.
At 3300-3500 RPMs, shouldn't the WRX be able to achieve better gas mileage by keeping the same 5-speed gear ratios, while adding an additional gear to lower RPMs to 2800-3000?
tl;dr: different gear ratios are a feature, not a bug. Some cars use more gears for acceleration, some use them for better gas mileage. You can't do both.
At 3300-3500 RPMs, shouldn't the WRX be able to achieve better gas mileage by keeping the same 5-speed gear ratios, while adding an additional gear to lower RPMs to 2800-3000?
You've exposed the classic trade off in transmissions. We want to get the spinning motion of the motor to turn into spinning motion of the wheels. Unfortunately, the engine has a maximum rotational speed (the redline is there for a reason).
The gears in the transmission are really just multipliers in the rotational velocity equation.
First, a picture:
In this image, you can see that the larger gear A (being driven by the motor) is driving the smaller gear B (used to turn the wheels). Yes, I know that there are more bits after this gear but for the sake of discussion let's forget final drives, etc.
In this example, you can see that each turn of gear A results in two turns of gear B. This translates to a gear ratio of 1:2 or 0.5. If you had this gear in your car, you could cruise at very low revs on the highway (but you'd never get up a hill!).
Saying it again, in words:
A low ratio gear will turn the car tires a low number of times for each engine rotation. A high ratio will turn the car times several times per engine rotation. Thus, a high gear ratio at the top end permits a high top speed. It also means that, at highway speeds, the engine revolves less per linear meter of road. Fewer revs == less gasoline burned per second.
Note: sometimes you will about hear "short" and "tall" gears. Short gears are the low speed gears (with high ratios) and tall gears are for high speeds (with low ratios). This inversion of terminology is one of the great joys of trying to discuss transmissions.
However, a high gear ratio has a lower mechanical advantage. This means that it's harder to accelerate the vehicle (tires have to turn faster to get the car to go faster). A fuel efficient gear ratio is also not a fun gear ratio (less zip).
When you really want to accelerate quickly, adding gear ratios down low (say a six speed rather than a five speed) lets you stay at a higher mechanical advantage for longer.
However, if you're focused on fuel efficiency, you can also take what is effectively a five speed and put a really low gear up top for highway cruising. This will get your mpg numbers up but will be extremely non fun (you might even be below the minimum revs required to spin up the turbo).
Back to the specific cars in question:
From looking at the WRX vs STI gear ratios, it's clear that the WRX has a low ratio first and second gear so that you only have to shift once to reach 60 mph. This is purely an effort to optimize the 0-60 time (marketing. Sigh). In the STI six speed, the gear ratios of the first five gears are spaced pretty evenly between the gear ratios of the first four of the WRX. This means that the STI won't feel that slog through first and second that we five speeders have to labor through. Even the sixth gear of the STI is still a higher ratio than the fifth gear of the WRX. Cruising won't be as relaxed but you'll be better able to accelerate from 50 mph to some higher number....
Most vehicles will only have a single overdrive, where the second to the last gear will be a 1:1 ratio (or something near it). The notable thing here is the Tremec 6-speed transmissions (used in the Camaro, Viper, Corvette, Mustang, and others) which have the double overdrive.
The main purpose of more gears is to allow the vehicle to stay in the torque/hp range for the trade-off to gain fuel economy. More gears means the engine can be tuned to perform it's best at the smaller rpm range. For instance, with more gears the rpm range in question may be from 1500-2250rpm rather than 1000-3000rpm (for normal operations ... we're excluding spirited driving here). If the manufacturer tunes for the smaller range, they can get more out of the engine while maintaining better fuel efficiency. With the broader range the engine has to be able to perform over the complete range, so the tune has to be done to accommodate.
Most vehicles will not have a double overdrive because they do not have the torque to maintain the speed at the lower RPMs. Given a 4-cyl engine which, while having great hp output at the higher rpms, does not have the torque needed to successfully propel a car if it were driven at 1500 rpm at 70+ mph. The car would either need to be dropped down a gear to accommodate, or the engine would be lugged with the driver mashing on the pedal ... both of which would defeat the purpose of having more gears in the first place ... that being to gain fuel mileage and drivability.
In most sport-oriented (and heavy towing) vehicles, the point of an extra transmission gear is to keep the engine operating between it's torque peak and HP peak. Thus, most models of vehicles that are base models will have a 5 speed manual (or 3 speed auto), while the sportier or heavy duty versions will have more gears to chose from.
Many 6 speed transmissions have relatively similar 1st (perhaps a smidge lower) and 6th gear (perhaps a smidge higher) ratios in comparison to their 5 speed counterparts. If you want to go through the effort, you can change the final drive ratio or tire size to go a little faster at a given engine speed.
Semi trucks have 10-18 gears because they need to be able to stay at just the perfect RPM to get max torque, and to be able to get up into the next gear (while the truck is slowing down) and still be close to perfect. Semi transmissions are usually in fact 2 transmissions in a row, and very close attention is paid to final drive ratio so that both transmissions are in a 1:1 gear at 60mph and the engine at it's peak fuel efficiency RPM.. Every time the power has to go through a set of gears, some is wasted as heat, and most transmissions in any gear except 4th all the power has to flow through TWO sets.. now multiplied by 2 transmissions, and you can easily lose 30% of your power (and thus fuel economy)
Lastly, just because your engine isn't turning as fast does not guarantee it will be more fuel efficient.. typically, the more torque the engine needs to produce, the more fuel it will need per volume of air, reducing it's fuel efficiency, and higher gears will reduce your engine speed, but increase the load on it, and it won't accelerate as fast either.
TANSTAAFL.. There ain't no such thing as a free lunch
If you have 20 gears, you would have to "shorten" each gear (ratio) to "match" the gear it came from and the gear it is going in.
So you go through "smaller gears" faster (more gears).
