lets say you have a 5 speed transmission in your car. Could you replace that with lets say a 7 speed transmission, to get better gas millage and performance?
For an increase in mpg you will need to know the transmission final drive ratio. Probably both the 5 speed and 7 speed will end up with about 69:1 in high gear so you won't gain anything on mpg.
Making a change like that would be complicated as those types of things do not just bolt up anymore and even transmission have computer management systems these days. More gears generally allow for more opportunities for a given car to always be in exactly the right gear for the right purpose at all times, so yes, in general, everything else being the same, your car will get better mileage with a transmission with more gears.
in a car In a passenger car, NO! Passenger vehicles and even light duty pickup trucks (1/2 ton) will gain maximum 10% mpg compared to automatics, or even fewer geared manuals. Heavy duty and big rig trucks is a different story. They require multi gears to keep the RPM's down, which reduces fuel consumption. If there were a 7 speed manual transmission for passenger cars and even light duty pickups, you would still have to rev up the engine from idle to about 3,000 RPM's prior to shifting up, so the fuel savings would be minute; probably 1% at best. You would also be adjusting and replacing the clutch more often. Taking in consideration our speed limits and overall vehicle usage, a four-five speed is just about right. For those that argue that a five speed manual is better, I would also remind the difficulty of maintaining speed around town due to engine RPM's being too low at 35 mph in fourth gear and too high in third gear. The same goes for 40 mph, 45 mph and 55 mph. In my opinion, five speed manuals are only beneficial above 60 mph (like an overdrive in automatics). I prefer four speed manuals for city traffic. Just my $.02
