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Is a 6 speed manual transmission really that much better than a 5 speed or 4 speed gearbox? If my car didn’t come standard with a 6 speed gearbox, is it going to be worth doing a trans swap?
Before we jump into the pro’s and cons. Let ‘s have a quick look at what a 6 speed gearbox actually is. As the name suggests, a 6 speed has 6 forward gears and one reverse gear (often with a reverse lockout). The first 3 gears are normally underdrive gears and multiply torque from the engine (more on this later). 4th gear is normally a 1:1 ratio, i.e. if the engine is doing 6,000rpm, the tailshaft will also be doing 6,000 rpm. 5th gear and 6th gear are normally overdrive gears, these gears reduce the rpm that the driveshaft spins at. For example, if 5th speed is a 0.25 overdrive, changing from 4th gear to 5th gear will drop engine rpm for 3,000 revs to 2,250rpm.
A brief overview of the benefits of a 6 speed manual transmission, include advantages like a massive increase in acceleration, more power when you need it regardless of road speed, quieter and more relaxed highway cruising, reduced engine wear and vastly improved fuel economy.
Indeed the performance benefits can be huge, but just replacing a 5 speed with a 6 speed trans won’t get you there. You will also need to change your diff ratio, to gear set with a numerically higher ratio.
For example, if you have a 5 speed car with a diff ratio of 3.25:1, changing to a 4:1 ratio will increase your acceleration by around 20% and will effectively be just like adding 20% more horsepower. This is due to increased torque, as the engine now has 20% more leverage to spin the driven wheels and accelerate the car forwards. Sounds awesome, doesn’t it! But the disadvantage, if you stick with the 5 speed is that you will be that you will be pulling 20% more RPM in 5th gear on the highway, which will increase engine noise, speed up engine wear and significantly reduce your fuel economy. But if you change to a 6 speed, you can drop into 6th on the highway and reduce your rpm, therefore getting all of the benefits of the lower diff ratio, while still having a massive boost in acceleration.
This combination of a lower diff ratio and a 6 speed trans, gives you a close ratio transmission. As the driver you will have more ratios to choose from at any given speed. Meaning it will be much easier to keep your motor in its power band ( the RPM that your motor makes most power), leaving you with more power to accelerate either out of a corner or just outright acceleration.
Of course it’s not all upside and there are some negatives in swapping to a 6 speed trans if your car isn’t 6 speed from factory. Popular 6 speed swap transmissions such as the T56, TR6060, CD009, Aisin, Getrag or Tremec don’t come cheap, and there are often other costs involved like a conversion bellhousing, clutch, flywheel and slave cylinder, not to mention the change in diff ratio to get the most out of your 6 speed. Plus, of course you will want to run a quality short shifter from CUBE Speed to get the ultimate 6 speed experience. There are also other considerations, such as the increased weight and physical size.
Overall there are massive benefits to be had from running a 6 speed stick shift. But in the modified car world the majority of us are trying our best to stick to some sort of budget and we have to decide what it is we want from our favourite ride and the best mods to get us there….wheels, turbo upgrade, coilovers, forged engine….the list goes on.