Posted: Fri Dec 07, 2007 10:42 am Post subject: Any point in having 6th gear
My mate drove the new diesel zafira some 520 miles and found that 6th gear gave less fuel economy (about 10-15% less than 5th) according to the trip computer in all cruising conditions from 70mph to 100mph.
its all very well being a tall gear (33mph/1000rpm) but that gives no oomph, worse economy according to fuel computer compared to 5th (and 4th at 70) and any gains in quietness are pointless becuase it's diesel anyway, especially if you're paying a £1 premium for every 80 miles travelled. Economy pretty crap anyway - 38mpg for a diesel is only 12mpg better than that VXR8 6.0 petrol!
Does anyone else find 6th gear a pointless addition to their car?
Joined: 08 Nov 2004 Posts: 1178 Location: End of the World
Posted: Tue Dec 11, 2007 1:55 am Post subject:
Well, if you are going 70-100mph, of course you are going to have shitty fuel economy. You tend to have shitty economy in anything going over 60mph. 6th gear is useful if you know your shift points to manage your fuel economy better. It is also useful in racer-type cars such as the Impreza WRX-STi and the Lancer Evolution. In those cars, the 6th gear allows the power band to be wider in each gear. Since they are turbo charged, they aren't the best on gas, having the wider power band apread across 6 gears helps a lot with fuel economy.
My Lotus Elise has a 6 speed. The performance powerband of the engine is between 5500 and 8500 RPM. At about 6200 RPM the cam profile shifts to a steeper angle, so above 6200 RPM the performance and acceleration are awesome.
When maximizing acceleration, any upshift keeps the engine RPM in the performance powerband if you get close to the redline. So, shifting from 3rd to 4th at 8500 RPM will drop right in at a little above 6700. When maximising acceleration, it is easy to keep the engine in this powerband.
When just driving around town, upshifting at around 4000 to 5000 RPM will keep the engine RPM above the 3000 RPM minimum. Driving below 3000 RPM doesn't work well.
The transmission seems to be designed quite well. When the steeper cam profile engages, it puts more stress on the drivetrain, so you would not want to drive it above 6200 RPM all the time. Also, with the steeper cam profile engaged, the fuel economy basically sucks. The way the gearing is designed, it is allowing both performance driving and city driving within the RPM range that is appropriate.
The 6th gear doesn't have much use below 150 KPH. 6th gear is a little quieter than 5th at higher speeds. Both 5th and 6th will probably take the car to 245 KPH, in 6th it's not running too close to the redline though. At the higher speeds, however, 6th gear often keeps the engine below the RPM of the cam profile change which occurs 6200 RPM. The steeper cam profile uses a lot more gas, so, in cases where there is a choice between driving above or below 6200 RPM, driving below 6200 is definately more economical. So, the 6th gear is actually quite valuable in the right circumstances.
Snip
It is also useful in racer-type cars such as the Impreza WRX-STi and the Lancer Evolution. In those cars, the 6th gear allows the power band to be wider in each gear. Since they are turbo charged, they aren't the best on gas, having the wider power band apread across 6 gears helps a lot with fuel economy.
The power band is a characteristic of the engine and has nothing to do with the gearbox.
Ideally the gear ratios need to be chosen to match the power and ecconomy characteristics of the engine. Changing gear takes time so having too many gears can actually reduce an acceleration time. This has to be balanced by keeping the engine as close to maximum power as possible. That gives the best performance. A taller top gear can then give better fuel ecconomy.
However, this is all rather theoretical as manufacturers tend to make a limited number of gearboxes and simply fit one that's available regardless of the ratios. That keeps costs down.
There's also the advertising benefit. The public think that more gears are better so manufacturers fit them whether they're needed or not. It looks good in the advertising blurb though.
When I chose my car, I looked at the gear ratios (amongst other things). They are equally spaced from 2nd to 5th so the revs drop in each gear is the same. That matches the power chracteristics equally in each gear. The ratios also give useful speeds in each gear. Hitting the rev limiter while overtaking is most annoying as well as slowing the process.
Joined: 02 Feb 2008 Posts: 861 Location: Worcestershire UK
Posted: Thu Mar 06, 2008 9:10 pm Post subject:
It depends upon the car and the driver. I find my 6th gear economical for 70mph cruising and can drop to 3rd or 4th to accelerate if required. It's worth taking note of the economy display to choose the best gear for a steady speed and the rev counter for rapid acceleration.
Different trim levels of the same car (with identical engine) can have different gear ratios, so that one (say a sporty version) has a better 30 to 50 time in 4th gear, but you pay with poorer economy than another less sporty trim level of the same car (and same engine) with a higher 4th gear.
Joined: 18 Jun 2008 Posts: 411 Location: The Sprawl
Posted: Wed Jul 02, 2008 5:23 pm Post subject:
People are actually arguing about this? lol
Exodus pretty much nailed it. The point to a 6th gear is the same as the point to a 5th, or a 4th, or a 3rd. You know, there was a time cars had only 2 speeds, right?
More gears means you can either push the car faster (whithout needing a push to get started, like a lot of the salt racers do in Bonneville), or accelerate quicker, or get better economy, or some combination of the above.
My car comes as either a 6 speed maunal or a 4 speed auto. The autos get worse economy because they are turning a much higher rpm on the highway. This is why BMW and M-B are working on 7 and 8 speed autos, and even GM has 6 speed autos being more common now.
Of course, if the gearing is poorly matched to the engine and car, it's going to be a problem. And that sounds like the original posters case. Engines do have gph sweet spots, and lower isn't *always* better, so if that, plus drag, plus power band are such that that car does better at a higher rpm, then in this case, it sounds like your 6th (or final drive) are simply too low. Change the diff, or the wheel/tire combo's rolling diameter, or mod the engine, and you'll see things change.
Joined: 02 Feb 2008 Posts: 861 Location: Worcestershire UK
Posted: Wed Jul 02, 2008 6:11 pm Post subject:
Usquanigo wrote:
.......................... Engines do have gph sweet spots, and lower isn't *always* better, so if that, plus drag, plus power band are such that that car does better at a higher rpm................
Yes, I have observed that with my Vauxhall Vectra Estate VXR, which is 6 speed. I enjoy driving, so I don't mind a bit of gear changing and hopefully get the best compromise between economy and performance.
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