Gear ratios - why 6 gears if the gears are very short?

[pawel]

Hello.
I have a question that has been bugging me since I bought a 2011 Renault Fluence last year.
The car has a 2.0 litre Nissan petrol engine with 140 HP and 195 nm of torque. It feels really nippy and flexible, at least compared to my previous cars. Picks up easily even from low revs, the acceleration is there even on higher gears. The engine is also really quiet, barely audible.
And here's my "issue". The car has 6 gears (which I was hoping for when searching for a new car), so that's good, right? The problem is that the gears are short; in fact, the sixth gear has the exact same ratio as the fifth (and top) gear in my previous car, a Peugeot 308, which had less power, less torque, and generally felt a lot more sluggish, but still had no problems at all on the motorway. So why would Renault/Nissan decide to put such short gears in a car that could easily do motorway speeds at relatively low revs? As is, the engine does 2.5 thousand revs at 90 km/h (56 mph). Which means that at motorway speeds the revs are pretty high and the efficiency drops. It also means that I need to change gears more often while driving in the city. Which seems an overkill because the car seems to have enough power/torque to manage gears nearly twice as high. As is, I normally barely reach 2.500 revs before shifting up. Which might contribute a bit to mileage, but then all those gains are lost at higher speeds. At this point, it seems that the 6 gears mainly mean that I have to change gears more often while still not feeling more comfortable at motorway speeds. So what's the advantage of having 6 gears when they are short? Perhaps someone can explain the logic here, because I might be missing something.
Thank you.

[GiveMeABreak]

6 gears are really used for better fuel economy and to reduce noise. It's like an overdrive and means in top gear, at motorway speeds for example, the car can run at higher speeds with lower engine revs.

The highest gear is important, i.e. 6th gear as it determines what RPM the car 'cruises' at. Obviously different engines with different power and torque will determine whether you need to shift down to overtake at higher speeds or not, but essentially that's it. It reduces the engine noise and allows the car to maintain cruising speed at lower engine revs and therefore saves fuel.

[Paul-R]

The six speed gearbox in our 308 functions much as I think you're expecting it to. The fifth and sixth gears are both overdrives and fourth gear is as high as you can go in a 30mph limited area. Fifth can be used in a 40 limit and sixth really needs about 50 - 55mph. The new 80Km/h limits on ordinary roads in France cause me a problem now as I have to stay in fifth gear more often which I'm sure is detrimental to fuel economy.

If your car's sixth gear truly is the same as the top gear in a 5 speed gearbox (and the final drive ratio and tyre size are the same) then I also find it curious and a waste of an opportunity. A sixth gear is generally fitted in order to give a vehicle a better official fuel consumption figure.

Our car is a diesel and gear ratios on diesel cars are generally wider spaced than on petrol cars but even so I would have expected your car's sixth gear to be a bit higher.

[GiveMeABreak]

They won't be the same ratios for a 6 speed as a 5 speed guaranteed. But the ratios will vary according to the specific box, as there are often variants of the same box that have slightly different ratios depending on the engine fitted.

[pawel]

The individual gear ratios aren't the same, obviously, because the number of gears is different. But the top gears (5th and 6th respectively) have the same ratios in both cars. The gears are just "shorter" in the Renaulr. So the "overdrive" is not such a big overdrive. The fifth (which I can easily switch to at 40 mph) is just barely an "overgear" and the 6th is an overdrive that corresponds to most cars' 5th gear (actually, some cars' 5th gear is taller than the sixth in this car - I've watched a lot of POV videos). Really frustrating.

They won't be the same ratios for a 6 speed as a 5 speed guaranteed. But the ratios will vary according to the specific box, as there are often variants of the same box that have slightly different ratios depending on the engine fitted.

[GiveMeABreak]

You are talking about 2 different vehicles - chalk & cheese and are not comparing like for like. If you compared the 5 speed gearbox from an earlier PSA car and then a 6 speed box on a later model, you'll pretty much find all the ratios are different when doing a comparison - that's the point. A 6-speed manual decreases the engine revs needed to change gears which means changing gears earlier and saves fuel.

You can't really compare different manufacturers' gearboxes for different cars, different engines, different Euro standards.

It's the same with the Automatics - the older 4 speed v the EAT 6 speed and now the EAT 8 speed - all contribute to smoother gear changes and gears are changed much sooner which saves fuel.

[pawel]

Still, driving on the motorway at 4000 RPMs in 6th gear sucks a bit, doesn't it? The engine is still relatively quiet, drowned by wind noise, but the mileage is not great.
But your point about decreased revs needed to change gears at least explains the intention behind this. They decided that motorway driving was not a priority (although they did put in cruise control with buttons on the steering wheel )
Anyway, maybe I'll just need to convert it to LPG.
Thanks for replying!

You are talking about 2 different vehicles - chalk & cheese and are not comparing like for like. If you compared the 5 speed gearbox from an earlier PSA car and then a 6 speed box on a later model, you'll pretty much find all the ratios are different when doing a comparison - that's the point. Decreased engine revs needed to change gears which means changing gears earlier and saves fuel.

You can't really compare different manufacturers' gearboxes for different cars, different engines, different Euro standards.