'eco' models like VW Bluemotion etc.

Why do they have 5 gears not 6?

I can only think it's so that, for any given speed, you end up choosing a higher gear than you would if you had more to choose from. Is that why?

Wouldn't an extra gear mean better economy once up to speed?

Its an absolute con? Stick an 'eco' badge on a focus with slightly slimmer width tyres or 'eco' plastic (harder) compound tyres?

If my Forester had a 6th gear it would cruise at c3k revs rather than closer to 4k revs on the motorway.

Exactly. I get the logic with higher gear ratios, but not why fewer of them is better.

Its not the number of gears that counts but what the gearing is.

Way back in when my dad had a some leyland car that was an " Economy" version. It only had 4 gears but the gearing was very high so that cruising on the motorway it was at low revs so very economical.

For economical cruising what you need is an overdrive top. Number of gears is more or less irrelevant

Not enough power to comfortably drive the wheels in such a high gearing I assume?

As above, it's the ratios, not the quantity.
5-speed boxes are also lighter and cheaper, plus a torquey engine doesn't NEED plenty of gears.

Good point, is there any info on the gearing out there? I used to get over the Gov's figures in my mkV 6 speed golf GT TDI. I dont know how but you could change up very early in the revs (lower labouring point?) so that would mean lower gearing? (Im guessing here!)

M_F beat me to it.

Also good point re weight saving…

Why don't the just fit them with 1st, 3rd and 5th? More weight savings and most modern cars have enough torque to cope anyway…

its gear spacing not gear quantity that matters.

you should find that any car, at the Nat Speed Limit of its country of origin, is in its most economical rev range.

i used to worry that my Diesel was at 2.1k on the motorway, when it pulls fine at lower revs, but 2.1k is precisely in the top torque range of the turbo band.

mines an "eco model" too, all that means for me is that the 2L TD engine is tuned to 90bhp, rather than 110bhp, like the rest of the range, but i get 55mpg, where as the rest of them are lucky to top 40.

what WOULD be useful on a diesel cars, is to tune it to Euro fuel standards, where as all production cars are tuned to burn 95% of Diesel availible, including crap yank diesel, and dodgey eastern european Heavy oils. (IIRC)

if you tune the car to the better stuff we have here, you can burn it more efficiently, but they would only work in the EU, and risk damaging the car if the fuel standards dropped too low…..

….as i understand it

which is what diesel "chips" do
very tempted to get one for mine, except 30% increase in power, mpg and torque is all very well, but the suspension isnt designed for it, and it would go through tyres much quicker too…

makes me chuckle, flowing some **** in a chavved up impreza, with the smell of unburnt fuel coming out his big box exhaust, because his "mate works for a tuning company innit"

But this one goes to 11!

haha
thats EXACTLY right

As already said….most of the smaller eco cars are little TDi engines & probably wouldn't have the power available at motorway speeds to warrant a 6th gear.

The new Seat ecomotive apparently has better aerodynamics, reduced weight, re-mapped ecu, longer gearing, thinner tyres than the normal version & Seat claim 76.3mpg.

A 'fuel ecomomy driving expert' earlier this year got 97.4 mpg out of it and managed 970 miles on one tank – 45.53 litres apparently!!
Autotrader tested it & even these 'normal drivers' managed 71mpg & 81mpg on an unfamiliar route!
Best I have out out of my 1.9TDi is 60 mpg. But that does have 50bhp more and I bet it's a damn sight quicker.

you should find that any car, at the Nat Speed Limit of its country of origin, is in its most economical rev range.

Due to its tuning, my celica does more mpg at 85 than at 70 lol. Peak torque at 3400rpm, which is nearly 90mph. The eco range from peugeot simply uses a longer 5th.

Back to the original point, If you have a "green" model you'll want to take a low power engine, pop it into a light body and add a gearbox that allows it to reach a relatively low max speed while placing extra-urban test rules squarly in the most efficient engine speed range. If you put extra gears in there, as said earlier there is extra weight but also you may shift the ideal test conditions (Speed) to an inefficient engine speed. Plus if you're shifting gears left right and centre you kill economy, so a small speed range and low power engine with lots of gears means lots of shifting (comparatively). Most of your 6 speed geared cars are the more powerful ones, as such the box will be meatier than required etc.

On a related note, I was reading somewhere that contrary to popular belief, with a turbo diesel it is better for fuel economy if you drive with the engine in the torque band, rather than keeping it off boost.

This is because diesels use more fuel the more the pedal is pressed, rather than related to higher engine revs, and with the extra torque provided with the engine on boost, you will only have to touch the pedal as opposed to mashing it.

God, that was a right mouthful, but there is a point in there somewhere!

as has been touched on, its not just a case of high gear = good otherwise all cars would have a mahoosive top gear and do 100mpg. its all about keeping the engine in its peak torque and tuning it to be most efficient at cruising speed.
this is probably why wee city cars are seldom as good as you'd expect in terms of long distance economy.

"Ride the Torque" yes 🙂

Makes sense now thanks