Gears And Fuel Consumption

[rockyracoon]

I went on a speed awareness course yesterday and one of the people running it said that a good way to stop yourself going over 30 was to stay in third gear and not go up to forth, one of my fellow speeders said this would cost more in fuel, chap said no as newer cars were made different. Who's correct.

Thanks for any replies.

[Baldric]

The engine would be revving higher in 3rd than it would be in 4th so I suspect your fellow speeder was correct.

[rockyracoon]

Mmm, this is what I think as well, perhaps he was just giving us a load of waffle so we don't speed again.

[rockyracoon]

Just to add, I tried it on the way home and it was flipping hard not to go into 4th.

[Graham-W]

As Baldric says, your engine would be at higher revs but not working as hard, therefore using less fuel.
My car doesn't like 30mph in fifth so I would use 4th. I think third would be on the low side for long distances at 30mph.
As with any fuel calculations, it depends on the circumstances.

[Graham-W]

There's a graph half way down the page comparing fuel use in 3rd against 4th at different speeds. 4th wins all the way down.
http://www.motoiq.com/MagazineArticles/ID/2277/Ask-Sarah-MPH-vs-MPG.aspx

[rockyracoon]

Thanks for that Graham, I wish I'd had that chart handy yesterday.

[woofgang]

if you need to stay in third when the engine is screaming, so as not to speed, should you be in the road at all? If the engine isn't screaming there's no problem staying in third.

[Old_Geezer]

Gut feeling is the faster you rev to hit a particular speed the more fuel you use. If such was not the case for newer cars how come we get more forward gears as time goes by ? I have six on my present car and use the top one to increase mph.

[rockyracoon]

Me too OG, 6 gears and use them quite often especially when on a good run, the car is very economical then, around town it's hell so for me to stay in third is a big fat no no.

[heathfield]

Which would you prefer? To not go over 30, or to save fuel?

[rockyracoon]

Hmmm, toughie that Heathfield! I suppose what I'll have to do is change gear and not go over 30 :)

[heathfield]

My dad had an early Ford V8. It had only three gears, and he only ever had to engage first gear to pull away when facing up-hill.

[SlackAlice]

Third gear would use more fuel than 4th and increase wear and tear on the engine gearbox.

Why not use cruise control to set a speed in 4th gear?

[sapelesam]

You only have to look at the rev counter, if the car is running at higher , which it will be, it has to use more fuel. Change up and keep an eye on your speedometer. This applies to all manual cars regardless of age.
My qualification, PHD in automotive engineering.

[ToraToraTora]

another illustration, if it where needed, of what complete brainless eejits run these courses! You couldn't make it up!

It will use more fuel!

If you want to keep to the speed limit I recomend a speedometer, they're terrific!

"As Baldric says, your engine would be at higher revs but not working as hard, therefore using less fuel." - PMSL!

[ToraToraTora]

"My qualification, PHD in automotive engineering."
yes sapelesam, you posted before I finished typing, correct of course!

[BRIGHT SPARK]

So if you have a DCT gearbox you will need to select race mode?

[Hopkirk]

Different scenario, but I have tried to see if differing driving affects fuel consumption.

I cruise at high speed most of the way to work.

I have tried gently driving at 50 to 60 for a whole tank.
After filling I drove my next journeys at a steady 70 until time to refill.

Result? No appreciable difference in fuel consumtion.

[chas2008]

the quicker you get into a higher gear the better unless the vehicle is struggling, but a competent driver would feel this through the vehicle.
Hopkirk,
i would disagree with your findings, up until a few months ago i drove a diesel Peugeot 207, sitting on the M1 at a steady 70mph (after filling up) it showed 600+ miles left in the tank, hit the road works where it went down to 50 and it showed 800+ in the tank, at one point i dropped to a steady 45 that showed 900+ miles in the tank....

[Teddio]

In diesel cars with manual transmission it is better to stay in 3rd gear in a 30 mph zone. Acceleration is better (eg. 20 to 30 mph), the engine will labour less on gradients and engine braking is better.
It is too simplistic to say that fuel consumption increases with engine rpm.