Disappointing fuel economy plus fuel tank size

Strider590

vikingaero wrote: »

High speed motoring burns fuel. If you drive at 50mph you'll use roughly 15% less fuel than travelling at 70mph. 60mph uses about 10% less.

The most economical driving is often on clear A-roads following the bumbling 40mph drivers.

40 mph is uneconomical for most cars......

It's slightly too fast for 3rd gear and slightly too slow for 4th gear, so your either revving too high or labouring the engine.... Either are bad for fuel economy.

The most economical is to drive in at a speed and in a gear that runs the engine at the point where it's just "on cam" (peak torque = 2.5k - 3k rpm in most normal cars). It stands to reason that if the engine is at it's point of peak torque/power that it's burning fuel at maximum efficiency.

cyclonebri1

Strider590 wrote: »

40 mph is uneconomical for most cars......

It's slightly too fast for 3rd gear and slightly too slow for 4th gear, so your either revving too high or labouring the engine.... Either are bad for fuel economy.

The most economical is to drive in at a speed and in a gear that runs the engine at the point where it's just "on cam" (peak torque = 2.5k - 3k rpm in most normal cars). It stands to reason that if the engine is at it's point of peak torque/power that it's burning fuel at maximum efficiency.

Totally wrong :T


40mph? proably the most economical speed to travel at, practical? oh no, try it.

quote,
"the most economical is to drive in at a speed and in a gear that runs the engine at the point where it's just "on cam" (peak torque = 2.5k - 3k rpm in most normal cars). It stands to reason that if the engine is at it's point of peak torque/power that it's burning fuel at maximum efficiency"

True for performance but alas not so for economy. Try driving at 5800 rpm all day and see what mpg you get


Too fast for ? too fast for ?? car dependant and irrelevant here

LandyAndy

Strider590 wrote: »

40 mph is uneconomical for most cars......

It's slightly too fast for 3rd gear and slightly too slow for 4th gear, so your either revving too high or labouring the engine.... Either are bad for fuel economy.

The most economical is to drive in at a speed and in a gear that runs the engine at the point where it's just "on cam" (peak torque = 2.5k - 3k rpm in most normal cars). It stands to reason that if the engine is at it's point of peak torque/power that it's burning fuel at maximum efficiency.

Driving at peak torque should be most economical which is why my Mazda6 gives better indicated mpg in 5th at 40 mph than in 6th.

It's a long time since cars had 4 gears.;)

The best aids to fuel consumption?

Correct tyre pressures.
Anticipation and reading the road to avoid braking.
Setting off slightly earlier and driving at say 60mph rather than 70 mph (you'll be surprised how little extra time it takes (if any) and how much fresher you are on long journeys).
Treating the accelerator as a tap on your bank account. The harder you push the further open the tap.:D

Strider590

cyclonebri1 wrote: »

True for performance but alas not so for economy. Try driving at 5800 rpm all day and see what mpg you get

5800 rpm would be revving the nuts off it.... While this may be peak power/BHP (also the point of maximum fuel+air mixture through engine), 5800rpm is a very long way off peak torque.

At peak torque the engine is operating at maximum efficiency.... As I said, this is typically 2.5k - 3k rpm.

Indout96

At peak torque the engine is operating at maximum efficiency.... As I said, this is typically 2.5k - 3k rpm

Which petrol cat is at peak torque at 2.5k !!!!!! Toyota's VVTLI engines hit peak torque at around 8k
Even my Tigra is not there until 4.6k

Strider590

That's why different cars are more efficient in different gear/speed combinations.

My old 306 2L for example, that hit peak torque at something like 3.5k rpm... It was uncomfortable in any gear at 40mph, but bring on 45/50 and it was fine again..... Overtaking average speed drivers was a complete pain in the !!!!, as you can imagine.

While my Vectra doesn't like 40, it's better than the 306 was.... The Vec however is most efficient at motorway cruising speeds.

smitchy73

Thor,
As others have pointed out, you need to fill your tank, then do your journey taking a note of the mileage, either by a simple start and finish and the difference, or reset your trip computer if you're car has one. Then when you're ready fill you're tank again to the brim, and work out how many gallons you have filled with, divided by the miles and that will give you you're mpg.
My car until I started working out myself was telling me about 51/52 mpg, but when I work it out it is actually 48mpg, still not too bad for a 2.0l diesel.

skiddlydiddly

Well if you can drive at say 3k rpm in 4th gear doing 40 mph but can just as easily drive at 2k rpm in 6th gear doing 40mph without labouring the engine and using the same throttle, which do you think is going to use the most fuel?

Just because peak torque is at a certain point, it doesn't mean you actually need to be using that many revs to maintain your speed without wearing the engine.

sebdangerfield

Surely this is purely subjective? Where peak torque is in one car will be totally different to the next, as will be the gearing so there really is little point in discussing rpm or speed unless were talking about one car.

I agree with cyclone. Why would peak torque produce the best fuel consumption? The fact it's the "peak" surely proves that it's not the most economical.

The best economical driving is done by the best driver with the best anticipation. Purely sticking at a certain rpm or speed shows a poor driver not dynamically changing their style to adapt to conditions.

forgotmyname

The old saying of drive slower doesnt work on these modern cars. Raised gearing, different power & torque bands changed all that.

My mondeo diesel 6 speed, gets better mpg at 60mph than it does at 50. Not enough power at 50 so your actually pressing the accelerator further to maintain the same speed.
Depends where your peak torque figure is.

Best economy is around the cars peak torque.

Peak torque is sometimes most efficient as that is where the engine is at its strongest. Most pulling power.

Say your car needs 100bhp and 100ft/lbs of torque to maintain 60mph.

Slowing down will reduce the amount of torque you have so the car slows, You increase the throttle to compensate using more fuel.

Try a car with a trip computer and have a try, Its mad increasing speed and getting more MPG.