What is more economical.

[waynne]

I'm having an arguement that I can't settle.

Which is more economical?
1) Build up speed to a constant 56mph slowly over a 1 mile distance and hold that speed for 3 miles.

or 2) Accelerate to 56mph briskly (but not at full throttle) over say a 1/4 miles distance and hold that speed for 3.75 miles.

Assuming the car is most economical at 56? Journey time will be shorter in the second scenario but which would use less fuel overall? In scenario 1 the mpg in the car is about 25-30 mpg while accelerating. In scenario 2 the mpg will be 15-20 mpg while accelerating but the car will return 56 mpg at a constant 56 mph.

Show you workings - this is not meant to be a maths question though? Its a real pig to argue this one and I just wondered what you guys thought about it? (Especially our resident physicists!)

[HDi fun]

I think 2 would be better in general.

This 56mph thing is a bit of an old chestnut. Here's why:

Years ago (before cats and the widespread adoption of fuel injection) manufacturers would tune their engines to run on a lean fuel mixture at whatever engine speed corresponded to 56mph in the highest gear. This is because the 56mph mpg figure was a known benchmark.

This also led to very poor driving characteristics at around that engine speed (in any gear) giving rise to flat spots and hesitation.

Further, these constant 56mph tests were conducted in a laboratory on a rolling road. No-one got close in real world driving.

This is no longer possible with catalysts in the exhaust as the 3 way closed loop cat requires a reliable 14:1 air:fuel ratio by mass (not volume). Running it lean will cause overheating and damage.

Hence we now quote figures for urban, motorway and combined cycle. These are generally more realistic and can sometimes be beaten if you really nurse the car along. Problem is it takes all day to get anywhere and you drive following drivers round the twist.

[aswearing]

would it also depend on the engien size? 1.2 getting to 56 as quick as possible would rev much higher in more gears than a 2.5, which would rev halfway in few gears?

[HDi fun]

It depends on so many factors. Driving style is the biggest factor.

Years ago I had the misfortune to be lent a 1.2 litre Fiat Punto. My usual car was a Nissan Primera 2.0, which is a 16v 150bhp unit. It usually turned in about 30mpg, or quite a lot more on a run.

This Punto, 1.2 litre 55bhp managed 24mpg on a hard run to Bristol and back (200 mile round trip)!

Why? Because I whipped its arse trying to get the bloody thing to go.

So you have to drive to the car's ability too. I'm sure my father could wring 55mpg from it on the same trip but it would take days.

[pgarner]

goto admit with hdi

my old civic used to about 35mpg including alot of motorway driving lucky if i hit 40

in the bora over the last 900 odd miles ive driven her since pickin her up i got around the same usually bettering.
simple reason am not battering hell out of it for preformance

with most engines now it would proberly work out the same if you got the speed up quickly or if you take it up slowly with modern injection

[HDi fun]

There's little point in buying a super economy car with a tiny little engine if you're not a super economy minded driver. You're better off with something that suits most of your driving style most of the time.

[pgarner]

yeap look at fingers wots under the near lee 5.2 theres no way thats going to be economical but a whole lotta fun
no point in having all that engine then tryiing to get a good mpg

[HDi fun]

But I bet he better fuel economy than if he tried to drive a 1.0 12v corsa the same way

[fingers]

Originally Posted by pgarner

yeap look at fingers wots under the near lee 5.2 theres no way thats going to be economical but a whole lotta fun
no point in having all that engine then tryiing to get a good mpg

Actually, the Near Lee isn't as bad as you'd think. I average at 25mpg. Not bad when you conside that my Toyota SpaceCruiser 2.0 does 18mpg at best.

[aswearing]

i did drive a 1.0 12v corsa, and anfter 3 months cheekily moved on..

if you put you foot the to floor i swear it just got confused.