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HDi fun · 14-09-2007, 10:07 AM

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.

14-09-2007, 10:07 AM	#2 (permalink)
HDi fun Moderator The Torque Meister Join Date: Apr 2006 Location: SOUTH EAST Posts: 1,384	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.