Driving slower to reduce pollution

[Richie Stan]

Driving slower, means you pollute longer.

What is the optimum speed for most cars (maximum distance, minimum pollution)?

[Toureman]

How can your first statement be correct as you will use/burn less petrol by driving slower?

[Richie Stan]

yes but to get to your destination it will take longer.

The goverment are thinking of reducing the speed limit to reduce pollution.

[bimbim]

42

[Buenchico]

Petrol engines vary between different makes but they're generally designed to work most efficiently at a 'moderate cruising speed'. This is typically around 50mph.

Diesel engines work with consistent efficiency across the range of speeds for which they're manufactured. i.e. in a diesel-engined vehicle, it doesn't matter what speed you drive at, the 'mpg' will remain almost constant.

Chris

[Catso]

The optimum speed for any vehicle depends upon many factors: the aerodynamics, engine efficiency, engine pwoer curves, transmission, load in vehicle, road surface, road incline... The list is endless. But, principally, aerodynamics and engine performance are the 2 biggest factors

The best speed is one that produces the best compromise of all the factors but simplifying all the variable, usually where the engine is producing the best torque (usually midrange revs). This is why rev counters often have a green segment in the midrange. It's the most economical engine speed. Which in turn dictates the vehicle speed.

Buenchico, I just don't believe that. I used to drive HGVs, and the user guides always - strongly - emphasise that you should drive 'in the green' for maximum economy. I know my own diesel car uses far more diesel if I thrash it.

[Buenchico]

My answer doesn't really differ that much from Catso's.

My reply said "Diesel engines work with consistent efficiency across the range of speeds for which they're manufactured". My old Citroen BX diesel (1700cc) would give me around 56mpg whether I was driving it around town at 20 to 30mph, driving along East Anglia's rural roads at around 50mph or on a long motorway run at 70mph. Those speeds were all within the 'design range' of the engine. I wouldn't have expected to get the same figure if I 'put my foot down' and drove at over 90mph because such speeds are outside of the design range.

That seems to be consistent with Catso's point about not 'thrashing' an engine. Obviously, fuel efficiency will decline with poor driving technique but speed alone (within design limits) shouldn't have any significant impact on the fuel economy of a diesel engine. (The rev counter on a diesel engined car never needs to go much above 2000rpm. If it's allowed to spend any great time above that figure, it's obviously going to waste fuel).

Actually, the only times when my fuel consumption figures varied much from 56mpg was when I was on very long motorway journeys at a steady speed of 70mph. These tended to be in warmer weather (when diesel fuel compresses more easily) and involved very few gear changes (meaning fewer high-rev situations). On those journeys the engine obviously ran with greater efficiency because I nearly always achieved 64mpg.

Chris

[Spartacus84]

I believe most cars are at their most economic at 56 mph.

At least thats what it used to be.