Safe Speed Forums

Aha, now I see what you mean. You are entirely right. If the next downhill is big and steep enough to warrant braking then that is indeed an efficient method. However, I had made the assumption (rightly or wrongly, it’s difficult to decide which) that one wouldn’t want to slow down wasting time and potentially frustrating following drivers. If one is that unconcerned about journey time then it may be more efficient to further cap the top speed (or reduce the average speed) of the journey.

For many, aside from the dangers (no power steering, brakes could fade), turning off the engine is not an option (my turbos would be ‘coked’ if I did that after a hill climb, heat soak could also be a problem), so those cars would have to remain idling. Slowing down in such a manner over the crest will result with the car spending more time at idle, thereby wasting fuel and going some way to undoing one’s efforts.

I guess this goes to show there isn’t a fixed method of best practice when dealing with hills.

I only pointed that up as an extreme example to illustrate the principle.

But in the real world, if you are specifically trying to minimise consumption then you can employ this technique to a greater or lesser degree, ie cresting hills at a lower speed than your average and conversely reaching a higher than average speed in the bottom of dips.

Either example uses some of the energy conservation theory put forward by Paul to reduce fuel consumption.

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CSCP Latin for beginners...
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Part of the reason for constant speed is the need to maintain relatively constant speed with the general flow of traffic, some of which tends to lose speed up hill and gain it down hill....
I try to maintain the engine revs over the point at which the turbo kicks in (approx 1800rpm on mine) and this, with a light throttle, seem to keep efficiency and economy high. The 56mpg was a brimful-to-brimful calculation, not a trip computer estimate.
I'm not sure about ride height, but most spoilers are more designed to increase down force than reduce drag and would therefore tend to worsen fuel economy. Body kits would be more likely to depend on function.

Lowering the car causes more of the airflow to be deflected over the car, causing low pressure under it and therefore downforce. But that air deflection causes drag.

The ideal from a wind resistance perspective would be a car with a smooth and aerodynamic top and bottom, with tall thin aerodynamic running gear to keep it well clear of the ground.

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CSCP Latin for beginners...
Ticketo ergo sum : I scam therefore I am!

You seem to be going the wrong way. I suggest that a lower ride height reduces effective frontal aera and underbody turbulence.

To give some context - Claire and I have owned quite a few BMW E30 325i s over the years. 350 miles out of a tank is about normal. Just one car has regularly returned over 400 miles per tank. It was one of Claire's and it had been lowered 2 inches by a previous owner (and yes it had a standard fuel tank!). We have always suspected that this gain was due to lowering.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I'm always keen to know how to get the best fuel economy out of a car and the technical discussion has been very interesting. However I think for the majority of motorists it's the simple things that would make the biggest difference, such as planning ahead and avoiding late breaking. It seems bizarre to me how some drivers continue to acclerate until the very last minute and then slam on when its clear the traffic in front is not going to move anywhere fast.

I'm currently getting 28mpg out of the Capri on the combined cycle - have had it as high as 34mpg on a run.

JT, I agree with everything you say except labouring a diesel! As far as I know, diesels don't have throttle butterflies so having the throttle fully open won't help with the pumping losses - I think this is one of the main reasons why diesels are so much more economical than petrol engines. Obviously, low revs do mean more complete cylinder filling but with a turbo diesel I agree that this should be pretty much 100% (or more!) once the turbo is spun up to speed. I seem to get better instantaneous consumption (at least according to the trip computer) at slightly higher revs than I would in a petrol engined car.

Paul, I have a lot of personal experience of identical vehicles with and wthout a rear spoiler. There's no question that the ones with the spoilers have worse fuel consumption - especially at higher speeds) and a lower top speed. Having sent plenty of drivers out on the same run in both, the ones with the spoilers always bring me higher fuel receipts! An easy test (if anyone wants to try it!) is a "coast-down" test. Get the vehicle on a flat level road at an accurate (GPS is fine) 70MPH, dip the clutch and time how long it takes to drop to (say) 50MPH. It doesn't take much more than a set of roof bars to make a very noticeable difference!

Citroen's C5 and C6 automatically reduce ground clearance over a set speed, one of the stated reasons is to improve economy. Does it work? I don't know. Hydraulic suspension makes this very easy.

Surely leaving the engine in gear downhill uses less fuel because the wheels drive the engine, so the emu thinks, okay, i can put even less fuel in, where if you take it out of gear it has to work to keep it idling... I'm probably wrong

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Mike

You're not wrong, however, in many instances that will end up being a false economy.

Minimise weight (get on that diet), minimise wind resistance, drive with minimum inputs into anything (steering, throttle, brakes), and minimum outputs (no radio, no lights, no A/C). Clarkson did a good demonstration on Top Gear in the London to Edinburgh to London on a single tank challenge.

The best way of considering the "hill question" is to forget cars for a second and think about roller coasters - the perfect example of conservation of energy. Other than the initial climb they are completely unpowered, but gain maximum momentum when going down, and convert momentum into elevation when going up at the cost of speed. If you drive like that with a light throttle (I would never disengage gears) then you get good mpg.

On the flat, most cars were optimised for maximum fuel efficiency at 55mph as that used to be the measure for official fuel economy and many still are, so top gear, 55 mph with a feather throttle and think like a roller coaster. Do not brake unless you really have to.

For those who are willing to compromise safety for fuel economy, then slip-streaming will dramatically reduce energy requirements by eliminating wind resistance - just ask any cyclist especially road racers: think Tour de France. Unfortunately if you are in a Car then it has another rather more despised name: tailgating. Drive 6' from the vehicle in front - especially if it is a big one, and it will almost suck you along.

Hell yes! Good example! Just imagine how much energy you would have to use and dump to make a roller coaster run at constant speed.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I save a significant amount of fuel compared with most other cars on the road simply by not driving with half a dozen england flags flapping out of the windows.