Safe Speed Forums

I was watching Avon and Somerset's "Safety" camera Partnership's latest TV advert which shows a driver cruising gently along, while a voiceover exhorts us to drive slowly and smoothly etc., then he drives past the scene of an accident, then you realise he's driving a hearse. All objective, dispassionate stuff...

One point it makes is that you should accelerate gently to save fuel.

Now I'm not disputing that at a given instance you will use more fuel at that instance if you accelerate harder.

But if you were able to accelerate to a terminal speed of (say) 60 for example then maintain that speed for a constant (say) ten miles, is it more efficient to accelerate more smoothly but take longer about it, or accelerate rapidly up to your terminal speed, and thus reach a constant speed sooner?

Has anyone ever bothered finding out if it's true? Or is it another of these "common sense" things, like 50 being safer and more efficient than 60, that isn't necessarily true?

It's almost certainly the case that you could accelerate too slowly to achieve maximum efficiency (i.e. spend longer than necessary in lower gears), so by definition there must be an optimum value.

I suppose in theory you could plot a graph of terminal speed and journey length (or duration) which would show what was the most efficient acceleration (?), though this would vary from car to car.

All I have ever read on the subject suggests that the most economical way is to accelerate to your intended cruising speed as quickly as possible.

Due to the high frictions of typical engines (when compared to all the other resistances together), they are most efficient when outputting near full torque (at midrangeish revs, not labouring at low revs). It makes economical sense to give it some beans, but not in the typical blow-up-your-engine boy racer style. There is of course a trade off with the additional drag, but seeing as the most efficient driving speed is usually around 50mph so the increased drag isn’t an issue.

Of course it won’t be very good if you’re hoofing it then immediately dissipating it all in your brake disks (and you’ll look like a \/\/4|\|£3r) – it’s all about planning; it’s even better if you can plan when to burn and coast (never with engine off). Sometimes I coast between gears when changing up through them when I’m caught behind slower accelerating traffic (well I have to engage the clutch anyway).

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

My on-board display reads 999.9mpg on the overrun and only 498 when coasting. I presume that the management cuts all fuel as opposed to a tickover feed?

Yes, that's exactly what happens. My car does it too. In fact, I've often wondered what's better - coasting and having the engine eat however much fuel it eats at tickover, or leaving it in the highest gear and using the over-run fuel cut-off. With the former, I get further, with the latter I use less fuel over a shorter distance

With regard to the original question, I'm nowhere near bright enough to work it out - cars vary too much and it's jsut too complex for me, but my gut feeling is that it doesn't matter. The "area under the curve" would be very similar if accelerating gently for a long time or harder for a short time.

THAT SAID, at least on a modern, normally aspirated engine, I would be inclined to keep the revs below (say) about 3000 so that it stays in its "closed loop" mixture control mode. I think most engines tend to run a bit richer outside that area. Also, a wide open throttle sometimes gives extra enrichment so I'd stay away from that too - even below 3000. On the other hand, for petrol engines, low revs and relatively large throtle openings promote good cylinder filling and thus volumetric efficiency!

I drive a 2 litre turbo diesel.

The most efficient engine RPM (regardless of gear) is around 2,000. This is just at the point when the turbo is starting to work so it can force more air in, hence more oxygen, and get more "bang per buck" for a given squirt of fuel.

In 6th gear this equates to 70mph.

So if I'm coming onto a motorway I'll generally do one of two things:

On a downhill sliproad I'll give it full power in 3rd, rev to around 4,200rpm (redline is at 4,300) then shift straight to 6th. This will then drop me right to 2,000rpm in 6th gear.

On an uphill sliproad I'll do the same but go to 4th, holding 4th until I've joined the carriageway so I've got a bit of acceleration available if needed. Once I'm into lane 1 I block-change straight to 6th.

Much less than 70mph in 6th tends to labour the engine...

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it is my role to agree with mole.. so i shall
a heck of alot of variables to consider here, which is always going to come down to a specific engine's power/efficiency curves against engine load/rpm etc.
if you had the data you could make some assumptions run some simulations, but the real world is unlikely to reflect the assumptions all the time.




hmmm thats a bit of an assumption too..... if the amount of fuel stayed constant when the turbo kicked in you would either now be running lean or have been running rich previously. it seems unlikely that any modern engine management (especially one protecting a CAT) would allow that.



[there's another aside here which nags at the back of my mind when crombie mentions 'hypermiling' (besides the atrocious terminology) and may apply if we decide pulling away slowly in a high gear is best..... which is being in a gear which allows you to respond to changing conditions... either by accelerating a bit harder or taking advantage of engine braking.... being in too high a gear compromises both options]

Ah! I can feel a bout of mutual back-slapping coming on here! I must agree with Ed about the safety issues regarding being in a high gear. In fact, a colleague of mine, some years ago was lucky enough to be sent on an advanced driving course with the famous John Lyons. He made exactly the same point to my colleague - you can make most compromises, but maximising economy AND safety are mutually exclusive (for exactly the reason you point out)!

a classic i see is on the infamous cov ring road where the merge on/offs can be short and present conflicting traffic at the last minute.
the limit is 40 which is comfortable in 4th but you often find 3rd a better option at the merges as it usually only takes a bit of throttle waggling either way to cooperate.... what usually happens however is one or both parties find themselves either unable to accelerate effectively and hence braking / losing time & concentration to change gear causing all sorts of turbulence to the following traffic!

PS fuel economic sounds like bad grammar to be.... fuel efficient sounds better !

last summer, you know when Golden Brown went cap in hand to Opec to ask them to reduce the price of oil, but for got to reduce duty on fuel, the price of unleaded became too expensive for me. It made me adjust my driving stlyee somewhat. Instead of leaded footed Grand Prix traffic light shoot outs, I adopted a type of driving more fitting with the grandadmobile I drive.

Out went the big heavy boots, in came the slippers. I didn't let the engine labour, but as soon as the engine felt like it could take an up shift, I shifted up and just rolled along. The reasult being 45.54mph around town doing sub 6 mile journies with the AC on. Which is OK for a 10 year old petrol. My friend calibrates gasoline engines for a leading motor manufacturer and drives the same, and if anyone should know how to get max MPG from a car it is him.

Think about how a blast furnace works. By forcing oxygen into it you get more heat/power from a given amount of fuel. That's how a turbo works on a diesel engine. The TDI engine also injects diesel directly into the cylinders rather than into the inlet manifold like on most common-rail designs. (At least this is my understanding, of course my sources may not be accurate!)

On a turbo petrol it's slightly different, it forces more fuel/air mixture into the cylinder so you can get the same "clout" from a 1.8 engine with a turbo as you could from say a 2.3 litre without.

The 2,000rpm figure is based on driving the same car for 75,000 miles and doing various experiments with instant MPG readouts.

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Magistrates rule #1: "Never let justice get in the way of a conviction."

ok then.



thought about it.


but its nothing like how either a petrol or a diesel engine works !

Only if you want to run at dangerously lean mixtures and risk overheat. I think Ed is right, for petrol engines anyway; the mixture is corrected with use of an air mass flow sensor.

I'm not sure how this works with Diesels, does the fuel/engine better tolerate the very lean mixtures at the low throttles? (AIUI, the air isn't controlled like petrol engines otherwise the lack of compression won't ignite the mixture).

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

can I suggest IAM advice of ages ago.


Please look at their website. We can have fun . without compromising our budgets adversely

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This isn't really my "patch" but I think diesels can run at 50 or 60 : 1 air : fuel ratios without doing themselves any harm. Traditionally, that's why diesels don't need a throttle plate - they just take however much air they need and as little fuel as they can get away with for the given load-speed combination. If you starve them further of fuel, they just don't produce any power but they don't damage themselves. Petrol engines are much fussier and like to run at abut 15:1 air : fuel ratio. With no load (or light load) they can get up to (about) 17 : 1 but much leaner than that and they start pinking and eventually destroy themselves. The lack of a throttle butterfly on the diesel means absolutely brilliant volumetric efficiency (i.e the cylinders fill very well each time the piston moves down the bore on the inlet stroke). This is further helped by the fact that they rev lower, allowing more time to fill the cylinders. A turbo helps the process even further.

On a petrol engine, at low loads, the throttle plate restricts the amount of air entering the cylinders to keep the ratio between it and the fuel within sensible limits. This reduces volumetric efficiency because the cylinders can't fill properly. I consider the turbo on a petrol engine to be the work of the Devil as they need to run lower compression ratios, which means that their off-boost volumetric efficiency is further compromised at light throttle openings. By changing up early and using larger throttle openings (but not labouring the engine) one keeps the revs down (to promote good cylinder filling) AND uses larger throttle openings so as not to restrict the airflow into the engine - thereby getting as close to the diesel as a petrol engine will allow.

This is hard for me to swallow, as I happen to like the sound and feel of a nice (normally aspirated!) petrol engine as it comes "on cam" , but it's a sad fact of life that diesels are the way forward!

Take a look at this engine's power and torque curves, for example.

Despite it's torque peaking at 4500Rpm (screw ethanol), that obviously isn't the most fuel efficient RpM to reference against any cruise control target. Do note, however, the ever-so-slightly lesser peak, around 2500 RpM.

If I didn't know anything else, I'd strive to set the cruise control so that this particular engine stayed at or under 2500 RpM.

Since I'd prefer to know more, I'd ask a GM technician, or someone familiar with the stock PCM settings, "at what RpM does the EGR Valve stop working?"
Keep RpMs under the lesser of the two.

While I have the tech's ear, I'll also ask, when coasting down, "at what engine RpM does DFCO engage?", and "at what RpM does it disengage to try to keep the engine from stalling?
I'd try to keep the engine in that RpM window.

When a car is equipped with a lean burn catalyst, the PCM will permit Air:Fuel ratios approaching 30:1 from time to time, provided the throttle is managed deftly, and NO acceleration whatsoever is called for; ANY additional throttle opening resets the target A:F ratio @ 14.7:1.

Generally, the heavier the acceleration/load, and the higher the RpMs, the richer the A:F ratio needed. (My old Caprice will cruise at 16:1 under 60MpH in 4th Gear, but before I had it reprogrammed, Wide Open Throttle Power Enrichment would cause the A:F to approach 10:1. With the reprogramming. WOT PE sets a target A:F of about 12.6:1.)Also, in reality, the more fuel combusted, the more energy wasted as heat.

Finally, when accelerating, I'd use 1st gear to get out of the blocks, then use the highest gear reasonably possible to keep the engine RpMs within the above windows. This may involve skipping a gear or two [-or three, in the case of nearly any GM engine attached to a Getrag Six Speed Gearbox].

If I had to guess without any of the above info, assuming a four cylinder engine like the one above, I'd stay between 1000 & 2500 RpM, whether accelerating, cruising, or decelerating. (Hey, that was a decent guess, no?)

I'd wager that Wayne Gerdes, or other hypermilers, strive to stay under 2000 RpM whenever reasonably possible, but I simply don't have that kind of time.

(Note that if your engine is equipped with both a manual gearbox AND a wireless throttle, the accelerator pedal doesn't decide how far the throttle should open by itself; the PCM makes this decision against a matrix table referencing Throttle Position, selected gear, and present speed.
In other words, the PCM will not allow the throttle to open or close too quickly.
Then again, if your accelerator IS connected to the throttle, or the PCM doesn't mind a huge delta throttle angle, you can try the Throttle Filter suggested in this article.)

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The old Audi 80 Avant TDi I had used to get about 60mpg - that was old fashioned B4 VAG TDi - quality engineering with no BS which had lower CO2 emissions (probably abnormally high particulates though) than most comparable diesels of today!

I remember you !

Aren't you the same person who talked utter rubbish about how nimble and efficient his car was?

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

That's pretty much the size of it. Most things that "improve" environmental performance in one way, make it worse in another.

I've had a variety of Peugeot diesels (identical vehicle) with and without particulate filters. The ones with particulate filters are always a bit thirstier (and thus kick out a bit more CO2) than the ones without. Similarly, your old Audi was probably lighter than the current equivalent but wouldn't have got as many EuroNCAP stars. It's amazing how much weight cars have put on in the last 50 years!

You pays your money and you takes your choice!

Older diesels assumed as much air as required would be available, the throttle controlled only the fuel supply.

Modern diesels are much more "managed".