Coasting: in gear vs. neutral
 

Hi,

I was driving a friend's Renault Scenic which has an onboard fuel computer and was surprised to see that when coasting downhill while still in gear (foot off accelerator), the current fuel usage was 0.0ℓ/100 km.

Do cars use less fuel when coasting in gear versus when in neutral? Or does it depend on the vehicle?

I've always coasted down hills in neutral (idle), thinking it was more economical.

Cheers & thanks

Even though the engine uses no fuel when coasting in gear, the engine drag slows you down and shortens your coast so quickly that you get better overall fuel economy by coasting in neutral even though the engine is running since you are coasting so much further when in neutral. Of course the best fuel economy can be gotten by engine off coasting in neutral. Coasting in gear has a place in FE driving, mainly for limiting speed on steep down hills, when coming up to a place you have to stop, or for reducing your speed when you are getting too close to the car ahead of you.

Ah right, this makes sense.

So even though no fuel is used when coasting in gear, the distance gained by coasting in neutral off sets the fuel that is burnt when idling down the hill?

I drive a 2006 VW Polo which has power steering, I don't think it'll be happy coasting with the engine off.

Check out the 100+ Hypermiling tips on this site, especially this one: The most efficient way to slow down

Unless it's a steep downhill, there's a limit to even how far you can run with the engine drag. Get a long hill in neutral and you go forever.

Best of all is EOC, of course, allied to skillful clutch starting to save wear on the starter motor.

How I find it :
If you are on a very long hill using a kill switch or turning off the ignition is useful while in neutral. I have a hill that I coast for 10km on that I use my kill switch on but not for the 200ft long coast as it passes so quickly that little to no gain can be realized.
In short its a lot of work for a little gain. Most small cars use about 1/3 gal. per hour at idle so that much is possible to be saved with continual obsessive engine off in Neutral coasting.
Using a kill switch(or turning off the key) is a distraction that best left for those purposely long coasts in my opinion.

Coasting in neutral is only effective in carburettor-fed engines, or in Diesels with all-mechanical injection. But it still wouldn't worth the increased wear on brake pads...

Not sure why you say that...? Coasting in neutral is effective on any vehicle when the conditions call for it... If one needs to use brakes when coasting in neutral, then it's not being done right.

That's probably the best guide. If you need to use the brakes, you ought to be coasting in-gear.

I coast around in neutral every chance I get , it and low rpm shifting are my favorite and most effective Eco-driving tricks that I employ.

I agree that if you need to hammer on your bakes you should not have chosen neutral at that time or place.
Some trial and errors may be needed but it is easy to master, you can shift between 5th and neutral in order to control your speed..

I meant coasting downhill :D

Thanks for the replies. So the general consensus seems to be, if you need the added engine drag (to avoid braking) then leave the car in gear when coasting. Otherwise, shift to neutral and idle down the hill?

Yes but not just downhill , you will find you can coast up to lights and anywhere where acceleration is not needed , you pick how much neutral coasting you want to do and do it.
Such as pulse n glide with a neutral glide.

I only usually coast downhills, I'll start trying in other places too.

When coasting, is shifting to neutral necessary? Or is fuel usage the same staying in-gear with the clutch fully pressed-in?

The result is the same, the engine is disconnected from the transmission gears.
When the clutch is fully engaged it is not wearing , just when being engaged or dis engaged.

Using it as smooth and gently as possible will extend its life, some drivers get 500,000km out of their clutch , mine has 210,000km on it and is working normally.

Try not to jolt it or ride the clutch when accelerating, the jolts can break it and the slow release will wear a slick surface on it that makes the clutch feel weak with little grip when engaging.
If you do wear a hard slick surface on the clutch from slow dragged out clutch work you can "fix it" by shifting proper for 2-4 months till it wears the slick surface off and replaces it with its normal surface,just by clean fast clutch work.


A fast shift will require less clutch time / clutch wear

Coasting in gear with the clutch fully pressed-in may still increase more of its durability than clutching, shifting to neutral, clutching again and engaging a gear. Clutch wear doesn't increase in the meantime it's fully pressed-in.

My take on the clutch thing is if you press in the clutch, your clutch bearing might wear a little faster but clutches need to be replaced anyways and it's usually the friction material that goes not the bearing. Shifting in and out of gear will put extra wear on the synchros which are much harder to replace than the clutch.

I've seen t-bearings give out first but typically only on cars that see a clutch last 250k. I think the highest I remember is a 200SX with 546k on the original clutch. 110 mile round trip commute daily.

regards
Mech

I don't worry about clutch/bearing wear, it is like $100 in parts and a fancy nut and bolt puzzle (that comes already assembled!), every ~150,000 miles/10 years. Whereas if you worry about driving to double your mileage (say from 23 to 46) you will save another ~$12,000 in fuel costs.

The longest-lasting clutch in a daily-driver with most city traffic commuting was 75000 miles in an Opel Corsa Caravan with the 1.0L 16-valve engine.

Wait guys, can't coasting in neutral at high speeds with an automatic fry the transmission?

In most cases it's fine. I can personally vouch that it's fine in a Honda. ;) If the engine is running, there's a pump that's lubricating the transmission.

Some automatics don't tolerate engine-off coasting. Hyundai is one. Some are ok with it. Some have "limits' like Honda: up to 35 mph for up to 50 miles. Since I never coast for 50 miles at a time, I feel it's ok to push the mph limit a bit. Then again, the gains from EOC at highway speed are much smaller than in the city, so it's less relevant there.

I believe that only applies to engine off neutral coasting.

It was actually a factory feature in the automatic versions of the Opel Astra, which had been briefly offered in the U.S. and Canada as Saturn Astra. AFAIK this very same transmission was used in nearly every late-90s compact sedan such as your Corolla :D

I've coasted in neutral (only engine running) in autos for probably a decade, several different cars, regular autos, CVTs, and 23+k in the fiesta powershift.

Engine off coasting in neutral?

You will never see me do that in any auto transmission. In almost ever auto tranny made there are two flanges on the back of the torque converter that engage the primary pump of the transmission. When you turn the engine off, the torque converter stops rotating, since it is bolted to the crankshaft, so you have no primary pump and no pressure for gear actuation or lubrication, and neutral needs activation to unclamp all of the bands that engage the planetary gearsets.

Would you run your engine without oil pressure intentionally?

You will never see me coasting in neutral, with the engine off, in any auto transmission, until the manufacturer guarantees me that they will replace the transmission if it fails.

They won't.

Engine on only in autos. Engine off is risking a several thousand dollar repair, in many cases more than the car is worth.

Manuals are fine with engine off coasting.

regards
Mech

Thanks Mech, this is the answer I was looking for. I never engine-off coasted in my Camry, and I'm glad now I didn't ;D

Have to agree. Was in the towing business back in antiquity. Lots of rules about about which automatics could be towed drive wheels on the ground. Some X number of mile at <Y speed others not at all. These assumed the transmission was in good condition. Best bet was flatbed, drop the drive shaft or dolly.

I wouldn't do so, but an uncle of mine once towed a Pontiac Trans Sport with the drive wheeld on the floor, using a towbar.

I'd rather use something like this if I couldn't get a dolly :D
http://cdn.trendhunterstatic.com/thu...-fixed-it.jpeg

:eek:

Is it just me or are space saver spare tyres getting smaller and smaller?