Hypermiling and wear on the clutch


#1

I drive a VW CC Sport 6 speed manual. The EPA gives it 21 in the city and 31 on the highway. I’ve been reading up on “hypermiling” using techniques like Pulse and Glide, etc. I find myself making great gains in gas mileage by coasting whenever possible which I do by putting the car in neutral, rather than just depressing the clutch (thinking that this may save wear and tear on the clutch bearing). My question is, when coasting and putting the car back into say 5th gear when I’m doing 50, how much wear am I placing on the clutch? Am I just eating the rewards of better gas mileage by having to replace the clutch earlier?


#2

Yes.


#3

If you’re doing it correctly you’re putting very little wear on the clutch.

By the way, you’re smart to be putting the car in neutral rather than holding the clutch in. Holding the clutch in would wear your throwout bearing out prematurely.


#4

If you match the wheel speed to the motor speed, using the tachometer and the gas petal, there should be zero wear on the clutch. If there is a jolt when you release the clutch or a significant change in motor speed then you are not using the correct technique.


#5

Whenever you’re coasting in gear (foot off the gas pedal) the car’s computer shuts off fuel flow to the injectors.

Actually, coasting in neutral uses more fuel than coasting in gear, since the engine needs fuel to maintain idle speed in neutral, but it does not need fuel while coasting in gear.

You’re wearing out your clutch release bearing for no reason. Leave it in gear. The VW engineers have already figured out the best ways to optimize fuel mileage.


#6

While I’m no fan of hypermiling, those who do claim they save more gas with the transmission in neutral and engine idling (ie no engine drag), than leaving it in gear where the injectors will shut off but you also have engine drag slowing the vehicle down.

I don’t know if it’s true or not. I am curious. Has anyone here measured the difference?


#7

If conditions allow for extended coasting with a manual transmission turn the engine off and coast in neutral. It’s called “Georgia overdrive.” I drove most of the distance from Flagstaff to Phoenix in neutral several times 40 years ago. Fantastic mileage.


#8

As was said, if you match the engine speed to what it will be after the clutch is engaged, then the clutch friction disk wear will be negligible. If you have a tach and of course, a speedometer, you can make a little chart for yourself or else memorize the numbers at various speeds and when using the top gear.

As for clutch TO bearings, I have been driving manual transmission cars for many years and have never lost a TO bearing. A TO bearing is sealed with lubricant inside, just like your alternator bearings which run for many miles and years w/o relube. If your VW is like mine, then the TO bearing is inside the transmission and is lubed with trans fluid like the other bearings inside the transmission. Worry about TO bearings may be an echo from the distant past when some TO bearings were carbon blocks or when metallurgy, manufacturing capabilities and lubricants were not up to today’s level of development.

Regarding fuel injection cutoff, I don’t know that to be true for all cars. My other car has an instant gas mileage computer readout. It maxes out at 99 mpg. Judging from the rise time to 99 when releasing the gas pedal only or doing that and also shifting to neutral, I don’t believe that the gas cutoff happens during coasting in gear. The mpg rise time to 99 is faster when coasting in neutral is begun.

For another thing, brakes are much stronger now. Dire warnings about coasting in neutral are another carryover from the past when front drum brakes needed a little help from engine friction to stop a car, more so in mountain areas than on the flatlands.


#9

@Wha Who:

The throwout bearings are a much better design these days, but still only designed to actually be used 10%-15% of the time when driving. These do wear out, and I’ve worked on a few cars with good clutches, but bad throwout bearings. Maybe that was from a rare bad part, but they do wear out. Hypermiling while holding the clutch in is a sure way to cook this bearing quickly.

All modern fuel-injected vehicles have fuel cut-offs when coasting in gear. Some even shut down cylinders when cruising at low power demands.

And, as far as brakes and coasting, brake rotors are much thinner than they used to be from the 80’s and 90’s. Warped brakes and brake fade is a real problem with a lot of modern cars. And once the brakes are cooked, you have very little else to stop with.


#10

If you are ‘cooking your brakes’ you are not hypermiling. The first rule of hypermiling is to drive as if your car has no brakes.
Anything that slows your car is a brake, engine braking, deep sand, driving through a pool of water, wind resistance due to high peak speeds. Your engine has to replace the energy lost through braking.
It is well known that a lot of modern efi cars go into deceleration fuel cutoff when the thottle is completely closed and the engine speed is above a certain rpm. The problem is that the fuel saved during DFCO does not always replace the momentum lost from the engine braking that occured during DFCO. The concensus of the hypermiling experts is that in gear pulse and glide doesn’t save gas over steady state cruising. Pulse and glide in neutral ICE on does save fuel. ICE off pulse and glide saves the most fuel.


#11

In the 1950s, when fuel price was not an issue incidentally, fuel mileage challenges were made and I recall a Florida student who modified an Austin Healey Sprite with very narrow high pressure tires, overdrive gearing and tuning to achieve 200+ mpg, But a 1,500 lb car with a 950cc engine has a good head start on fuel mileage. Is there anything available today that compares to the Sprite?


#12

Actually, gas prices were an issue in the 1950s. They only seem cheap with today’s dollars.
Remember, 7 cents would get you a Coke from the vending machine in those days.


#13

@BLE: I agree with you that the “brake fade” argument against driving in neutral is overblown. Assuming you aren’t towing anything, most any newer car has lots of excess brake capacity.

That said, the safety issue of “sometimes” having PS,PB needs to be addressed. Ordinary driving of a vehicle without these two modern conveniences may or may not be safely accomplished depending on the strength of the driver and/or the nature of the car in question. For unexpected evasive maneuvers, however, it seems an issue for everybody: evasive maneuvers usually require decisive action NOW, and having to (a) remember if you have PS/PB and (b) modulate your control inputs accordingly is a concern.

I toyed around with hypermiling back when I had a MT, and I probably would have deleted the PS if I ever got serious, so that there’s a constant force/steering deflection ratio. I also would have put an extra-large booster reservoir on that would allow 6+ stops with boost.


#14

Quote from BustedKnuckles: “The throwout bearings are a much better design these days, but still only designed to actually be used 10%-15% of the time when driving.” Unquote

It appears that you have access to engineering information that only an auto engineer would have available. What is your source for this number?


#15

"Actually, gas prices were an issue in the 1950s. They only seem cheap with today’s dollars.
Remember, 7 cents would get you a Coke from the vending machine in those days.
"

Gas prices in the U.S. have remained pretty stable over the last hundred years, once you correct for the declining value of the dollar. Before gasoline came along we complained about the price of feed.