coasting down hills? or speeding down them?

[comrade_charlie]

I drive a turbo 87 300zx, nissan

Any way everyday I have to drive up and down 3 or 4 big hills, then going to my friends house I have to do the same. It is really annoying because I dont know how to drive for the best gas mileage out of my bad gas mileage car, and im always on empty

should I coast down the hill then use gas to go up it, if I coast down and use gas to go back up do I have it in 3rd or 4th gear with vacuum around 20, or have it in 5th gear and have the vac closer to zero? higher rpm higher vac vs. lower high lower vac

Or use a little bit of gas to speed up going down the hill in 5th gear with the vac at 20 and then ill have to use less gas to go back up the hill meanwhile slowing down to the speed limit. kind of like using gravity to shoot me up the hill.

These are pretty big hills so I dont see an advantage in coasting down the hill to climb a huge hill.

basicly show I speed down the hill and shoot up it, or coast and drive up the hill in 3rd, 4th, or 5th gear and at what throttle percentage is best?

[R.I.D.E.]

Neutral coast downhill, 5th gear uphill. Only downshift if the hill is too steep for partial throttle in 5th gear, which would have to be a pretty steep hill for a 300 turbo.

Too much speed downhill let completely off the gas, downshift if you are going too fast in 5th, no fuel used if you use gears downhill, small amounts if you are coasting in neutral.

0 vac is most efficient as long as you are not wide open throttle, which causes enrichment.

regards
Gary

[comrade_charlie]

Alrighty thanks.

[R.I.D.E.]

Think of it like a roller coaster.

(This is assuming you will not get a ticket for excess speed.)

You pay for the first climb with bad mileage.

After that your best possible mileage would be to shut your engine off and let your inertia do the job.

Flying downhill stores the energy in the mass of your car. Using the same mass and velocity to go back uphill still would require no energy if you shut the engine off, and let it go like a roller coaster. Ideally the hills would decrease in height and you could do everything but the first climb with the engine off and no fuel used whatsoever.

Now lets talk reality. You don't want a ticket or a wreck, so make the appropriate adjustments you deem necessary to try to come as close as you can to the roller coaster scenario without getting yourself hurt or cited to appear in court.

Shutting the engine off means no power steering or brakes, which in hill country in a car as heavy as the 300 could be an issue.

Don't risk your life for a few ounces of gasoline.

regards
Gary

[1993CivicVX]

It depends how fast you get going down the hill in neutral and how long and steep the hill is going back up. The more detailed info you can give on the hills' length and steepness etc, the better I can give you a recipe for fuel savings. General principals are you want to slowly be losing velocity while going up the hill so that you are not going less than 30mph by the time you reach the apex. If you are reaching 55mph in neutral going down the hill then you shouldn't gas it down the hill. If you are only reaching 40mph in neutral down the hill, then you might want to gas the whole thing (down and up) according to the above principles.

If you are coasting down the hill, it's important not to wait too long before hitting the gas again. When you should get on the gas again depends on how steep and long the hill going up is. The bigger the hill, the sooner you should get back on the gas so that you can reach the top at 30mph without the need of low gears.

Hope that helps.
I also understand some cars use lots of gas when in turbo--if that's the case with your car, then you might want to do whatever it takes to avoid engaging the turbo.

[bobc455]

This is an interesting question.

A few years back, I was talking with a co-worker who liked to fix up old motorcycles. One of the things I found intriguing was that cruise controls on those bikes was a constant-throttle style control. He went on to explain that the constant-throttle driving was best for gas mileage.

I have no proof one way or the other on this, but ever since he planted the thought in my brain I can say that it does "feel" right.

It is impractical and unsafe to drive a car with a constant throttle position, however I also follow the general guideline to gently lose speed when going uphill and gently gain speed when going downhill. (I have an automatic, and don't like going into neutral for coasting so much - that works much better with a manual).

-BC

[Jay2TheRescue]

I can remember years ago when I drove ambulances one of the trucks we had was built on an International DT4700. The truck had the optional cruise control, but also had a throttle lock, as it was standard on the chassis. The ambulance manufacturer did affix a huge sign right beside the throttle lock that said something to the effect that the throttle lock should not be used for safety reasons. I was almost tempted to have the throttle lock removed from the vehicle, just so someone who didn't know what they were doing wouldn't play with it, but it was decided that drivers would not be allowed to operate that vehicle without special training and approval of the officers.

[JanGeo]

Yeah air drag not being a big factor in that old rocket the next thing to be watching is the turbo enrichment of the A/F ratio - you may get into the rich not so chemically efficent but more thermally efficent operation mode of the engine which probably is still not going to get you the best mileage.
I would guess that some light throttle in the valleys building some speed for the up hills with moderate throttle upwards slowing to crest the peaks at about 30mph would be about the best and keeping it close to legal is always a good idea.
A good rule of thumb is the less the motor turns the better the mileage.

[Project84]

Despite all the techinical jargon I would recommend the following.

Coast down the hill at an acceptable speed, before you hit the low point give the car some gas in a high gear to pick up a little more speed to help start the climb up the next hill. This will hopefully keep you from having to downshift.

Using a SGII this is what I've found works best for my car (which is nothing like yours).

[spotaneagle]

Originally Posted by bobc455

This is an interesting question.


I have no proof one way or the other on this, but ever since he planted the thought in my brain I can say that it does "feel" right.

It is impractical and unsafe to drive a car with a constant throttle position, however I also follow the general guideline to gently lose speed when going uphill and gently gain speed when going downhill. (I have an automatic, and don't like going into neutral for coasting so much - that works much better with a manual).

-BC

this method is definately best for engine bearing(lugging), and your transmission, who knows