Downhill mileage

I live at 8900feet in the mountains and drive 19 miles downhill to work. I can put my 5-speed Eclipse in neutral and coast all the way down without ever putting it back in gear. It idles at 1000rpm all the way down. The trip back up in the afternoon is an arduous affair and I am limited to 4th gear - sometimes 3rd to get back up.

TWO QUESTIONS: Forget the braking aspect-I have ceramic pads and drilled and slotted rotors. Q #1: Going uphill in 4th gear, does the car use more gas if I floor it in 4th to maintain a steady speed versus or back the accelerator off a bit and still maintain the same speed. The degree of incline is such that it doesn’t make any difference in the mph if I floor it or not, but you can tell a difference in the sound of the engine when it is floored. Q #2: Coming down the hill, if I decide to use my transmission gearing to keep me at a steady speed instead of braking, the rpm’s are high (about 3500 rpm) of what seems like to me of “sucking air” for 19 miles. Is this bad on my engine? Is it using just as much gas at 3500 rpm sucking air as it would if I were on level ground holding it at 3500 rpm?

Many cars, when going down hill in-gear, use no gas at all. If yours is one of them, then you are using some gas while coasting in neutral, but you would use no gas while coasting in-gear. And no, the gas consumed is not tied to the RPM. It is more tied to the combination of rpm and load. In this case, the load would be negative, but the best you can do is no gas used.

i agree!!but when youre going up hill it will use more gas the more you press the accelerater pedal

Q1-I’d think you’d use more gas with it floored.
Q2-You use much much less with the car turning 3500 rpm with the throttle closed that driving on level ground at 3500 rpm. Very little (in some cars, 0) gas is used with the throttle closed. I’d do that to save my linings, much more expensive than the little extra gas you might burn.

Well, its a 1998 Eclipse. Do you think they had that kind of technology back then? I’ve heard of zero fuel burned with diesels when you back off them, but not with gas cars. I’m not the expert. One note is that my gas mileage hasn’t suffered too much. I’m still getting about 35 mpg average with a trip up and down every day.

Yes, it might not have the fuel shutoff that some cars have, but even without that it’s not going to use much more gas than at idle, the closed throttle really limits the amount of air drawn into the engine. If you had a vacuum gauge, you see it at extremely high vacuum (very little air per stroke) on the downhill runs in gear.

Yes, they had closed throttle fuel cutoff in 1998. Basically any car with fuel injection and a catalytic converter has the fuel cutoff. My '85 Accord SEi (Honda’s first fuel injected street car) had the fuel cutoff. If you have your foot off the throttle and the RPM over ~1200 your car will use no fuel. The only downside is if you do that for miles and miles in the winter the engine will cool off and you’ll have no cabin heat. I’ve been there.

You’re coasting 19 miles and riding the brakes the whole time? That’s stupid! I don’t care how fancy the brakes are, you’re risking boiling your brake fluid and losing them completely. Use the engine for braking – you’ll burn almost no gas doing that, and it’s much safer. It will do no harm to your engine to “suck air” (it does that all the time, anyway).

Agree, McPhil; this forum has responsible posters giving responsible advice. I live in mountain country and use engine braking to help slow the car and not overtax my brakes. Ever since gas prices went up some drivers are trying to get something for nothing and endangering their lives in the process!!

I would like to see dtracy post his/her question to an experienced truck driver, who always releies on engine braking to help his brakes.

dtracy, you are out to lunch; act normal and stop trying to second-guess the Mitsubushi corporation’s engineers and thousands of good transport drivers. It may save your life!

OK…OK already. Your point is made. Expert advise is what I was looking for and obviously, you guys know more than me. The point probably needs to be made to more people. Maybe others reading this will learn also. I really do appreciate the advice - no one seems to be posting on the other side of the issue, so the ney’s have it! – dtracy