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Old 07-11-2008, 06:55 AM   #1
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Question about hills

What do you think about mild acceleration down a hill to gain speed and coast up the hill immedietly following? I'll drive at about 58 but when I go down a hill I'll accelerate up to 65-70 gaining momentum so I can coast up the hill following it. My wife insists I'm wasting any gas i save by accelerating, i say it takes more gas to coast down the hill and then have to give it gas to get back up the hill. I have a 92 Daytona 5 speed 2.2 turbo DOHC and have picked up about 4 mpg around town since I started using some of the tips on this board, but my car is too old for a scan guage and I'm not ready to go crazy and figure out a different way to get instnt info on my car yet. I'm thinking about a scan guage for my winter beater 07 Town and Country, but I'll wait until winter to get that when I start driving that again. Any thoughts or ideas are apprecited.

Have you guys heard about water for gas?

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Old 07-11-2008, 07:07 AM   #2
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Daytona should be able to use a Chrysler "Traveller" or "Navigator" trip computer, provided you don't have a modified callibration in the SBEC or are using bigger injectors etc... There's a constant in the SBEC code that needs to be right for it to be accurate, many re-cals don't bother changing it.
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Old 07-11-2008, 07:14 AM   #3
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Some people have proposed that in an automatic, accelerating downhill is more efficient. However, with a manual transmission, it's far better to coast downhill and accelerate uphill.

Either way your engine has to produce the same amount of work. If you do it downhill, you do it at a higher speed, which means higher RPM and more engine friction. If you do it uphill, lower RPM helps, as well as the increased throttle angle. A wider throttle opening reduces pumping losses (that's because of the reduced effort the engine has to make to suck air past the closed throttle).

An easy, inexpensive way to get some live data is with the DIY fuel rate monitor I have linked in my sig. It doesn't tell you MPG, but it does give you relative fuel rate.
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Old 07-11-2008, 07:23 AM   #4
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My cars are all manual transmissions and I always coast down the hill and accelerate on the uphill. That is what has seemed to work best in my case. My wifes car and the one I got for my 17 year old son to drive are automatic, but I don't drive them enough to know what works best on them. I drive them the same way I drive mine whenever I drive them. I guess it is because I am in the habit of driving that way.
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Old 07-11-2008, 07:29 AM   #5
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a while ago, someone was talking about this very thing and they posed a pretty important factor. if you accelerate down a hill, gravity is actually helping you along. I guess kind of like a teather ball. if it is already going that direction, you just give it a nudge and off it still goes. going up a hill is just the opposite, fighting gravity as you go.

I know this isn't the only factor involved but I figured I'd bring it up. also remember that at higher speeds, your drag increases tremendously. I have heard that the threshold is around 53MPH but also it is dependant on the cars design.

also remember that it will take quite a bit of gas savings to pay for that speeding ticket if you get caught going down a little coutry road at 70 to pick up speed for the next hill.
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Old 07-11-2008, 07:46 AM   #6
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That's what I'm thinking Beef, a nudge when I'm going 58 up to 65-70 then coasting up the next hill keeping the engine from lugging in 5th or having to downshift to fourth, but from what I've read hear so far that may not be the case. I'm not real worried about cops around here, it's a small town and I know most of them. I sell cars so if I can't talk my way out of a ticket at 14 over then I don't have my "A" game that day or the cop is giving me a ticket regardless. I've talked the state boys out of tickets mainly by admitting I knew I was driving a little fast but didn't realize I was going that fast. I guess I really want to believe mild (stress the word "mild") acceleration down a hill would be better because otherwise my wife would be right about something car related and I'll never hear the end of it. Either way it's a chance for me to do a little speeding at very little cost!
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Old 07-11-2008, 07:49 AM   #7
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Correction: I've talked a (one) state boy out of a (one) ticket, but that was a few years ago, I very rarely approach 80 mph anymore.
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Old 07-11-2008, 08:21 AM   #8
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I have a very significant hill I drive over every day. It is about 1200 feet in elevation in 6 miles. There is no way I can drive over this without downshifting at least 1/3 of it. It seems that when downshifted, I have to ease up on the throttle to maintain the same speed. Lower RPM+higher gear (od)= 2/3 throttle. Higher RPM+3rd gear= 1/2 Throttle. Does this indicate in any way how much fuel is being used?
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Old 07-11-2008, 08:42 AM   #9
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that is a good point as well. anytime you can avoid downshifting is usually an FE advantage.

I have an auto (in my car at least) and I have noticed that gunning it (no more than about half throttle) to get me up to 40mph and then letting off and easing up to speed will get me into 4th (my top gear) and keep me there and has yielded better MPG numbers. I have been trying this latelty and hopefully I will update my gas log at the beginning of next week or so.

I know that had nothing to do with the question but I figured I'd throw it out there.
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Old 07-11-2008, 10:26 AM   #10
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Quote:
Originally Posted by wapiti_j View Post
Lower RPM+higher gear (od)= 2/3 throttle. Higher RPM+3rd gear= 1/2 Throttle. Does this indicate in any way how much fuel is being used?
It depends on RPM. Fuel rate is mainly a product of RPM and throttle. Generally, in my experience, lower RPM + wider throttle always beats higher RPM + less throttle, but that differs from one car to the next, and could differ for specific situations like yours.
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