Save my relationship


#1

Help! My boyfriend and father are at each other’s throats! The question is do you save gas if you downshift going down hills? My boyfriend is convinced that when you down shift and the RPMs go up, you are using more gas, even if your foot is off the accelerator. My father is as equally convinced that when you do the same downshift while going down hill, you save gas; that increasing the RPMs without pressing the accelerator does not increase gas usage. I need some definitive proof of who is right! Just saying so will not convince them that one or the other is correct!


#2

Cars are not all the same. On most modern cars the there is a minimum flow of fuel that will keep flowing when your foot is off the gas. It does not change when you downshift. In the old days of carburetors it might have.

Now my car is a diesel and drive by wire (computer) the computer shuts off the flow of fuel anytime my foot is off the accelerator and my engine is at or above idle (950 rpm) So at that time I am getting infinite mpg.  

In sort, with most modern cars you are not going to use more fuel by down shifting and you may use even less.   You also will reduce the wear on your brakes and may prevent them from overheating which is a very bad thing to have happen.

#3

Perhaps I should add that my car is a 2007 Honda Accord, gas driven vehicle.


#4

What JEM says is true, but you also wear the clutch and tranny when you downshift. Now those are more expensive than the brakes, but on the other hand if your brakes fail due to overheating then you are history. I try to keep the brakes well serviced and the fluid relatively fresh with no condensation in it. On lengthy downslopes it is worthwhile to downshift and use engine braking, on the short ones I don’t bother.


#5

I am afraid I have to disagree with the previous poster. The fuel injector pulse width in your vehicle is determined by many things. Throttle position sensor, incoming air, engine temp, etc. When the throttle is closed the ECM will reduce injector pulse width to a minimum to keep the engine running. I believe however, that your car will use more fuel based on the fact that the higher the engine revolution will require the ECM to injection that minimum amount of fuel more times. Hope it helps.
~Michael (dartman69)


#6

I’ll second Dartman’s right on the money opinion.


#7

Some interesting opinions, but not one person here is going to calculate the actual fuel savings by their methods. I estimate the better method, whichever it may be, may save all of 1.5 cents per hill. I might be a little high on that amount. Why not just let the guys work things out themselves?


#8

quote (Meehan):

… On most modern cars the there is a minimum flow of fuel that will keep flowing when your foot is off the gas. …
I think you are wrong. Under decel conditions, throttle closed, car in gear, rpm above certain value, fuel injection is usually cut off, except for an occasional “squirt” to keep the cat temp up. Other occasions for cutting fuel is if red line is exceeded, or turbo press gets too high. Injection pulse length is a function of many variables.


#9

I agree, but the difference is probably very small.


#10

Rule of thumb here in Beautiful Golden, Colorado, is to use the same gear going down a hill one might use going up. Keeping brakes from over heating and speed under control are the key idea behind the choice of down hill gear. jkd


#11

have them do a test one drives his way and the other drives his way and see who uses more gas.


#12

I had some friends that you couldn’t win a discussion with. It didn’t diminish their worth to the point where I didn’t get along with them. The thing is that neither one of them actually knows and they should only get hot about the things that matter, like global anything, religion and politics.


#13

I used to own and Chrysler Town and Country mini van with the fuel mileage display. I have done that part where you’re going down hill and shifting into low gear to slow down. Fuel mileage actually goes down. I tried the same in different situations.

Going downhill

Shift into neutral 50+ mpg (DO NOT ATTEMPT THIS!! IT IS VERY DANGEROUS)
Leaving in D4 setting 40 - 45 mpg
Downshifting to 2nd 30-35 mpg

Hope this helps