Engine/Transmission (1998.5 - 2002) Was: V10 Vs I-6 wishful, Now: MPG coast

[jponder]

The original thread has developed into people stating their mileage under different conditions. That is a good source of info but there are some issues that have been said that ought to be explored. My position on cruise Vs constant throttle is defined by my response to RussW1911 who PM,ed me and follows



jponder wrote on 06-30-2003 06:35 PM:



quote:

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RussW1911 wrote on 06-30-2003 08:19 PM:

Quick question. I saw in your post about the guy talking to the lady with the V10... ... You said if you are using Cruise it was probably costing you 2 MPG. If you could will you explain this to me. I am always trying to improve my gas mileage and if cruise is lowering it I will try not using it. I guess I always thought cruise helped the mileage..... Thanks for the info Russ Wallis



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Cruise control has lots of disadvantages. I think iflem always likes to point out how they surge even on level ground, which is why he desensitized his by blocking its ability to change once set, he got better mileage yes and he attributed it to the reduction of surging and yes that helped but with a perfectly functioning cruise it could only match a constant throttle device on level ground and it couldnt do it there. When you are moving you have Kinetic energy 1/2 MVsquared and potential energy Mgh. Think of a marble on a hill or your ram in neutral. As you leave the top of the hill you are converting potential energy into kinetic energy. At the bottom of the hill you will have a lower potential energy but a higher kinetic energy and energy is conserved minus friction and wind. Its natural! Think of the way a marble would got through a series of hills slow at top and fast at bottom. Cruise tries to do something unnatural It tries to maintain a specific speed as you go down the hill with cruise it lets off the throttle and your engine brakes you all the way down and then you have converted all that energy, that should be speed to climb the next hill, into heat from your engine braking. now you have to climb the next hill and your cruise hits the throttle to maintain your speed. It used your engine to slow you down going down the hill(same as hitting your brakes ) and now it is going to try to accelerate up a hill. That kills your mileage. Now look at a constant throttle cable or holding your foot in one place; at the top of the hill you are slower and at the bottom you are faster and that speed caries you up the next hill. Just try it one day for **** and giggles and you wont believe the difference in your mileage.



Then Big Dawg posted this

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RedRam...



I wish I got 26mpg consistently... .



My highest mileage ever was on 2-25 of this year and was 27. 12 mpg. That was hand figured with easy driving habits that would've annoyed even an 80 year old grandmother. Yes this is a wild number but I have 26. 85 mpg on the tank before that one. I've changed my driving style a little but my combined mileage usually is above 20 and usually 21. I was worried when I first got my truck last September because my mpg was low but it started to come up at about 4000 miles. I have quite a few figures in the 22 to 24 range. I hand calculate my mileage and keep records and receipts for every tank since I bought the truck. I also try to coast in gear as I have been told that the truck does not use fuel while coasting in gear (standard trans. ). I used to drive my TDI (diesel) Golf that way too. I have a friend that told me the Golf didn't use fuel when coasting in gear. I thought he was just old and goofy until he hooked up a laptop to the Golf computer and charted fuel consumption as I drove. That will teach me to respect my elders. I have since learned to pay close attention when he talks. He also has a 60mpg lifetime average on his diesel Jetta. THAT'S AVERAGE I SAID! I also frequently drive by the fuel computer. This seems to help.



Here are my last five tanks combined city/hwy. :

20. 67

20. 17

20. 38

21. 93

22. 71



Here are five back to back tanks in February (combined city/hwy) when I was trying to be the MPG King (This is extreme gas mleage gettin' style drivin'):

23. 58

22. 30

24. 80

23. 33

26. 85

27. 12

22. 89



The biggest drain on fuel appears to be getting the truck moving but if you accelerate very slowly this can be minimized.



Pulling U-Haul's biggest trailer loaded (all hwy) I averaged 17. 85mpg.



You can look at my sig and see the truck's specs. I weighed my truck the other day with a freshly filled fuel tank along with me and my 95 pound honey. Front axle was 4040lbs and the rear axle was 2780... . 6820 ttl. No 410's... No 4x4... No fuel sucking automatic... No DRW... So my truck is bound to get decent mileage. The fuel I buy is always very fresh and the two places that I usually fill up get five loads of diesel a day. I'm reeeaaallll careful where I buy fuel... . Don't want any surprises.



One other thing I've noticed. Increasing your speed over 60 or 65 will drop your mileage quickly.





Last edited by Big Dawg on 06-30-2003 at 07:41 PM

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That followed by Big Dawg saying that coasting in gear took no fuel while coasting in neutral did. Well Big Dawg I think you are a mileage nut like me but i DONT BUY THAT. It might be true that if you are coasting at 60 MPH in neutral you are usig the same fuel as idling but I think if you are in gear and your foot is off the pedal then you may be using no fuel but the engine is slowing you down which isnt the same as coasting.



Anyone have any thought on this!

 

[fest3er]

Originally posted by jponder
... It might be true that if you are coasting at 60 MPH in neutral you are usig the same fuel as idling but I think if you are in gear and your foot is off the pedal then you may be using no fuel but the engine is slowing you down which isnt the same as coasting.

Anyone have any thought on this!

It gets down to how the injection pump handles the go pedal being at idle whent he engine is turning faster than, say, 1200 RPM. On many modern fuel-injected gas engine, when you go downhill, in gear, foot off the gas, the system turns off the injectors, so no fuel is being used.

Does anyone know for certain how the VP, VE and P7100 pumps operate in this regard? And, for that matter, the HPCR system, which could be programmed to operate like gassers in this one respect.

Fest3er

 

[jponder]

Yes I had heard this before on the TDI page I think. If you are not providing any fuel for combustion then you arent coasting right? The engine is slowing you much faster than if you were in neutral. What is the difference in a truck going down a hill in gear foot off the gas and the same thing with the key off. Nothing that I can see. I know it seems trivial but if this idea of coasting in gear has merit then it would certainly mitigate the negative effects of cruise in a diesel. In my old Bronco 2 Gas burner I would see a 4 MPG difference in foot vs cruise, but that is not happening with the diesels.



Oh I went back and read your post again. If you have a gas engine shut off all fuel then you are going to get alot of engine braking. You wont get near as much on a diesel. Is this just a consequence of the turbo suppling some pressure on the intake stroke?

 

[fest3er]

Originally posted by jponder
... Oh I went back and read your post again. If you have a gas engine shut off all fuel then you are going to get alot of engine braking. You wont get near as much on a diesel. Is this just a consequence of the turbo suppling some pressure on the intake stroke?

No, it's a consequence of the gasser having a throttle plate that largely shuts off air intake to the engine, thus creating 'vacuum' engine braking. A diesel has no throttle plate and little back pressure, so there's nothing to slow the piston down, thus there's very little engine braking on a diesel.

Fest3er

 

[ToolManTimTaylor]

Fest3er , Nailed it.



Waay off diesel coast but the same block was a wanna be diesel at one point



I know for a fact that GM TBI Injected 350 C. I motors SHUT OFF the two injectors before the throttle body when you let off the gas to coast. then at 1200 they start again. (You can prove it with an A. M station @ 1680 am or somewhere high in the stations. ) Anyway I think thats why the 350 TBI injected motors would eat valve guides. Mostly every other motor I know keeps injectors running on decel to provide at least some sort of burn to combustion chamber releif going as to not create sooo much vacuum on decel like the tbi motors that were trying not only to draw air past the throttle plate in a closed condition but the actual throttle body itself which was a bad flow match to begin with.

 

[WadePatton]

Now that we've covered coasting, I have a simplified comment on cruise.



On level ground with no surging--cruise can be just as good as your foot for MPG.



And IF cruise keeps you going at a slower rate of speed than your foot--like 65 instead of 70, then you'll see MPG gains.



BUT cruise can't see a hill coming and won't accelerate you into it, nor will it see the peak of the hill and lift to carry over at the top. It's at its absolute worst when it holds you back going down and then you're right into another hill.



FOOTFEED baby!