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Pleased with mpg

Reebs said:
I don't see why they wouldn't. And it is just the interia of the car turning the motor over that is keeping it alive when engine breaking so zero fuel should be used as I understand it. I've never read up on it though. :shrug: :)
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I was wondering this because if they did continue to spark wouldn't there be an extremely lean mixture (almost all air) if there were still any trace fuel vapors in the intake tract? I suppose it would just be completely negligible for the couple of revolutions until any traces are burned off, but it just made me think.
 
96RedSE5Sp said:
My Scangauge. It gives real time MPG figures, as well as a bunch of other stuff.
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You do realize that its not 100% accurate and anytime your mileage jumps like that you really arent getting that super mileage. The high mileage you see when leaving the car in gear is way more accurate than the 300 you see while coasting.
 
How do you know this is not accurate? My Scangauge steadily and consistently reads 300 to 325 MPG when I go down hills of a mile or more.

I've read a lot about these gauges and have never come across anything stating that the gauges are not accurate if you put your car into neutral.
 
Well first of I'll tell you that I returned my scangauge because even with the same size tires on the stock rims the speedo on the gauge was still off by 3mph which would mean all of the mpg functions I wanted would also be off. So yours might very well be off as well without you even knowing it. That being said its a known fact that keeping the car in gear to engine brake down the hill is the best way to save gas besides just turning the engine off and coasting down the hill. Your car isnt any different so dont actually believe your car that gets 25mpg on the highway can hit 300 at any time.
 
Considering it gets 4 or 5 MPG during acceleration and zero MPG while idling at a red light, 300 MPG while coasting is very believable.
 
Calculate fuel-flow rate from pulsewidth


  • (Fuel flow rate) ≈ (pulsewidth) × (engine speed) × (number of fuel injectors)
Looking at it another way:
  • (Fuel flow rate) ≈ (throttle position) × (rpm) × (cylinders)
Looking at it another way:
  • (Fuel flow rate) ≈ (air-charge) × (fuel/air) × (rpm) × (cylinders)
Substituting real variables for the 5.0 L engine at idle.
  • (Fuel flow rate) = (2.0 ms/intake-stroke) × (hour/3,600,000 ms) × (24 lb-fuel/hour) × (4-intake-stroke/rev) × (700 rev/min) × (60 min/h) = (2.24 lb/h)
Substituting real variables for the 5.0L engine at maximum power.
  • (Fuel flow rate) = (17.3 ms/intake-stroke) × (hour/3,600,000-ms) × (24 lb-fuel/hour) × (4-intake-stroke/rev) × (5500-rev/min) × (60-min/hour) = (152 lb/h)

Now.. take all of that, and then realize that when you engine brake your car the injectors are SHUT OFF. You are using ZERO FUEL... as in.. none.. you're telling me that by IDLING and USING fuel that you are getting MORE mileage than using NO fuel and still covering the same distance down that hill?

I don't know if that scangauge calculates fuel efficiency by comparing pulsewidth to speed or what, but if it did then when there are NO pulses (injectors shut off) it wouldn't have anything to compare to...

Either way, its irrelevant... your most efficient means would be engine braking...
 
Hey guys, I have actually thought about this now that I live in a very hilly area compared to flat Ohio.
So when I crest a hill at like 75-80mph and have a long 1-2mi. downhill I'm better off leaving it in 5th to engine brake? I had started throwing it into neutral until speed came back down to 60-65 but started thinking of this and it makes sense what you guys were saying. I'll engine brake from now on.
-J
 
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