Quote:
Originally Posted by mcrews
Same way that coasting at 65 at 1 rpm doesn't double the mileage from the 750 rpm. It's not a simple math equation. I would actually assume it is based on what is required to rim the engine. Engine load, gearing etc.
the issue, imho is consumption of fuel during the brief moments of coasting TO THEN justify a hack that might have unintended consequences.
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The math IS pretty simple, you just have to pause and consider what you're mathing.
One mile at 50mpg followed by one mile at 30mpg should average 40mpg, yes?
If so, then it follows that one mile at 5,000mpg followed by one mile at 30mpg would be 2,515mpg, and as we approach engine-off coasting, our average goes toward infinity. We know this is not the case though.
We're not interested in miles per gallon per mile, we're interested in gallons per mile. Burning 0.020 gallons over one mile (50mpg) followed by burning 0.033 gallons over one mile (30mpg) results in a total fuel consumption of 0.053 gallons over two miles, or 37.7mpg.
Doubling your MPG while coasting might look significant, but it's important to figure out exactly how much fuel you're actually saving. It might or might not be significant over a tank, depending on how much coasting you do.
This is why L/100km is a better metric than MPG. Going from 15 to 17mpg actually saves 50% more fuel than going from 60 to 90mpg; MPG obscures this.
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Just as a thought exercise, if we assume 15mpg @ 60mph climbing one side of a hill for 30 minutes, and coasting down the other side (assume also at 60mph) for 30 minutes, just how much fuel can you save by reducing fuel consumption while coasting? Note that we're assuming 50% coasting in your normal driving - it's probably
far less than this.
15mpg @ 60mph = 4 gallons / hour
If you coast at 0.40 gallons per hour on the other side at 3000rpm, you're going to use a total of 2.2 gallons to go 60 miles (again, with 50% coasting), resulting in 27.3mpg.
If you coast at 0.20 gallons per hour @ 700rpm, you're using 2.1 gallons to go 60 miles, resulting in 28.6mpg.
Approximately what percent time are you coasting, would you say?