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Old 06-16-2022, 07:13 PM   #1 (permalink)
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Alternative MPG Calculation

I just wanted to verify my thinking and math with the tools I have. I run a Holley Terminator X and I can take logs of short trips and sections of drives. In the logs I can see my overall fuel flow in lb/hr and speed in MPH. I setup a spread sheet of a log of my daily commute to work and the average speed was 35.87 mph and found the average fuel flow to be 12.75 lb/hr for the whole trip. My question is the formula based on these values. I have been using this formula.

Code:
(speed)/(fuel flow)*(weight of gas/gallon)
My understanding is a gallon of gas is 6.3lbs. In the above case I got 17.44mpg but at the pump doing the tradition calculation I have been multiplying the miles by 1.09. I have comparred the Jeep ODO to a GPS and the GPS reads higher by about 9%. So when I multiply the speed by 1.09 and do the calculation again, I get just over 19mpg.

Does my barely passed highschool math check out or am I wrong? I have been making some lean tunes and so far the best I have gotten was 16mpg for the last 2 tanks using a corrected speed and 13mpg before that, so I am a bit skeptical I did this good.

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Old 06-16-2022, 07:25 PM   #2 (permalink)
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Did you mention the average speed of 35.87 because that's all you have to work with?

I see your thinking, but I don't think that would be a very accurate way of calculating it.
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Old 06-16-2022, 07:35 PM   #3 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
Did you mention the average speed of 35.87 because that's all you have to work with?

I see your thinking, but I don't think that would be a very accurate way of calculating it.
I got all sorts of values I can use. I converted the data log to a excel spread sheet and there is about 40,000 lines of data that are roughly at 0.024 seconds per point. I attached pics of the different data points that are possible if setup. The Fuel Economy data point does not work on mine because it requires an electric GM trans that I do not have.
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Old 06-16-2022, 08:06 PM   #4 (permalink)
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If you have it plot each point as a mpg number (instant speed / instant fuel rate) then average that it would be accurate, but probably the wrong units. It might be easier to have excel convert it to gallons/hr then plot it

I thought maybe all you had were averages, which wouldn't necessarily give the whole picture.

*I didn't look at your pictures since I am on my phone. I will when I get home though In case I missed something.
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Old 06-16-2022, 08:18 PM   #5 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
If you have it plot each point as a mpg number (instant speed / instant fuel rate) then average that it would be accurate, but probably the wrong units. It might be easier to have excel convert it to gallons/hr then plot it

I thought maybe all you had were averages, which wouldn't necessarily give the whole picture.

*I didn't look at your pictures since I am on my phone. I will when I get home though In case I missed something.

The only fuel flow I can get is in lb/hr. So how could I convert that to gal/hr?
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Old 06-16-2022, 08:45 PM   #6 (permalink)
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Divide by 6.3 (or whatever the weight of gas plus % ethanol is).
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Old 06-16-2022, 08:51 PM   #7 (permalink)
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If you only wanted the average for the trip/data recorded same you probably could have used the averages. Just use the average flow rate and average speed and the overall time to calculate total. So if you averaged 35.87 mph for one hour and 12.75 lb/hr (2.023 gal/hr) then you would get the average for the data. Which would be 17.7 mpg

Sorry, I just realized that after re reading everything.

If you do it the other way you would be able to see how your fuel economy was fluctuating and with some more math you could tell when and where you were getting the best fuel economy
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Old 06-16-2022, 09:04 PM   #8 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
If you only wanted the average for the trip/data recorded same you probably could have used the averages. Just use the average flow rate and average speed and the overall time to calculate total. So if you averaged 35.87 mph for one hour and 12.75 lb/hr (2.023 gal/hr) then you would get the average for the data. Which would be 17.7 mpg

Sorry, I just realized that after re reading everything.

If you do it the other way you would be able to see how your fuel economy was fluctuating and with some more math you could tell when and where you were getting the best fuel economy
Its been nice having these numbers since I can setup certain inputs to offset target AFR or ignition timing by flipping switches and I can see what has the best effect on fuel economy and just use averages of certain points instead of the whole log.
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Old 06-18-2022, 05:19 AM   #9 (permalink)
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Does your car return fuel to tank or is it returnless? If it returns fuel, does your fuel flow calculation account for that?
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Old 06-18-2022, 09:21 AM   #10 (permalink)
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Quote:
Originally Posted by Drifter View Post
Does your car return fuel to tank or is it returnless? If it returns fuel, does your fuel flow calculation account for that?
It is a return, and I am not sure how it calculated flow, but my guess is it takes a calculation based off of the fuel injector flow specs that had to be entered in manually or pick a preset factory injector part number, the fuel pressure, and the duty cycle of the injector. Mine is a return system with a regulator roughly 12 from the tank.

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