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Old 05-20-2013, 01:36 AM   #1
SXGT
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Default wack MPG

I did a little road trip today and thought it would be a good idea to check my MPG. I did 70 miles to TN and when I was there I picked up a 115lb passenger and drove in the city some, and the country side. I dropped them off and went to fill up and had a calculated MPG of 21. SO i was expecting 23 MPG driving from exit to exit on the interstate only. I did 75mph average and when I filled up it was 19 MPG....can different pumps make that much of a difference on where/when they click?

Also when I fill up I've been getting over flow as the gas pump clicks off on its FIRST click. not only over flow out of the fill up port but also the over flow tube that dumps out on the bottom of the bumper.

I know this car has had the gas neck replaced. I wonder if I am loosing gas when the tank is full, or if I am loosing gas on right hand turns or something odd. My CEL code is Evaporative Emissions Purge Small leak detected. Which everyone says its just a bad gas cap.

I have my tires at 42psi and I know my air filter is clean. my Automatic 97 GT was always good for 24MPG. I did 360 miles in one tank once on it and I dont think there is anyway possible I can get 230 miles out of a full tank on this 01 GT 5 speed. Granted the 97 has 215/55/16s all the way around.

:/ I am going to go back to the 87 octane tune for a bit.
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Old 05-20-2013, 03:21 AM   #2
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You cannot determine if you are losing or gaining MPG from one or two tanks of gasoline. You cannot duplicate the exact driving conditions of one tank of fuel in the morning to the next tank of fuel that afternoon.

Different gas pumps and whether the ground next to the pump is level can make a big difference in when a pump clicks off. The most accurate MPGs are calculated when you refill from the same pump. I've filled my Mach 1 tank from pumps that I can set on the maximum rate and others that would click off if I used the lowest setting. Add your overflow, filler neck issue and you have no idea how much gasoline was actually burned by your Mustang.

"Averaging 75MPH" means nothing. You can travel 75MPH for one hour and cover 75 miles. You can travel 150MPH for 30 minutes and cover 75 miles. You can drive 65MPH for 15 minutes and travel 16.25 miles, then drive 78.33MPH for 45 minutes to average 75MPH.

Traffic, wind and slight grades can have an enormous effect on MPG. Back in 2004, I filled up at a gasoline station across the street from a freeway on ramp a few miles west of Palm Springs, California. When I left the station, the traffic light was green and I was on the freeway in 500 feet. Air conditioning on and cruise control set at 72MPH, I didn't touch the brakes or cruise control until I ran out of freeway at the West end of the 91Fwy at the 110 Harbor Fwy. Again, the traffic light was green and I refilled at a gas station 1/2 mile from the end of the 91. My MPG was 27.5.

The only way I could ever hope to duplicate would be to do it again at 3:00 AM because of the traffic in Southern California.

I now live in an area with very little population and traffic. I have a ScanGauge II plugged into my OBII port that can display instant MPG as well as average and tank MPG. I have tried to duplicate MPG on 30 to 100 mile runs on deserted 2 lane highways. By deserted, I mean I don't see another vehicle driving on either side of the road for 30 to 100 miles and I can't duplicate MPG. It's always a little different.

I suggest you buy a ScanGuage II, calibrate it and set it up to display instant and average or tank MPG. You will be surprised what squeezing the throttle just a little harder from stops a few times will do to your tank average. The same goes for driving into a 5MPH wind or with a 5MPH wind. Going from 5,000 feet to 4,000 feet altitude (almost a constant grade) over 23 miles gets me an extra 1.5MPG. Going the other way from 4,000 to 5,000, I lose about 1.9MPG.

Don't forget the coefficient of drag squares when you double speed. That means speed costs MPG. As an example, I eeked out 24.8 MPG out of my F250 Powerstroke at 43MPH over 25 miles (lowest MPH that kept the automatic trans in top gear), 21MPG at 50MPH over the same course, 19MPH at 55MPH, etc. (All with the cruise control and zero traffic.)

Using the ScanGuage II, you can learn some tricks to improve MPG like:

Accelerate on while going downhill and bleed off the excess speed on the following hill.

Drafting behind a semi at 80MPH on I-15 in Utah gets you an extra 4MPG.

If you can monitor you instant MPG, you will naturally learn how to improve mileage without trying.

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Old 05-20-2013, 06:34 AM   #3
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I have never gotten over 240-250 miles per tank in the 02.
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Old 05-20-2013, 07:18 AM   #4
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Me either in an 03.
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Old 05-20-2013, 08:14 AM   #5
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LOL
Miles per tank is a TERRIBLE way to compare mileage. WAY TOO MANY variables to be a valid method for comparison.
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Old 05-20-2013, 09:41 AM   #6
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What do you do to measure accurately then?
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Old 05-20-2013, 11:54 AM   #7
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I usually do use the same physical pump every time but that wasn't possible with me out of state. the results for my average week are consistently 18-19 mpg. Lots of elevation changes on this trip TN too.

It seems like the 97 GT was more consistent no mater how i drove it, the 01 seems super touchy.
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Old 05-20-2013, 12:25 PM   #8
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My driving habits vary very little and I can almost set my watch by when my low fuel level light is going to come on based on my trip odometer and I usually wind up getting right at 13.5 gallons per fill up. 17 miles per gallon, + or - a mpg, variables be damned.

I don't think I've ever cracked 20mpg.
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Old 05-20-2013, 12:25 PM
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