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Question- 20% Decrease in MPG Suddenly after Track Event


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Most of you know that I recently did an awesome weekend track event at Roebling Road near Savannah, GA.

 

On the way down there, my SGT got its usual 25 to 26 mpg on an extended trip 250 miles. However, after the trip, I only noted 20 to 21 mpg per the trip computer which is usually pretty accurate. I took it to the dealer today to change all the fluids (brake & oil plus went to Royal Purple 5/20 this time), and had them check out this issue. They came up with nothing.

 

It is not running rough or anything...seems to be performing OK.

 

Any thoughts??

 

Andy.

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If average grade is running uphill to go home that will affect it; as will headwinds. Also, following a track day, you may have just driven faster going home!

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was this mileage per the onboard computer or did you do the old fashioned miles divided by gallons?

 

 

I generally use the onboard computer as a benchmark so that the number comes from the same place.

 

 

Did you have a headwind on the way home? How was your tire pressure?

 

Inspect the brakes (including the E-brake), you may have one hanging.

 

Not much in the way of wind...very calm day on Sunday late afternoon.

Tire pressure was fine (nitrogen fill when I purchased new set of tires in late December)

I did not check specifically the rear brakes, but I did rotate the tires about 5 minutes prior to leaving the track and getting directly on the highway to Atlanta...Didn't notice anything unusual and was listening for sounds for the first 15 minutes of travel to make sure it was ready for the 250 mile trip home.

 

 

If average grade is running uphill to go home that will affect it; as will headwinds. Also, following a track day, you may have just driven faster going home!

 

 

I was heading "uphill" from near sea level to Atlanta at 1050 ft above, but I usually only lose 1 mpg, maybe 2 at most. Thus, I would have expected 24 to 25 mpg range, not 20 1/2 to 21 mpg. I did have the cruise control on both ways set to 79 mph.

 

I know I was hitting much higher fuel consumption on the track - average per the same computer was 9.5 to 10 mpg. I wonder if it is just an internal calculation skewd by this consumption rate, and if disconnecting the battery for 24 to 48 hours will reset the memory?

 

Any thoughts?

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Just a WAG (Wild Ass Guess), but your car's computer "re-learned" new fuel and timing tables during it's track venture and continued to use those same values on your trip back home. Give it a few more days of normal day to day driving and it re-learn to be a street car again and your mpg will return. Or it's something really seriously bad(LOL, j/k) :drop:

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I think I did re-set the gallons used, etc. on the dash, but I will double check and reset again this weekend. I was wondering if the computer has some sort of internal algorithm for that calculation that got skewed when I ran off a lot of gas in just a few short miles on the track. If so, I thought that maybe disconnecting the battery may cause the algorithm to reset and learn a new baseline or something.

 

Dans07SGT - how do you like your JBA Ceramic Shorties? Did you notice any diference in your SGT? I just had a set installed on my SGT in December.

 

Andy

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Or it's something really seriously bad(LOL, j/k) :drop:

 

 

LOL....that is what I was thinking the entire way home..."Holy cow - I killed it!" :rip:

 

 

The only noise that I kept hearing on the trip home was very faint...I turned the radio off to listen more closely...I could just barely make it out after about 10 miles. My car kept saying over and over again while driving on the highway...."80 is too slow....80 is too slow...80 is too slow"!! :shift:

 

 

Andy.

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I think I did re-set the gallons used, etc. on the dash, but I will double check and reset again this weekend. I was wondering if the computer has some sort of internal algorithm for that calculation that got skewed when I ran off a lot of gas in just a few short miles on the track. If so, I thought that maybe disconnecting the battery may cause the algorithm to reset and learn a new baseline or something.

 

 

 

Andy

 

Andy,

It will come back after you go through a tank or two of gas if you reset after each fill up . Disconnecting the battery will NOT change things , only after reestablishing a new baseline will it get back on course. It is commonly referred to as " Releasing the Thirsty Beast " . Unfortunately it takes a little longer to settle back down to a normal life once one has tasted the " track life " for both the car and the driver. You're experiencing the same things that every " Track Virgin " does. Just be glad there were not any sacrificial alters that the other drivers could have strapped you down on to/in to . ( like having to buy the beer or washing their cars ).

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I've never noticed a mileage drop after a day at the track and I watch mine like a hawk as I cannot afford to waste a drop. Every time you reset the computer via the dash bottons the mileage is 'reset', but the only way for the compute to return an honest number is to reset with every fill-up.

 

If the mileage has never been reset or hasn't been reset in many miles/fill-ups a day of blasting around a track isn't going to have a significant impact on the mileage.

 

My 1st time on a track with the SGT I filled the tank at the track and reset the mileage to see what it cost me. If I rember correctly I pulled down ~15mpg on the track (it's been awhile since a visit to the track so I maybe remembering wrong, the point is I was still impressed with mileage). I then filled the tank before heading home and reset again and saw the typical 25-26mpg on the highway.

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Albino500's comment is probably the case

The on-board computer generally averages the data over the past 3 tanks

For piece of mind calculate actual mpg the old fashion way

Or just do more track days and have a constant smile on your face

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Andy, why not just do a old pencil and paper computation. Fill it, drive it, refill and check the miles divided by gallons. This will be an accurate current average rather than some type of computer averaging. If you are averaging your previous around town mileage there is probably no problem. If you are experiencing a significant decrease there is a problem which you will want to address right away. An engine that is running too rich can be a real problem.

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Andy, why not just do a old pencil and paper computation. Fill it, drive it, refill and check the miles divided by gallons. This will be an accurate current average rather than some type of computer averaging. If you are averaging your previous around town mileage there is probably no problem. If you are experiencing a significant decrease there is a problem which you will want to address right away. An engine that is running too rich can be a real problem.

 

 

Agree. You'll know in a week or two whether your mpg has truly dropped.

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Andy, why not just do a old pencil and paper computation. Fill it, drive it, refill and check the miles divided by gallons. This will be an accurate current average rather than some type of computer averaging. If you are averaging your previous around town mileage there is probably no problem. If you are experiencing a significant decrease there is a problem which you will want to address right away. An engine that is running too rich can be a real problem.

 

 

Just for peace of mind, I'm going to take it out some this weekend if the weather cooperates and check it the old fashioned way. I figured the computer should give similar readings for before and after assuming it wasn't influenced by a rolling calculation type of algorithm. I figured the MPG was an instant calculation since last reset where the miles to empty is a rolling average base on the most recent 500 miles or so if I remember correctly.

 

I'll check it and see what I get and post results here....

 

Andy.

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Andy,

 

My fuel gauge went "wonky" after tracking. It's still weird, reading empty with a half-tank, then jumping to reading full after a shut down and restart. I think I screwed up the fuel sensor with the sloshing fuel or something, but I just keep an eye on the mileage and refuel as necessary!

 

Track mileage was considerably less than highway, but even it suffers under my lead foot!

 

Sam

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My 1st time on a track with the SGT I filled the tank at the track and reset the mileage to see what it cost me. If I rember correctly I pulled down ~15mpg on the track (it's been awhile since a visit to the track so I maybe remembering wrong, the point is I was still impressed with mileage). I then filled the tank before heading home and reset again and saw the typical 25-26mpg on the highway.

 

. 15 at the track! Holy cow, the last time I set mine at the track I averaged 4.9-5.5 mpg. Do you have some sort of on track super saver fuel economy mode. Lol.
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. 15 at the track! Holy cow, the last time I set mine at the track I averaged 4.9-5.5 mpg. Do you have some sort of on track super saver fuel economy mode. Lol.

 

 

Yes, I've rethought this since I posted. I cannot confidently recall the number as it been a couple seasons since I've been on the track (hope to get back on it this season), but I do recall being surprised by how much better it was than I expected. It definetly was not 4-5mpg. Obviously different tracks, conditions, and driving styles will dictate mileage.

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Uncertain if this may be helpful or not

DTE (Distance To Empty) calculated using running average mpg based on past 500 miles (where I was thinking 3 tanks but not used for average MPG calc)

No clue how instantaneous average mpg may or may not be related to DTE, but there are obviously static table values used to generate the various output data

 

Mustangfueluseagep66-67_zps47f1fc4c.jpg

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Thanks for the post Lgs ShelbyGT - there you go Andy . By the way , I get 4.5 MPG at the track and consistently 21-22 MPG for daily driving ( I can't drive 55 ). Also one other thing to note when tracking your car is NEVER let it go under a quarter tank on the gauge - makes the fuel injection very fussy - bad juju .

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Hey Guys,

 

Tell me if you think this could have anything to do with my loss of gas mileage. I just had the JBA Shorty headers and the Steeda UDP installed on Dec 21st. When I took it over there for the work, they were supposed to "tune" the vehicle (see my other post about "Going for 400 HP Naturally Asp."), charged me for it ($150 dyno time and $100 tuning charge), but said it tuned up fine and didn't need any adjustments. Between the installation and the date I left for the trip, I only put about 150 miles on the car. I found this out because when I added new tires, I had to reprogram the car for the slightly smaller diameter size to get the speedometer back to 100% accurate like the stock tires were. I had to call them to ask them if they made any tuning changes without using my SCT tuner...they stated that they didn't make changes to my tune.

 

I spoke to another shop who will be doing any future work for me, and he said that only getting 9 more HP is very low for adding both the JBA Shorty headers AND Steeda UDP, and probably because of not adding the proper tune. I told him about this decrease issue, and he said that the motor may be over compensating for the changes and now might be running rich. He said it could have taken the 300 mile range for the computer to start making substantial adjustments with the new equipment. He felt a new tune would get the HP improvements and get the fuel consumption back where it was normally in the 26 MPG range that I was used to getting.

 

I did notice that the exhaust tips were coated more than ever with black layer of film, which may indicate it is running a little rich. Also, one of the other drivers did note that when I hit the gas hard that I did leave a trail in my "wake".

 

That seems to make some sense to me....anyone have any thoughts?

 

Andy.

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Running rich will certainly decrease mileage. Running too rich will not only decrease mileage but will also have less potential for added HP. Tuners make HP by leaning the engine. Unfortunately, some lean it too much and then there is no safety margin, just like tuning for 93 octane. I would still try the old paper and pen method of miles driven by gallons purchased just to see how it is performing. Have you contacted Steeda re the tune? Have you tried a local tuner to diagnose?

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I'm getting lined up to see a local tuner who specializes in only Mustangs & Shelbys...probably week of Feb 11th. I figured that is the best way to figure this out and get it set correctly for my specific mods and keep it as safe as possible.

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  • 3 weeks later...

UPDATE -

 

I took my car to the tuner (LD's Performance) and Larry Day did a wonderful job on my car. The MPG has returned to normal -- averaged 24.4 mpg with combo highway/city driving.

 

He built me 3 different tunes.

 

1) 91 octane

2) 93 octane - moderate

3) 93 octane - aggressive

 

He set the 91 octane / most conservative in place for me when I left the shop with the vehicle and told me to try a different one each day over the next week or so and let him know my thoughts on the improvement.

 

Glad to see that everything is back to normal!!

 

Andy.

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