Just another MPG Thread!
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Funny enough, wide gaps actually increase efficiency IF the ignition is capable. It exposes more of the flame kernel. I have a 1997 S10 4.3L that was factory gapped at 0.060". Thing used to rattle like a can of marbles uphill in fifth on regular. I used to buy premium at 88 cents a gallon in 1998. Later models dropped the gap.
Tuning, fuel, and emission controls are different now, allowing for very good power ratings with economy and near zero emissions.
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Just bought a really nice 2013 XLT with the 5L engine, and was curious to see what kind of fuel mileage it gets.
Checked the odo by doing a measured 98 mile check, which showed 91.7 miles on odo, which is 7% off, giving a correction factor of 1.07.
1069 odo miles driven since first fill-up divided by 64.9 gallons gives 16.5 mpg.
Truck's mpg total shows 17.1 mpg.
Applying odo correction gives 1144 miles, divided by 64.9 gallons, gives actual 17.7 mpg.
This number includes a good bit of local suburban driving, three days of slow, congested Savannah driving, and 296 miles of 83 mph (less an hour or so of 15 mph) heavily-congested interstate driving.
The 277 (odo) 296 (actual) mile trip back (@83/15 mph) took 24.7 gallons, giving 16.7 mpg. A combination of very fast and very slow congestion doesn't give very good highway mileage!
Stable 80 mph reading on speedometer showed stable 83 mph on GPS, gives correction factor of 1.04.
Checked the odo by doing a measured 98 mile check, which showed 91.7 miles on odo, which is 7% off, giving a correction factor of 1.07.
1069 odo miles driven since first fill-up divided by 64.9 gallons gives 16.5 mpg.
Truck's mpg total shows 17.1 mpg.
Applying odo correction gives 1144 miles, divided by 64.9 gallons, gives actual 17.7 mpg.
This number includes a good bit of local suburban driving, three days of slow, congested Savannah driving, and 296 miles of 83 mph (less an hour or so of 15 mph) heavily-congested interstate driving.
The 277 (odo) 296 (actual) mile trip back (@83/15 mph) took 24.7 gallons, giving 16.7 mpg. A combination of very fast and very slow congestion doesn't give very good highway mileage!
Stable 80 mph reading on speedometer showed stable 83 mph on GPS, gives correction factor of 1.04.
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Joined: Mar 2026
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From: Pennsylvania
Since I'm new to this forum just wanted to post a couple variables in MPG on my 2022 FX4 King Ranch 5.0. Just bought it at 58,000 miles with the original plugs still installed. I tested it several times in very similar conditions and it averaged 16.8-17.6 on the same exact route on ECO mode.
I changed plugs the other day and gapped at .040 using OEM plugs. I let them seat in for a couple days and did the same test of about 14-15 miles same route with a full tank each time and the MPG averaged 18.3 to 19.7 after 2 tests. I would have to say that's a reliable increase to 4+ year old OEM plugs, I changed them at 61,500 miles.
I changed plugs the other day and gapped at .040 using OEM plugs. I let them seat in for a couple days and did the same test of about 14-15 miles same route with a full tank each time and the MPG averaged 18.3 to 19.7 after 2 tests. I would have to say that's a reliable increase to 4+ year old OEM plugs, I changed them at 61,500 miles.
What size and brand of tire are you running?
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Joined: Dec 2021
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From: Ellenton, FL
The lie o meter is inaccurate by about 7% optimistic, if it was never corrected.
If you used it before the plug change and are using it now then an increase in observed mpg is valid, but as said would be more accurate on a longer trip, and is best measured tank to tank.
Calculate the mileage after each fill up the old way to find a 100% accurate current value.
You can search here to find threads on accessing engineering mode to correct the lie o meter if you are so inclined.
Not trying to pick the flysh*t out of the pepper, just offering another viewpoint. I'm sure its doing better with the new plugs.
If you used it before the plug change and are using it now then an increase in observed mpg is valid, but as said would be more accurate on a longer trip, and is best measured tank to tank.
Calculate the mileage after each fill up the old way to find a 100% accurate current value.
You can search here to find threads on accessing engineering mode to correct the lie o meter if you are so inclined.
Not trying to pick the flysh*t out of the pepper, just offering another viewpoint. I'm sure its doing better with the new plugs.
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Joined: Sep 2023
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I read a few threads on this board when I got my truck and most settled at the correction value of 934, or 93.4% of displayed value. The difference? 6.6% . I'm sure the trucks in that cross section of samples varied in engine size, gearing, stock to lifted, larger tires, etc. so pretty typical.
I used the 934 correction value on my truck and it was dead on for city/mixed driving, and .1-.2 pessimistic for highway driving.
I offered this amount in approximation of 7% to the OP as a starting point because it was confirmed by several others and myself. Obviously if he's inclined he would have to start with hand calculations after several fill ups.
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Joined: Dec 2021
Posts: 362
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From: Ellenton, FL
Did you buy your truck new? Maybe the previous owner did the work.
I read a few threads on this board when I got my truck and most settled at the correction value of 934, or 93.4% of displayed value. The difference? 6.6% . I'm sure the trucks in that cross section of samples varied in engine size, gearing, stock to lifted, larger tires, etc. so pretty typical.
I used the 934 correction value on my truck and it was dead on for city/mixed driving, and .1-.2 pessimistic for highway driving.
I offered this amount in approximation of 7% to the OP as a starting point because it was confirmed by several others and myself. Obviously if he's inclined he would have to start with hand calculations after several fill ups.
I read a few threads on this board when I got my truck and most settled at the correction value of 934, or 93.4% of displayed value. The difference? 6.6% . I'm sure the trucks in that cross section of samples varied in engine size, gearing, stock to lifted, larger tires, etc. so pretty typical.
I used the 934 correction value on my truck and it was dead on for city/mixed driving, and .1-.2 pessimistic for highway driving.
I offered this amount in approximation of 7% to the OP as a starting point because it was confirmed by several others and myself. Obviously if he's inclined he would have to start with hand calculations after several fill ups.
Last edited by Grouper63; Jun 11, 2026 at 09:56 AM. Reason: additional info