Miles Per Gallon Drop

[Ruby5.0]

So today I decided to change the oil for the first time since buying the car new back in April. I was going to change it after the first 500 miles to a full synthetic like Royal Purple but was advised by them to make sure the rings were fully seated before going to their oil, which they recommended be around 2K miles. So at around 2200 miles I made the switch to 5W20 standard Royal Purple with a Motorcraft oil filter. I also reset the oil tracking gage to 100% so I am starting with it set right. I took the car for a drive today of about 70 miles and had filled it up Friday night. I noticed today my MPG dropped significantly. As in about 14-15 MPG. Driving the same road during the week, I usually get 21-22 MPG. Granted there were some more red lights, but nothing to account for that much of a drop. BTW I have an auto trans with the 3:15 axle, so MPG should be good. Has anyone else seen the MPG drop like this after their first oil change ? I will be getting an after cat exhaust, CAI and tune soon, but want to get this MPG issue resolved first. Thanks for the help guys !!

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[GTPONY]

Do the math, don't rely on the computer to tell you MPG.

 

[sm32589g]

gees I'm getting like 13 mpg with manual PP but then again my daily commute is mostly 2 miles stop and go then 1.5 miles cruise at low speed. eh...and somewhat heavy foot.

 

[thewoodscrew]

my dash display is usually 1-1.2 mpg higher than my hand calculated mileage

 

[JohnDoe]

Don't use the computer

 

[randyGT]

My computer is pretty dead on..... I like it.

my dash display is usually 1-1.2 mpg higher than my hand calculated mileage

 

[Ruby5.0]

Your right guys, I should just do the simple math to see what the actual mileage is. Since I was using the previous computer mileage as done since day one, I figured it would at least get me a good baseline to work from. But when I saw the mileage drop like that with a new oil change and especially with Royal Purple oil, it was a shocker. I wasn't even driving fast or hard. I guess I can use math going forward, but the only reference point with the factory oil and driving is with the computer. I have a feeling 25mpg is gonna be a hard thing to do regardless of how nice you drive the car.

 

[ConspicuousConsumption]

The 'don't rely on the computer' guidance doesn't make sense in a machine nearly entirely managed by a computer. You know?

 

[GT Pony]

My computer is pretty dead on..... I like it.

Mine is pretty close too. The dash MPG has been consistently reading ~5% higher than hand calculated MPG. That's as close as I've ever seen it be in any car with the on-board MPG displayed.

 

[danny0441]

same thing happened to me before my first oil change i was getting 19 mpg on my daily commute now im i cant make it to 18 mpg i seem to be at 16-17 which is not a big deal 20 bucks still gets me through monday-friday but still its kind of annoying oh well hwy mileage changed as well i could easily do 30mpg cruising at 62 mph now at 62 i get about 27 mpg (first world 5.0 problems)

 

[Ilovemyv8]

Waited til it hit 5000 miles and changed it with Pennzoil platinum. Mine is a auto gt. My mpg has not changed at all..... around 24 average. :clap2:

 

[dcrypter]

same thing happened to me before my first oil change i was getting 19 mpg on my daily commute now im i cant make it to 18 mpg i seem to be at 16-17 which is not a big deal 20 bucks still gets me through monday-friday but still its kind of annoying oh well hwy mileage changed as well i could easily do 30mpg cruising at 62 mph now at 62 i get about 27 mpg (first world 5.0 problems)

I was pretty much the same. I started off at about 19 then slowly crept down to around 18.3~ at which point the blower went on and it crept down to around 17.5~ and now it has gone down to about 16.3. I didn't buy a 5.0 for the gas mileage though(nor the blower heh) and TBH my '12 5.0 bone stock was only getting 16.5 so I'm not concerned.

 

[mjhousto]

I have an auto GT with 3.55 gears. First oil change was at 5,700 miles. The dealer put in the recommended semisynthetic 5w20. I was averaging around 22.4 on the factory oil and noticed a bump to 23 mpg after the change.

 

[jrt6]

my dash display is usually 1-1.2 mpg higher than my hand calculated mileage

Looks like its the same case for me as well. I use Fuelly to keep track of my MPGs, and right now my fuelly average says 16.6 MPG, but the car computer says 17.7.

 

[Dawg]

It's likely just coincidence that it was timed with your first oil change, but refinery problems have caused a switchover to winter blend a few weeks early this year. Winter blend has less energy, hence less mileage.

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