12-15% Drop in highway and city/mixed driving MPG. What could be the reason?

[gmupatriot]

Fractionally taller tire means car actually travels MORE than the odometer registers. A small amount, but that and winter gas vs summer. How's your alignment? Was that checked when new tires installed?

I had the car tires balanced and alignment done a day before my trip last week. I had just installed the Steeda IRS Subframe Bushing Supports and Subframe Alignment kit, so an alignment was necessary. Here you can see the alignment sheet.

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[Paul McWhiskey]

At 54K You might gain a bit with a new set of plugs. At least pull them and reset the gap. The dream in the owner's manual of 100K plug changes is just that. Don't forget to check the air filter.

Then, after these checks, pay more attention to other things that are available on the instrument cluster.

Pedal to the metal!

 

[gmupatriot]

At 54K You might gain a bit with a new set of plugs. At least pull them and reset the gap. The dream in the owner's manual of 100K plug changes is just that. Don't forget to check the air filter.

Then, after these checks, pay more attention to other things that are available on the instrument cluster.

Pedal to the metal!

I will check the plugs. I been wanting to for a while but never got around it.

 

[ORRadtech]

People keep mentioning the winter gas, which is a thing. But he took the trip last year in the same time frame so that's probably not a big contributing factor. Weather may be though. This year was extremely cold and windy leading up to the holiday. I don't remember last year being as bad.
OP, possibly put a scan tool on and check fuel trims, catalytic converter operation, air/fuel ratios, sensors, misfires, etc that could be present but not enough to set codes.

 

[K4fxd]

Could be you are getting E15 fuel.

 

[gmupatriot]

People keep mentioning the winter gas, which is a thing. But he took the trip last year in the same time frame so that's probably not a big contributing factor. Weather may be though. This year was extremely cold and windy leading up to the holiday. I don't remember last year being as bad.
OP, possibly put a scan tool on and check fuel trims, catalytic converter operation, air/fuel ratios, sensors, misfires, etc that could be present but not enough to set codes.

I recently acquired forscan and the OBDLinkEX tool. The only thing I have done so far is reset the transmission learning tables. How can I use forscan to check fuel trims, cats etc? If I need to use another tool what would that be?

Thanks

 

[gmupatriot]

Could be you are getting E15 fuel.

I always use 93 from either Costco, Exxon and sometimes Sheetz.

 

[NightmareMoon]

Worst gas mileage I've seen so far was on a trip into a strong headwind. Terrible MPG.

How closely you follow traffic ahead is a noticeable factor. If there's nobody directly in front of you, driving feels great, but your gas mileage will be a few MPH worse. If you're following a large truck closely it will be much higher.

Driving at a higher average speed (above a point) or too slowly will be worse.

I have a very heavy right foot. Born with it. My gas mileage always sucks

If your trip includes lots of slowing down and speeding up (inconsistent bunched up traffic), well obviously that's going to impact your MPG a lot. Any use of the brakes or accelerator will reduce MPG averages.

I would expect tires to be worth 2-3%. Winter E15 gas could be a bit more than that even (ethanol is lower energy density than 'regular' gas, so it takes more for the same power).

Check your plugs and air filter, and then stop worrying. If the car seems fine, it probably is.

 

[gmupatriot]

Worst gas mileage I've seen so far was on a trip into a strong headwind. Terrible MPG.

How closely you follow traffic ahead is a noticeable factor. If there's nobody directly in front of you, driving feels great, but your gas mileage will be a few MPH worse. If you're following a large truck closely it will be much higher.

Driving at a higher average speed (above a point) or too slowly will be worse.

I have a very heavy right foot. Born with it. My gas mileage always sucks

If your trip includes lots of slowing down and speeding up (inconsistent bunched up traffic), well obviously that's going to impact your MPG a lot. Any use of the brakes or accelerator will reduce MPG averages.

I would expect tires to be worth 2-3%. Winter E15 gas could be a bit more than that even (ethanol is lower energy density than 'regular' gas, so it takes more for the same power).

Check your plugs and air filter, and then stop worrying. If the car seems fine, it probably is.

Thanks, you are right about speed. 90% of the driving was around 80MPH or 85MPH. I am sure if I did 70-75MPH my MPG would have increased. In addition, I did encounter slowing down and speeding up as people moved out of the left lane. Another thing I didn't consider is the drive through western VA and Tennessee is through very hilly region. I am sure if I was going on a rather flat section of the country I would be able to get higher MPG.

I know that MPG should not be my concern driving a V8, however, I think what got my attention was the lower MPG compared to what I have achieved in the past. Just wanted to make sure my car is healthy and is making full power, I mean that's what I paid for (460hp) and that's what I want to make sure its making!

 

[Takeshi]

Thanks, you are right about speed. 90% of the driving was around 80MPH or 85MPH. I am sure if I did 70-75MPH my MPG would have increased. In addition, I did encounter slowing down and speeding up as people moved out of the left lane.

I know that MPG should not be my concern driving a V8, however, I think what got my attention was the lower MPG compared to what I have achieved in the past. Just wanted to make sure my car is healthy and is making full power, I mean that's what I paid for (460hp) and that's what I want to make sure its making!

I mean IMO there is TOTALLY nothing wrong with wanting good MPG out of a Performance car. If your stuck in traffic and commuting ; are you really gonna use all that horsepower next to the Donuts and Coffee guy in the Panda Mobile next to the Highway?

Also Tires do make a good difference can account up to 1 MPG from my experience (if your average is 20 mpg that's a 5% difference). I had on average 26-27 MPG on Dashboard actual was 25.X MPG Max on all season tires. Versus 24.3 MPG Max on Micheline PS4S tires. With the PS4S I do notice I like to go harder on the throttle and corners just because the car could just take that much more beating to the tarmac with grip.

Another Factor is traffic conditions over the course of time I used the all seasons there was less traffic 2020's Pandemic roads were clear so not much inputs were needed. 2021 comes around and we have more stop and go.

Traffic, Temperature, winds and winter blend can account for the other 1 MPG = 5%

Overall I had to drive the Mustang like a grandma on Michelin PS4S summer tires to get 19.9 mpg avg fuelly tracked vs 20.9 MPG AVG fuelly tracked on the All seasons (driving fairly aggressively [Orange color helped]).

Overall I think your enjoying your car to the max of every possible aspect ; unless your in California then you really do start worrying about gas $$ and MPG.

 

[twbthird]

Too many variables for an apples-to-apples comparison. Worth noting that we made a trip (not in the Mustang) during the recent very cold weather and that car's usual ~25 MPG dropped to ~15-18 MPG driving into a strong head wind on an extremely cold day. Normal mileage obtained on the return trip on a warmer day with little wind.

 

[shogun32]

To spare you further anguish, I will be happy to take the keys off you.

 

[gmupatriot]

To spare you further anguish, I will be happy to take the keys off you.

 

[Qcman17]

I recommend scope the block, pull the trans and yank the diff. But most importantly disconnect the centre dash speaker it’s the most likely culprit.

 

[ice445]

Definitely put fresh plugs in it at this point and retest.

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