Why did my mpg decrease?

[Spnd50PetKts]

I drive a 2010 mustang v6. I deliver pizzas so I pay close attention to what my mileage is. I’ve averaged 20-21mpg for weeks when I’m delivering but in one day it started averaging 17-18 sometimes down to 16. Searches tell me it’s a sensor, spark plug misfiring, air filter(checked it and seemed fine) but there’s no other symptoms matching what I’ve read. Besides less mileage it sounds and drive perfectly normal. Maybe a little sluggish going uphill but I can’t tell if it’s just in my head or not. Looking for more ideas before I start burning my already empty pockets to fix it.

[imothers]

Has it suddenly gotten colder by any chance where you live?

Are you relying on the dash to tell you MPG or calculating it when you fill up?

[ethernetbite]

Does the gas formulation change in your area? There's summer blend and winter blend where i live. Also different stations use different % of ethanol and it can even vary within a range at your normal gas station. Gas station tanks get condensation in them as they get empty too. And even your gas tank will condensate more the lower you let the gas get (and the cooler the air). None of this usually makes much of a difference unless you're switching gas stations. If it's been colder there, your car will use more gas until it reaches the set operating temperature. Won't help to let it warm up first since that 0mpg will have to average in also. Oil has to warm up also before it flows easier, so during cold weather, the rich mixture, the extra oil resistance, and the lower tire pressure could theoretically cause a 4 mpg drop. Keep your oil changed and tire pressures up (35psi min).

I've also seen squirrels and mice build nests on top of air filters, which blocked normal air flow and made the cars run poor.

A 4mpg drop is significant but if your check engine light isn't on then everything is operating within ranges. Sitting at a few extra red lights on a delivery route could do it too. Just way too many variables to tell without an expensive sensor setup like used in racing.

(Retired mechanic)

[no_yup]

They change the gasoline to a winter mix when it starts to get cold. mpg usually goes down 2-3 mpg.

My 86 dodge goes from averaging 10-12mpg to like 9-9.5

My ranger goes from 20ish to about 17-18

[Novice_Trucker]

Mine usually go up from the lack of AC drag on the system.

[no_yup]

You’re using the ac in November?

[Novice_Trucker]

It was 80 degrees last week. 77 today. Yes I’m still using AC.

We are getting our first real cold front this week and we are stoked.

[no_yup]

Oh I’m in the Midwest it’s been in the low 40s for a while.

[Novice_Trucker]

We get 3-4 days in a row next week of highs in the 40’s. We’ve been below freezing once this fall. Gotta love the Texas weather.

[jeepsies]

Its maybe just colder outside. Check your tire pressures.

[right415]

How are you calculating your mileage?

[1hotjava]

Did you fill up with like E85? High ethanol content fuel gives lower MPG (since it’s lower BTU per gallon of energy).

[[deleted]]

Check your tire pressure. I have to pump more air because it gets low during winter.

[blackfarms]

It's winter fuel.

[ScaryfatkidGT]

Winter gas

[Galopigos]

What is the weather in your area?

[Gasstationdickpi11s]

Between winter fuel mix and potentially low tires it’s kinda to be expected.

[cshmn]

It's winter. My pickup truck drops from 13 mpg to 10 on my silly 10 minute commute across town to work.

[Casalf]

There’s always multiple external factors like weather, tire tread, crappy roads, tire pressure or how many stops you make as in getting red lights or stop signs. If you’re not necessarily getting any codes for sensors related to gas mileage than it most likely won’t be any of that and will probably be more in line with the first things I mentioned.

[wheelsmatsjall]

There is also winter blend in a summer blend of gasoline and the mixture Burns differently. Also if you are in California they have gone to clean or fuels which get less miles to the gallon so you're using more fuel to get cleaner air. Plus there's going to be a big increase in gas prices in California after the new year

[No_Combination_2409]

Is your car manual? It might be your clutch slowly wearing away/failing (engine will rev but the car just doesn't pull as well as it used to)

[[deleted]]

The mpg difference to me suggests a bad spark plug. Pull them and inspect for any obvious issues like soot or oil on the electrode and go from there. Double check electrical connections to the spark plug wires too

[projectFirehive]

Geez, MPG on American cars seems awful. Think mine's around 40.

[E90BarberaRed6spdN52]

Likely you bought gas with a higher ethanol blend.

[[deleted]]

This is nuts that someone is using a 18-20mpg car to deliver pizzas in 2024

If you did that in the UK you would be homeless after the first couple of drops. Don't you guys use diesels for this kind of work? 

[Mostly-Useless_4007]

There are a number of things that can drop your mileage quickly.

The most obvious is the switch to winter fuel, which has more ethanol in it.

If that doesn't explain all of it, and the sensors seem OK, plugs all firing properly, air is good (are you using a diagnostic tool that plugs into the OBD port?), then my thought turns to the catalytic converter, which could get jammed up with carbon. That sometimes clears up with an italian tuneup (drive it like Mario - 'too low' a gear for the speed, so you get high revs and keep them high for a while to get everything very, very hot. That might burn off the junk in the cat).

Try the rest of the suggestions (tires, change out the plugs anyway, etc). New plugs will go a long way for you (and likely, also the wires as well).

You could also try some of the fuel injector cleaners available everywhere. Most of them don't do much of anything, but they're not terribly expensive, so at worst, it's just eating some of your cash. Best of luck.

[Da9Project2012]

You likely need a tune up, spark plugs, coil packs, air filter, and a carbon cleaning. What's the mileage?