MPG went down- QUICK!

BigPip

BigPip

Well-Known LVC Member
Joined
Feb 12, 2011
Messages
206
Reaction score
8
Location
Central Fl
Back in July I took a 12 hour trip (one way) and we averaged 24 mpg and a few weeks ago, we took a 3 hour trip (one way, again) and we averaged 22-23 MPG. This morning we were in the car for 5 hours and we could not beat 18.6 mpg. What happened?

I filled up with Shell Gas before I left and about 1/2 way here I filled up again to confirm the digital read out was correct and it was.

What is causing such bad MPG so quick? It is still running great.
 
Lots of things could have happened. Let's start with how the car was operated in conjunction with the computer.

Was the computer reset consistently through the three tests? The computer uses the 500 mile average unless you reset while in motion to see a brief actual mpg measurement. The longer on the highway will increase your average mpg until 500 miles have been travelled and from there it will not change much until you start city driving.

Were you driving with or into the wind? My person experience shows wind is a big factor. Speed is the other factor. It is all about the drag.

Hilly terrain and cruise control can wreak havok on your mpg. 'Deadfoot' driving will get you better fuel mileage in hills than the cruise control.

Temperature is a factor becasue air density changes. denser air more drag.

Load in the car the same for your 3 tests?

Was the car driven in the same manner each trip? You did not let a teenager take your car did you?

Cars are rated for mpg at 55mph with a 150lb. driver on level ground in the summer.
 
So, cruise control even on a flat terrain would cause more fuel consumption,how so?
 
That is not what I wrote, but yes a steady throttle will get you better gas mileage than cruise because cruise varies throttle position to keep a steady speed vs. a steady throttle and varying speed.
 
Are you just going by the dash? Thats for amusement only.

24 is not exceptionally good, and 18.6 is not exceptionally bad. Something might be wrong, but different conditions and speed could also easily explain it.

Is the transmission working ok, shifting and locking up etc?
 
Dash was showing 18.4, gas receipt with some math showed 18.1.

SAME driving habits, 76-78 mph steady With Cruise Control (again and always!)

After I ran the car just about empty, I filled it up with Mobil Gas and I am back at 23.8 mpg hwy (same driving habits,again). Say what you want but I will never use Shell Gas again.
 
First of all you are not concerned about getting good gas mileage by driving that fast. 24 mpg (US gallon) at over 70 mph is darn good. Does sound like you got some bad gas and you should have noticed your car running rough. I use Shell gas all the time where I am. I avoid Petro Can and other independent stations. They dont move enough gas and with the ethanol collecting water it gets contaminated.

Personally, I set my cruise by motor rpm keeping it under 2200 rpm. This gives me around 67 mph depending upon wind direction. My averages when commuting 1000 miles a week were 27 mpg.

The dash displays are really quite accurate. The gas pumps are going to vary more than the computer.
 
bradyelich said:
First of all you are not concerned about getting good gas mileage by driving that fast.
Click to expand...

Well, that was obvious when I bought a 275hp V8- I sold my Corolla to keep my Lincoln, I love this car, I don't mind the less than stellar mpg. My point was the mpg dropped considerable with the same exact driving conditions and habits.
 
I had a similar drastic drop in MPG and stalling after a 16+ gallon bad gas fill-up. 2-3 tanks of Sunoco Ultra cleared it up.

I usually get low 20s MPG, but typically run >>80 MPH.
 
bradyelich said:
First of all you are not concerned about getting good gas mileage by driving that fast. 24 mpg (US gallon) at over 70 mph is darn good. Does sound like you got some bad gas and you should have noticed your car running rough. I use Shell gas all the time where I am. I avoid Petro Can and other independent stations. They dont move enough gas and with the ethanol collecting water it gets contaminated.

Personally, I set my cruise by motor rpm keeping it under 2200 rpm. This gives me around 67 mph depending upon wind direction. My averages when commuting 1000 miles a week were 27 mpg.

The dash displays are really quite accurate. The gas pumps are going to vary more than the computer.
Click to expand...

Gas pumps are CERTIFIED and its ILLEGAL to be wrong fyi. The gallons readout is dead nuts.

My truck is a 2010 F250 with a 6.8L v10 gas motor. At 80 mph I get the same as 70 mph on the highway - 14 -15 mpg non stop. With my enclosed trailer its 10-12 MPG. With the trailer I can do 65 - 85 with no difference in mileage at all 10-12 mpg based on terrain.

Shell gas blows also, I get 2 mpg less in the truck with Shell gas vs BP, Mobil or Marathon.... its almost as bad as KwikFill or Country Fair lol.

When driving in the truck I have to do everything I can to get better mileage... k&N air filter = 1 mpg better avergage. Mobil gas = 1-2 mpg better average. Tires properly inflated are a BIG factor in MPG as well.

Running the GUMOUT brand WHOLE FUEL SYSTEM CLEANER every tank or every other tank I notice a HUGE difference in mpg. I mean like 2 mpg in my truck. Its $8.99 but it makes this crappy new gas run so much better!

Also, if I run 87 octane I get better mpg than 89 or 93 in my truck.... I experimented the whole way last road trip 700 miles out then back. 5 different fill ups lol. The brand of gas matters alot though.
 
Getting back on topic.... If your car sits for a long period of time before you use it and you dont run some kind of fuel treatment for the ethanol the gas breaks down and you get crap mileage as well.

Things to check when mpgs are low:
*Tire pressure
*Tire wear (looking for alignment problems)
*Brake calipers hanging up (touch the rotor if its blister your finger hot its not right, it should be warm/hot but not horrible - happens to me alot... mud lol)
*Spark Plugs - If the car sits they can get rusty/corroded from these crap fuels!
*Brand of gas - Sometimes smaller names like Marathon get better mileage than Shell
*Octane - Does your car require Premium and you put in regular 87? lol It happens Especially if the wife fills up the car lol ..... ughh
*Oil changes - when was your last oil change!!?? Running a full synthetic like Mobil 1 you can go 6000 - 8000 Miles before changing but running cheaper oils you will need to change every 3000 - 5000 miles to keep your motor in top shape. Lots of people get dollar store oil and wait 8000 miles before changing it and wonder why its like sludge in the oil pan.... :(

This is a good start... There are other factors as well but thats a pretty good check list... a sudden huge drop I normally find I had mud in my brake caliper pad slides and it caused the pad to hang against the rotor ... take apart sand, clean, re lube and re assemble..... went from 9.5 mpg to 14 mpg one road trip!!!!!
 
I've read in I believe an issue of Car and Driver something that said winter gas is less efficient than summer gas. I'm rarely on the interstate, so my numbers look real crappy but in the summer I was averaging 13.8 now that it is winter I've been hovering right at 12 flat. This is 95% city driving. Also I did a tuneup last summer and always run 93 octane premium from BP.
 
So what if it is illegal to have altered gas pumps - it is illegal to SPEED as well. The pumps only stay certified 1 in 10 tested.

My testing shows 20% decrease rate in mileage for every 10 mph faster than 55 mph. If you speed and talk fuel mileage you are a contradicting yourself. I drive for fuel mileage and I get better than what my car is rated and that is my goal. I buy less fuel, tires, oil, brakes, etc. becuase of my driving skills.
 
lol gas pumps are certified for a reason, they get re-certified and random checked as well - they are accurate. The stupid read out in the dash isnt as accurate as the amount of fuel dispensed into the tank.

Winter driving generally gets better mpg... your driving slower and your less on the throttle trying not to spin in general.

In NY, winter fuels have anti-freeze in them like the HEET products that contain alcohol so they dont freeze. I have no idea if this impacts fuel mileage.

In NY the gas pumps are dead accurate. Maybe if your filling up in some podunk inbred hillbilly ass little redneck town in the middle of nowhere at Joes gas station you might get a crooked pump.... or on the Indian reservations where water is added as well... lol
 
You must log in or register to reply here.

Members online

No members online now.
Total: 439 (members: 0, guests: 439)
Back
Top