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The week of Thanksgiving, I drove from Atlanta to just south of Tampa (Parrish, Florida) on one tank of gas with about 20 miles to spare. Atlanta to Parrish is approximately 498 miles. I used cruise control and drove at a steady 80-85 MPH the entire way. There was little to no traffic. It was chilly in the 40s when I left Atlanta and in the 70s when I arrived. I stopped twice on the way down, both times to use the facilities, then got back on the highway. It was a straight shot down I-75 from Atlanta to Parrish. The car registered that I was getting just shy of 25 MPG.
Naturally, I thought I could make it back to Atlanta on one tank of gas, but I didn't. I filled up the night before, and left Parrish in 40 degree weather. Again, I used cruise control and drove at a steady 80-85 MPH the entire way with no traffic. No stops this time; drove straight through - well almost. The car this time registered that I was getting 21.4 MPG. I had planned to "test" the calculation of MPG shown on the dash and risk it, but chickened out and pulled over in Stockbridge, GA at the Costco to fill up. Stockbridge is about 30 miles south of where I live in Atlanta. It was in the 40s when I left Parrish and in the 30s when I got home.
Since I had about 6 hours while driving to contemplate the difference in MPG, I asked for the elevation difference between Atlanta and Tampa - it's about 1,000 feet. So, I don't think that was the issue. The temperature was about the same on both trips.
When I filled up in Parrish, it took 19.5 gallons.
When I filled up in Stockbridge, it took 19 gallons.
Anyone have a thought on why I could make it Parrish on one tank with two stops, but couldn't make it back to Atlanta on one tank?
The week of Thanksgiving, I drove from Atlanta to just south of Tampa (Parrish, Florida) on one tank of gas with about 20 miles to spare. Atlanta to Parrish is approximately 498 miles. I used cruise control and drove at a steady 80-85 MPH the entire way. There was little to no traffic. It was chilly in the 40s when I left Atlanta and in the 70s when I arrived. I stopped twice on the way down, both times to use the facilities, then got back on the highway. It was a straight shot down I-75 from Atlanta to Parrish. The car registered that I was getting just shy of 25 MPG.
Naturally, I thought I could make it back to Atlanta on one tank of gas, but I didn't. I filled up the night before, and left Parrish in 40 degree weather. Again, I used cruise control and drove at a steady 80-85 MPH the entire way with no traffic. No stops this time; drove straight through - well almost. The car this time registered that I was getting 21.4 MPG. I had planned to "test" the calculation of MPG shown on the dash and risk it, but chickened out and pulled over in Stockbridge, GA at the Costco to fill up. Stockbridge is about 30 miles south of where I live in Atlanta. It was in the 40s when I left Parrish and in the 30s when I got home.
Since I had about 6 hours while driving to contemplate the difference in MPG, I asked for the elevation difference between Atlanta and Tampa - it's about 1,000 feet. So, I don't think that was the issue. The temperature was about the same on both trips.
When I filled up in Parrish, it took 19.5 gallons.
When I filled up in Stockbridge, it took 19 gallons.
Anyone have a thought on why I could make it Parrish on one tank with two stops, but couldn't make it back to Atlanta on one tank?
What brand ond grade of gas did you use on each trip? I assume you used premium, but was one 91 vs 92 or 93 octane. Lower octane, 91 vs 93, doesn't do any damage to your vehicle, the ecu just retards timing slightly to compensate, but that smal timing change could explain the difference in effeciency. The brand of gasoline sholdn't make any difference with gas mileage either, even if you used non Top Tier gas on the drive home. And I assume your tire pressures and load was the same, meaning no extra luggage or things in the trunk, just the turkey...
What brand ond grade of gas did you use on each trip? I assume you used premium, but was one 91 vs 92 or 93 octane. Lower octane, 91 vs 93, doesn't do any damage to your vehicle, the ecu just retards timing slightly to compensate, but that smal timing change could explain the difference in effeciency. The brand of gasoline sholdn't make any difference with gas mileage either, even if you used non Top Tier gas on the drive home. And I assume your tire pressures and load was the same, meaning no extra luggage or things in the trunk, just the turkey...
All good questions. I filled up at Costco in Atlanta with premium before I left. Once I got to Parrish, I filled up with premium at a Racetrac only because I didn't have enough range to make it to Costco in Bradenton. I topped off my tank at Costco the night before I left to come back.
I may have had a few extra pounds of stuff in the trunk on the way home. Tire pressure was unchanged on both trips; 35 psi all around. A friend suggested that there may have been a southerly breeze on the way down as there was a cold front pushing through about that time. While that is plausible, I'm not certain a breeze could account for almost 3 MPG difference, but maybe so?
This was my mileage on my last trip where I reset it and watched it. This includes 3,000 feet of mountain climbing on the way there and coming down those mountains on the way back. Cruise set to 80, but there was traffic on I-81 which brought the average speed down.
It's slippery, very efficient at speed, size considering. Varying speed on the other hand, is gonna hurt. Driving from the Whidbey house to Seattle (and vice versa) would give me numbers that varied by 3 or 4 miles per gallon, based on the time of day. Head down in the evening at a nice steady cruise controlled speed, I'd get my best numbers. Going the other way in the afternoon traffic, those numbers go down, how much depended on traffic.
Head and tailwinds are no small matter. Frontal winds on a slight oblique are the worst since you maintain roughly the same frontal area but add the cross wind component, increasing rolling resistance. There are a lot of ways to quantify the mpg loss; but you're looking at 4 times the energy expended per doubling of speed when overcoming aerodynamic drag. You typically aren't doubling speed but are getting some fraction of that working for, or against you. Considering the average wind speed in the US is 7 mph, it's meaningful even where there is seemingly little wind, and significant when the winds increase.
@SW20S Excellent numbers. I never saw that, my best was 26. That's an actual at the pump number, I'm old, still like to do the math. Not that the car wasn't getting it right, it usually closely gibed, in between the pump numbers and what it was saying in the display.
It's slippery, very efficient at speed, size considering. Varying speed on the other hand, is gonna hurt. Driving from the Whidbey house to Seattle (and vice versa) would give me numbers that varied by 3 or 4 miles per gallon, based on the time of day. Head down in the evening at a nice steady cruise controlled speed, I'd get my best numbers. Going the other way in the afternoon traffic, those numbers go down, how much depended on traffic.
Head and tailwinds are no small matter. Frontal winds on a slight oblique are the worst since you maintain roughly the same frontal area but add the cross wind component, increasing rolling resistance. There are a lot of ways to quantify the mpg loss; but you're looking at 4 times the energy expended per doubling of speed when overcoming aerodynamic drag. You typically aren't doubling speed but are getting some fraction of that working for, or against you. Considering the average wind speed in the US is 7 mph, it's meaningful even where there is seemingly little wind, and significant when the winds increase.
@SW20S Excellent numbers. I never saw that, my best was 26. That's an actual at the pump number, I'm old, still like to do the math. Not that the car wasn't getting it right, it usually closely gibed, in between the pump numbers and what it was saying in the display.
Agree on the wind impact assume constant wind velocity and direction it would have a doubling effect from one direction to the reverse and at 80 plus mph, as you point out it is a squared function.
I have not heard anyone speak of the warmer to colder weather in the beginning then the colder weather (constant) back to ATL. Colder air, as you all know, bears significant changes (delta) in MPGs when the starting weather is cold (engine taking way too long to warm up; this was also one of the recent MB recalls). I would like to know if you factored in the weather change. You can research it; I live in the NE corridor and during the winter months, my gas mileage is atrocious.
This was my mileage on my last trip where I reset it and watched it. This includes 3,000 feet of mountain climbing on the way there and coming down those mountains on the way back. Cruise set to 80, but there was traffic on I-81 which brought the average speed down.
This is the average over both legs of the trip.
I notice on your display it was hot then - 95 degrees - where it was in the 30s for me. I know gas expands as it gets warner, so I would think this would work against your milage. Also, I thought our ICE cars got better gas milage as the temps drop?
I should have mentioned in my original post that I drove in ECO mode both ways. Not sure how much that adds to MPG, but thought I'd mention.
crabman & DonS550 pointed out 2 very important factors in fuel mileage. I like how crabman pointed out the crosswind enigma. To further his point, a direct crosswind would add a headwind component, because if you didn’t correct for the crosswind, you would be blown off the side of the road. So what do you do to correct? You crab, man.
As for Don and the density of air in high pressure systems. Not only do they make the air thicker, and increase drag, they make the fuel consumption go up, due to stochiometry.
Denser air has more oxygen molecules per volume. Hence the fuel air mixture has to be richer, to obtain peak combustion conditions.
Exactly! colder air can cause fuel economy to drop anywhere from 24% to 34%. One more factor is the oil viscosity as the oil can take more energy to pump throughout the engine, hence increasing fuel consumption.
I notice on your display it was hot then - 95 degrees - where it was in the 30s for me. I know gas expands as it gets warner, so I would think this would work against your milage. Also, I thought our ICE cars got better gas milage as the temps drop?
I should have mentioned in my original post that I drove in ECO mode both ways. Not sure how much that adds to MPG, but thought I'd mention.
No just the opposite which is an ingredient to his numbers which given the difference in the temperatures are a bit apples to oranges.
I notice on your display it was hot then - 95 degrees - where it was in the 30s for me. I know gas expands as it gets warner, so I would think this would work against your milage. Also, I thought our ICE cars got better gas milage as the temps drop?
I should have mentioned in my original post that I drove in ECO mode both ways. Not sure how much that adds to MPG, but thought I'd mention.
Absolutely that plays a role. I would expect the same trip right now with averages in the 40s I would get 25-26 MPG. My concern is his reported 21, that seems very low...and his temps were in the 70s...so not cold
All good questions. I filled up at Costco in Atlanta with premium before I left. Once I got to Parrish, I filled up with premium at a Racetrac only because I didn't have enough range to make it to Costco in Bradenton. I topped off my tank at Costco the night before I left to come back.
I may have had a few extra pounds of stuff in the trunk on the way home. Tire pressure was unchanged on both trips; 35 psi all around. A friend suggested that there may have been a southerly breeze on the way down as there was a cold front pushing through about that time. While that is plausible, I'm not certain a breeze could account for almost 3 MPG difference, but maybe so?
I would be more inclined to believe there was a greater concentration of alcohol in the fuel that resulted in lower mpg. More alcohol = reduced mpg
Absolutely that plays a role. I would expect the same trip right now with averages in the 40s I would get 25-26 MPG. My concern is his reported 21, that seems very low...and his temps were in the 70s...so not cold
The only thing I can think is the quality of the gas from Racetrac? I always fill up at Costco, but had to go to Racetrac when I arrived because I didn't have the range to make it to Costco. Racetrac is a reputable connivence store-style establishment, so I want to believe I got good quality Premium gas there - but maybe not?
[QUOTE=MBNUT1;9075127]No just the opposite which is an ingredient to his numbers which given the difference in the temperatures are a bit apples to oranges.[
An engine does run more efficiently when it is hot hence better gas mileage. However working against that is that the engine has to turn the A/C compressor to get cool air into the car while using the heater is almost free since the waste heat from the engine is used to warm the car. Where the increased hot efficiency and the power consumption turning the compressor balance out is a question to be considered. My uninformed guess would come down on the side of the A/C compressor being a net minus for gas mileage.
As for the temperature being apples to oranges, a dip in temps from the 90s to the 70s shouldn't account for a 7 MPG drop...we're not talking about him driving in -20 here.
As for the temperature being apples to oranges, a dip in temps from the 90s to the 70s shouldn't account for a 7 MPG drop...we're not talking about him driving in -20 here.
From the post "It was in the 40s when I left Parrish and in the 30s when I got home." Which btw is the trip where he got the reduced mileage.
That's correct - On trip down from Atlanta to Parrish, it was in the 40s when I left Atlanta and in the 70s when I arrived in Parrish. It was the trip down where I was able to make it on one tank. It was the return trip back to Atlanta where I couldn't.
A cold front came through while I was in Florida. It was in the 40s when I left Parrish and was in the 30s when I arrived back in Atlanta.
Given all that you guys have suggested, seems like it was either the gas from Racetrac; the temperature warm-up from Atlanta to Parrish on the trip down; that fact there was a likely a southerly breeze due to the cold front blowing through on the way down; or a combination of all three.
There are so many factors, combined, they would easily account for the difference.
If I'm reading this correctly, you're more curious, than having a real concern. Probably a good way to look at it. What I see is someone who enjoyed a nice drive in the most comfortable car I've ever owned. IMO, no animals were killed in the making of that movie, so everything is right in the world, until such time as you see aberrant mileage numbers that say otherwise.
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