ConocoPhillips vs Mobil ULSD

[invest3]

I filled up at ConocoPhillips in St. Louis for my 651 mile road trip to northern Michigan, averaging 47.5 mpg running @ 75 mph, no a/c. I filled up at Mobil for the ride home, averaging 50.8 mpg running @ 75 mph, same route. The only difference was the air temp was about 10 degrees warmer coming back but I was also running the a/c for the last 350 miles. My TDI has a ventectomy so I know the #'s are accurate. What gives?

 

[sparkplugg]

I filled up at ConocoPhillips in St. Louis for my 651 mile road trip to northern Michigan, averaging 47.5 mpg running @ 75 mph, no a/c. I filled up at Mobil for the ride home, averaging 50.8 mpg running @ 75 mph, same route. The only difference was the air temp was about 10 degrees warmer coming back but I was also running the a/c for the last 350 miles. My TDI has a ventectomy so I know the #'s are accurate. What gives?

ExxonMobil in Houston is selling B5. Don't know about conoco phillips. Not saying this is why, but just a potential difference that may have some significance here.

 

[jettawreck]

Perhaps traffic and headwind/tailwinds would easily account for such a difference. I doubt its the fuel source. Moderate headwind vs tailwind will make a substantial difference.

 

[invest3]

Perhaps traffic and headwind/tailwinds would easily account for such a difference. I doubt its the fuel source. Moderate headwind vs tailwind will make a substantial difference.

I also had my sunroof vent open and windows half way down for the leg up to Michigan. I'm wondering if that could have penalized me 3.3 mpg?

 

[ruking]

I filled up at ConocoPhillips in St. Louis for my 651 mile road trip to northern Michigan, averaging 47.5 mpg running @ 75 mph, no a/c. I filled up at Mobil for the ride home, averaging 50.8 mpg running @ 75 mph, same route. The only difference was the air temp was about 10 degrees warmer coming back but I was also running the a/c for the last 350 miles. My TDI has a ventectomy so I know the #'s are accurate. What gives?

Any one of a number of variables and/or a combination of some to ALL of them can be responsible. Given your description of conditions FUEL brand difference is probably not a cause. Now this is NOT to say it plays NO part. It is very unclear what that percentage IS (if any)

 

[MikeMars]

... I'm wondering if that could have penalized me 3.3 mpg?

Yes, easily.

 

[Rual817]

Gotta also remember ...you went North. You went slightly up hill, to drastically up hill.
StL is 460ft
GB - 581, Frankfort - 633,
Groveland - 988
Crystal Falls - 1477
elevation travel alone is definitely enough to be the deciding factor, but there are definitely other factors as well to consider.

 

[LarBear]

I've read on this forum about folks who'd apparently rather drown in their own juices than turn on the a/c, but from personal experience and reading sources I have reason to respect at highway speeds the a/c doesn't cause more drag on the engine than open windows. Modern vehicles are designed as a whole for low aerodynamic drag. When the windows are down, even partially, they're increasing drag, that's what all the noise and interior buffeting is indicating. If you want a real treat roll the back windows down with the fronts up at 60-65 mph.

Yes the a/c uses power, but increasing the drag coefficient by 5 or 10 percent or more likely requires more power to push the car through the air. Just doubling speed from 30 to 60 mph requires four times the power to push the vehicle down the road everything else being the same.

I used to drive from Seattle to Vancouver, BC, regularly, and mileage one direction was almost always higher than the other direction, although the speeds were the same. AND, a headwind does more harm to mpg than a tailwind helps usually.

 

[JohnWilder]

Power versus drag

I've read on this forum about folks who'd apparently rather drown in their own juices than turn on the a/c, but from personal experience and reading sources I have reason to respect at highway speeds the a/c doesn't cause more drag on the engine than open windows. Modern vehicles are designed as a whole for low aerodynamic drag. When the windows are down, even partially, they're increasing drag, that's what all the noise and interior buffeting is indicating. If you want a real treat roll the back windows down with the fronts up at 60-65 mph.
Yes the a/c uses power, but increasing the drag coefficient by 5 or 10 percent or more likely requires more power to push the car through the air. Just doubling speed from 30 to 60 mph requires four times the power to push the vehicle down the road everything else being the same.
I used to drive from Seattle to Vancouver, BC, regularly, and mileage one direction was almost always higher than the other direction, although the speeds were the same. AND, a headwind does more harm to mpg than a tailwind helps usually.

At low speed, rolling friction and other things dominate. At higher speed air drag dominates. Probably about 40 or 45 mph is where the air drag overtakes the other sources. When speed of a body is doubled in a fluid (everything else being constant) the force required is a square function (4 times), however power required is a cubic function (8 times the power is required to move a body through a fluid at double the speed). Diesel engines do not loose much thermal efficiency when throttled. (Actually most diesels deliver their best T/E at half to three quarters of full throttle. Spark ignited engines deliver their best T/E unthrottle) For this reason the fuel penalty at higher speed is very noticable. A head wind or a tail wind will have a very large effect.

 

[jettawreck]

Moderate headwind/tailwinds will make more of a difference than AC use (or not).

A few hundred feet in elevation change in a couple hundred miles shouldn't really make much of an impact.

 

[bobbiemartin]

A while back I was able to speak to a guy that works for a large regional petroleum distributor. In the course of our conversation, I asked him about Diesel, cetane ratings and the difference between brands. As he explained it, none of the major brands cared about Diesel fuel, they pretty much sold what was available. He claimed all the Diesel sold in this area (and I'm guessing most areas) came from the same source. He did warn that some places didn't really watch for algae and/or contaminants in their storage tanks, but other than that it was all the same. He told me that several of his commercial accounts had requested cetane ratings, but that it was virtually impossible to determine. The wholesale suppliers bought from several sources and the cetane ratings varied. Unlike gasoline octane ratings, the distributors didn't care about Diesel cetane, so the wholesalers didn't care and the suppliers didn't care. Therefore, there is no reason to supply a consistent product. Basically his contention is that even day to day, the cetane rating (at the wholesale level) changes. So just because you always buy at gas station X, you are receiving a changing product. The way it was explained to me, virtually every tank is different and varies depending on where it originally came from. So it is not surprising your mileage/performance varies.

 

[jettawreck]

I buy fuel at several different locations and my mileage numbers are very consistant tank to tank seasonaly. Day by day, it varies due to temps/wind/etc, but the averages work out predictably.