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Spektyr · 08-03-2022, 09:57 PM

I just completed a round trip from Wichita, Kansas to Dallas, Texas and noticed a hard-to-explain jump in MPG on the return trip.

Starting from a full tank I drove south on I35 to the Sunoco in Pauls Valley, Oklahoma (because it has 93 octane) and then from there on to the Buc-cee's in Denton. On the return trip I filled up at Buc-cees and stopped at the same Sunoco in Oklahoma.

Same route both ways. Drove down on the 1st, back today (the 3rd).

On the way down the first leg (219 miles) averaged 28mpg, and the second leg (114 miles) averaged 29mpg.

On the way back the first leg (114 mile second leg of the first trip) averaged 34mpg and the second leg (219 miles) averaged nearly 35mpg.

Variables:
The second leg of the trip down was driven 4mph faster than the first leg (which is what prompted me to keep a close eye on things on the way back - faster should be lower mpg). The return trip was driven at the same overall speed relative to the speed limit. I picked a number I was comfortable with and set the cruise control to the speed limit plus or minus that number. So for example, if my number was 1mph, I would drive 76 in a 75, 71 in a 70, etc.

Wind was blowing out of the south both days, but I haven't heavily investigated how hard it was blowing along the route. So I had a headwind going to Dallas and a tail wind coming back.

Temp started around 80F in Wichita and rose to around 100F when I arrived in Dallas. It was about 104F when I left Dallas and about 101F when I arrived in Wichita.

There was about 100 miles of light rain on the way down as I went through Oklahoma, but mpg was consistent before, during, and after the rainy stretches.

Car had about 2150 miles on the odometer at the start of the trip, which totals around 700 miles.

I don't drive particularly aggressively, and I didn't vary my driving style significantly between the two trips. By filling up at the same gas station in the middle of the journey I (more or less) eliminated fuel quality as a significant variable. (I only run 93 octane, but maybe someone put the wrong stuff in a gas station tank.)

The countryside does slope downward as you go south as an overall trend, but it's only a few hundred feet over the course of 350 miles, so really not significant and I got better mpg going "up" so definitely didn't matter.

My theories are:
Wind played enough of a role to change mpg. Not sure if that's even realistic.
Engine break-in. Even though I had a couple thousand miles on it already, perhaps those 300 miles southbound were the magical "end" of some wear-in that significantly improved efficiency? Seems unlikely.
Temps - my brain says it remembers reading once that cars tended to be more efficient when it wasn't crazy-hot outside, but I might have that backwards.

Anyone have ideas/theories as to why I'd see a 5-7mpg jump in a couple days?

08-03-2022, 09:57 PM	#1
Spektyr Senior Member Join Date: May 2021 Drives: GR86 Location: Kansas Posts: 323 Thanks: 168 Thanked 249 Times in 139 Posts Mentioned: 1 Post(s) Tagged: 0 Thread(s)	Weird MPG I just completed a round trip from Wichita, Kansas to Dallas, Texas and noticed a hard-to-explain jump in MPG on the return trip. Starting from a full tank I drove south on I35 to the Sunoco in Pauls Valley, Oklahoma (because it has 93 octane) and then from there on to the Buc-cee's in Denton. On the return trip I filled up at Buc-cees and stopped at the same Sunoco in Oklahoma. Same route both ways. Drove down on the 1st, back today (the 3rd). On the way down the first leg (219 miles) averaged 28mpg, and the second leg (114 miles) averaged 29mpg. On the way back the first leg (114 mile second leg of the first trip) averaged 34mpg and the second leg (219 miles) averaged nearly 35mpg. Variables: The second leg of the trip down was driven 4mph faster than the first leg (which is what prompted me to keep a close eye on things on the way back - faster should be lower mpg). The return trip was driven at the same overall speed relative to the speed limit. I picked a number I was comfortable with and set the cruise control to the speed limit plus or minus that number. So for example, if my number was 1mph, I would drive 76 in a 75, 71 in a 70, etc. Wind was blowing out of the south both days, but I haven't heavily investigated how hard it was blowing along the route. So I had a headwind going to Dallas and a tail wind coming back. Temp started around 80F in Wichita and rose to around 100F when I arrived in Dallas. It was about 104F when I left Dallas and about 101F when I arrived in Wichita. There was about 100 miles of light rain on the way down as I went through Oklahoma, but mpg was consistent before, during, and after the rainy stretches. Car had about 2150 miles on the odometer at the start of the trip, which totals around 700 miles. I don't drive particularly aggressively, and I didn't vary my driving style significantly between the two trips. By filling up at the same gas station in the middle of the journey I (more or less) eliminated fuel quality as a significant variable. (I only run 93 octane, but maybe someone put the wrong stuff in a gas station tank.) The countryside does slope downward as you go south as an overall trend, but it's only a few hundred feet over the course of 350 miles, so really not significant and I got better mpg going "up" so definitely didn't matter. My theories are: Wind played enough of a role to change mpg. Not sure if that's even realistic. Engine break-in. Even though I had a couple thousand miles on it already, perhaps those 300 miles southbound were the magical "end" of some wear-in that significantly improved efficiency? Seems unlikely. Temps - my brain says it remembers reading once that cars tended to be more efficient when it wasn't crazy-hot outside, but I might have that backwards. Anyone have ideas/theories as to why I'd see a 5-7mpg jump in a couple days?