MPG acted strange: San Jose to Los Angeles and return

2013 model sport.
I have driven this route periodically on I-5 and am familiar. My car has 103k on it, mostly freeway. All fluids are maintained well. During Pandemic I drove once a month but when my other car broke down recently, I drove this instead.

All of a sudden my forward trip of 369 miles only gave me 23.27 mpg! The lowest I ever had was around 26 on a more likely loaded car. I actually ran out of gas - I had to top it off in the middle of the trip. Typically I use Costco gas at both ends.
Return leg of the journey, 382.8 miles, it gave me 29.28 mpg. 27-28 are typical.

I did not do any maintenance in LAX including tire pressure. Only 2 adults with 2 suitcases on both trips.

Forward journey started around 5am so we turned the heater on till about 10am or so (trip is around 6-7hrs including stops). This might be the only difference. I did experience a little vibration at speeds higher than 70mph - on return had wheels balanced. Tires are good too - previous owner used Walmart tire. The trip was during the Thanks giving break and there was a wind/gust warning but we didn’t experience much.

What might have happened?

Old gas vs fresh gas?

did you hit more traffic one way than the other?

… or, was the first leg of the trip more “uphill”, with the return trip being more “downhill”?
When I visit my brother and SIL in The Poconos, I get significantly better gas mileage on the return “downhill” trip.

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I’m going to guess that you drove against the wind one direction and with the wind (or in the absence of much wind) on the return trip.

Were winds forecasted out of the south and/or west on the day of the trip to LA?


This drive was from where to where?
There’s elevation, wind, and many here will scoff at this speculation:
Sometimes a batch of gas with lower heat content comes along.

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This is almost a non-problem . There are so many things that effect fuel mileage that there is no reason to expect to get the same MPG every time.

I once got 43.2 on a highway trip . I have never got that again so there must be something wrong with my vehicle .


Fresh gas as stopped filling full tank - also I had to use this car till emptying the tank before the trip. Right before the trip, I put in 1/3 of the tank at Costco.

On Thanksgiving day, there were small sections with traffic on I-5 - still doing around 50-60.
Then there was an accident so was driving at about 20mph for about 20-30mins.

San Jose to Los consumed 23mpg.
Returning to San Jose was 29mpg

There’s been some strong southerly winds here in San Jose in the past month. I’ve definitely noticed it when riding my bicycle. Generally the winds here are from the west and north, winds from the south are a little unusual.

Forecast was only for around LA. A smaller area.
At speeds about 70mph, there was this vibration from the car - I thought that it was due to wheel balancing. I got it done after returning to San Jose.

Could be that a wheel was out of balance. Wheel weight might have fallen off etc. Not an unusual thing given I find them on laying the side of the road around here very frequently. Wind seems the more likely cause imo.

Trucks are more sensitive to wind. One time I was driving I80 across northern Nevada going east and got 25 mpg, which is considerably higher than normal for my truck, then going the other direction a few days later I got 13-14 mpg.

On avg, what mpg do you normally get?

17 mpg at 65 mpg w/no wind

Avg of 25mpg and 13.5mpg = 19.25mpg → still higher than 17mpg

Wind, elevation, gas quality, winter blend, who knows? When I used to drive 200 miles to school from Minnesota to South Dakota, mileage was always better one way over the other. Don’t remember which-its been 50 years. Never figured it out. Maybe the car just needed a good workout to blow the carbon out or something. As long as it came up again going home, everything is fine. For some things we need to die first to find the answer and it’s not worth it.

This is the 70’s model f150 4wd with a v8 and no overdrive? It gets 17 mpg?

The OP did mention vibration at 70+mph and having the wheels balanced after the return trip but yeah, I’m inclined to go with wind.

Mom’s prius showed lower MPG on a tank purchased from a major grocery chain, the next tank from her usual station onwards was back to what she’d come to expect. Way lower than it normally would be that time of year.

I checked the historical values - elevation does not seems to be an issue as I had the opposite values to the one I reported here (29 and return at 23.57 - return is almost always without cargo. Another was 31.54 and return at 27.35). Besides MPG were almost always 27-29 irrespective of direction.

Weight of the vehicle does not seems to have a consistent impact.

So it is wind force, quality of gas or something else - unless you see a trend in the data below - first row is to LA:
31.54 29.03 28.89 28.30 26.32 27.76 29.36
24.02 23.57 28.59 25.17 27.92 28.43 26.72