MPG: SJC to LAX and return

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mazda
I have driven this route periodically on I-5 and am familiar. My car has 103k on it, mostly freeway. Tranny fluid flushed by the dealer at 86k. Had PS, Brake Fluid and Coolant done. I typically drove once a month but when my other car broke down recently, drove this instead.

All of a sudden my forward trip of 369 miles only gave me 23.27mpg! The lowest I had was around 26. 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.
On return, 382.8 miles gave me 29.28 mpg.

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

Forward journey started around 5am so we turned the heater on till about 10am or so. 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?
 
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We're the spark plugs changed at 75000 like the schedule says to? I'd not, that would be what I would bet
 
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