bizarre fuel economy reading?

[brillo]

So I wanted to test my highway mileage on a relatively short trip to the north part of town after getting my new tires (michelin PSS). The route was all toll road so I figured it was a good steady state 70mph test. My car has perrin tune stage 1 tune, perrin filter and a oil cooler. Otherwise its stock.

So after 33 miles round trip my mpg calc (to my major surprise) was 23 mpg (pump showed 1.44 gal). I was like WTF? I made the mistake of not filling up at the same place both times (dumb) so I have to think the pump had an issue. The weather was cool but not cold, no wind, and my car had recently gotten ~25 mpg in my last tank of driving (mixed city/hwy).

Is 33 miles to short a distance to test? Gas pump issues? What else could I be missing?

[N1rve]

No...? I don't see what the problem is...?

[Luis_GT]

I get 27MPG if I do 200 miles of highway...

[brillo]

The problem is my hwy mileage was 23 mpg which is way to low based on historical observation and the cars EPA ratings.

[Rayme]

You are trying to calculate your Miles per Gallon.

You drove 33 miles, and returned 23 Miles Per Gallon.

You don't even drive enough to average the mileage of 2 gallons of fuel.

Don't believe the gauge - It's not as accurate as manually calculating your fuel usage.

[N1rve]

Originally Posted by brillo

The problem is my hwy mileage was 23 mpg which is way to low based on historical observation and the cars EPA ratings.

Well your car is running PSS, stickier tires, the stock tires were Prius tires with low rolling resistance.... soo that would account for it.

[brillo]

I was manually calculating it. I went 33 miles per the odometer and the pump said I burned 1.44 gal of fuel

[993Fan]

It's very difficult to consistently fill up with modern self-stopping pumps, even at the same pump, especially if they're set up for vapor recovery.

And even if you were to take an average over several tank fulls, there would be so many variables involved with each tank-full that you still wouldn't know for sure what the true "highway" mileage was.

[brillo]

Tire rack shows the PSS makes a - 1% difference vs their most efficient tire. My cars computer mpg was 32.5mpg when I stopped to fill up.

[N1rve]

Have you checked your tire pressure lately?

[ntron1]

run a tank...33 miles is not nearly enough....

[brillo]

Yep pressures are 34/35 psi

[Cyau]

No one has mention the tune yet?

Most tuning are gear toward performance, not efficiency.

[brillo]

I wouldn't think an NA tune would affect closed loop cruising. I've gotten 30ish before on long hwy trips with the stock tires.

Going to guess 33 miles is to short a distance to test.