dramatic difference in gas mileage on a trip

Slight elevation change?

Colder on the drive home?

Fuel bladder makes it difficult to accurately calculate MPG based on fill-up?

Fuel pump fill-up variation?

 

Also:

Wet road one way?

Different breeze conditions?

No bladder on that car, but do check the elevation difference. It doesn't take much change to show up in Prius mpg results.

OTOH, a 6 to 7 mph speed difference could be enough to explain the entire mpg difference.

 

Fuzzy1, thanks for the clarification.

I just noticed the first trip was 65-70 and the return was 75. Having ONLY a 5mpg drop is fantastic. I would say the change in speed is almost entirely the cause. The aerodynamic drag increase alone at 75mph vs. 67.5mph is 23.5%.

 

Speed difference, plus possibly a slight grade difference, wind direction? Mostly speed.

 

There's something wrong when you get run over for abiding by speed limits.

I'm not sure it's that black-and-white, I'd hope not.

 

Just find a slow truck and follow it. Added bonus: nobody will cut you off to get in behind the slow truck.

 

Another factor can be the gasoline energy content which can vary about 7% MPG (assuming E10), if you filled up. You did not tell us if you filled up, but Houston is reformulated gasoline (lower energy). Most of Texas is not reformulated gasoline, so that means greater chance of variations. Whereas those of us in reformulated gasoline areas can be assured of pretty much constant energy content even between octane grades.

 

Depending on the dates of the two trips, a dramatic cold front passed through Texas. I've had a 40 degree swing in temps before/after Christmas.

 

Based on my rule of thumb for fuel consumption vs elevation change, I figure the descending leg would get 0.7 mpg higher than the climbing leg. So that doesn't account for what you saw.

I'll agree that this is the best answer so far.

 

Having done multiple trips(10+) between Houston to Dallas, (~250 +/- 15 miles), Speed is the biggest factor. I have driven at 65 all the way, added about 30 minutes to the drive (total drive time 3 hours 55 minutes), and attained 42 MPG(Calculated manually at the pump). (A/C on the entire time in the texas heat, temp: 95+) Driving same day back at 75 MPH+, MPG dropped down to 37 saved me time(Drive time 3 Hours 22 Minutes), which was more valuable to me than the fuel savings. Driving slower I had 4 bars of gas on the gauge when I got there, I had 2 bars lefts when speeding. Lets do the math:

Trip 1(65MPH): 250 miles / 42MPG = 5.9523 Gallons used. Drive time 250 Miles / 65 MPH ~3 Hours 55 Minutes
Trip2(75MPH+): 250 miles/ 37MPG = 6.7567 Gallons used. Drive time 250 Miles / 75 MPH ~3 Hours 22 Minutes

Difference in Fuel used 6.7567 - 5.9523 = 0.8044 More Gallons Used
Cost: 0.8044 x $2(National average 1/7/2016)= Spent $1.6088 More.(Values will be different depending on your local gas prices, Gas during last summer was higher, national average was $2.780 Actual cost was higher for me)
Time Saved: ~30 minutes
I believe saving that time is worth more than $1.6088 for me.

Applied to your scenario:

Trip 1(65-70MPG): 185Miles / 46MPG = 4.0217 Gallons used. Drive time 185miles/ 67MPH ~2 hours 45 minutes
Trip 2(75MPH): 185Miles / 41MPG = 4.5121 Gallons used. Drive time 185Miles / 75MPH ~ 2 Hours 28 Minutes

Difference in Fuel used, 4.5121 - 4.0217 = 0.4904 more Gallons used,
Cost: 0.4904 x $2 (National average 1/7/2016 )= Spent $0.9808 More (Values will be different depending on your local gas prices)
Time saved: ~17 minutes.
Do you believe this is worth the cost of extra time saved of getting back home?


You are correct, for speeding, 75+ MPH, our fuel efficiency isn't bad, for the motor.

 

Use cruise control and keep speed consistent and you automatically gain 10 mpg.

 

As stated above, cruise control will add to your fuel efficiency.

You are correct again, that for the nominal value of fuel cost, it's not worth being worried about speed vs MPG.

If I take a trip greater than 150 miles, I will speed and save time. 10-15 MPH faster will save about 20-30 minutes over a 2 hour time frame. I'd rather be at the weekend getaway sooner than later. If your not pressed for time, and want to enjoy the drive, cruise control/ consistent speed is your best friend.

Either way, we get some of the best fuel economy on the road.