Road Trip MPGs: 75-80 MPH vs. 65-70 MPH

We all know how fast you drive makes a difference in fuel economy, but I realized just how big a difference that is this weekend after 600 miles round trip to perform at a beer and music festival in Mammoth, CA. On the way there I cruised between 75-80 MPH and I got a pretty pathetic 17.3 MPG (actual, but the dash was close at 17.4).

Once a year for the last 15 years, I take my lifted 25 year old 4Runner on 34” tires, 4:88s, and full skids and bumpers on that same drive To Mammoth, and I always get ~18 MPG… and that’s with a kayak and an awning on top, and loaded up with several hundred lbs. of camping gear, tools and other crap. Then again, I’m only driving ~60-70 MPH tops in that truck. I was still surprised tho, that my mostly stock ‘21 Tacoma got worse fuel mileage than the old 4runner, (despite driving the Taco 10-15 MPH faster the whole way).

So on the way back I was in no hurry and decided to see what kind of MPGs I would get if I did 65-70 the whole way back (BTW, I’ve made this drive dozens of times and have always gotten virtually the same MPGs going there as coming home). And I got 21.5 MPGs on the 300 miles home vs. 17.3 on the way there (hauling ass). I didn’t think going 10 MPH faster over the course of a half day road trip would make a 4 MPG difference, but that’s what happened.

On a side note, we had an absolute blast playing at the festival!

 

I set my cruise to 66 and mostly get 22.5 mpg. Got 25.5 with the stock tires.

 

I have a 20.3 mile each way commute to work. I've played around with my speeds over the last 4 months and varying amounts of congestion, slow grandma/pas and truckers.
something about the 55 to 65 mph range gives the best MPG compared to going in the 30-40s and over 70mpg

 

Depending on frontal area and coefficient of drag the average vehicle gets 20% less fuel economy at 80 MPH than 65MPH. From 55 to 80 is 28%.

 

This is why I'm getting a new front bumper, to help with the aerodynamics of my Taco.

 

UH??? we haves a smart guy on the forum.

 

Looks like the festival was well attended.

From the web on the subject of Automotive Vehicle Drag:
This drag is due to the viscosity of the air.
The drag force is generally modeled as: D = 0.5•rho•V^2•S•Cd; where
rho is the air density
V is basically the speed of the car
S is the frontal area of the car if we assume that the wake drag is dominant, and
Cd is the parasitic drag coefficient.

So the drag is changing exponentially with the square of your velocity V, all other things unchanging between the legs of your journey. Was there also a difference in elevation (i.e. the destination at a higher elevation than your starting point)?

This doesn't tell the complete picture. If the travel path is not level, additional weight will be a compromising factor to fuel economy.

And maybe you just felt "lighter" following the gig due to the appreciation of the audience.

 

To double your speed, you need to square the power. So if going 50 mph takes, say 50 horsepower, to go 100 mph will take 250 hp, so yeah, going faster takes more power, more fuel

 

This particular drive was the best stretch of road I could possibly do this “test” on because I’ve driven it so many times and I always checked my fuel mileage. Like I mentioned, I’ve always averaged virtually the same MPGs on the way there as the way back in any vehicle I’ve driven. Both directions always average within .1-.2 MPG, and I always use the same amount of fuel to get there as I do to get back. I think it’s about as accurate of a test as possible without going full Mythbusters on it.