Optimal highway speed for fuel efficiency?

So I just did my first long trip in my 2018 Tacoma , went from Phoenix to San Diego. I averaged around 17 MPG which is the lowest I've ever got. Usually just driving around town during the week I at least get 19-20 MPG. Now I was averaging around 85 mph during this trek which has me wondering if if that's why it was lower. Is there an optimal highway speed you guys have noticed ?

 

55mph

its a bell curve for speed vs mpg

 

But I want to go fast

 

It’s a truck, don’t worry about the mpgs.

 

Yeah, when going over 70mph the mpg’s drop...there’s a lot of parasitic drag being shaped like a brick. You can always lower the truck to pick up some mpg’s or back off the throttle...

 

These trucks are heavy. And while the cd isn’t awful, the frontal area is a lot bigger than most “cars”, which is compounded by the raised ride height and exposed tires.

You can still get pretty decent mpg at lower speeds, but in my experience ~75+ and you’ll fall off a cliff.

TLDR: these trucks have a tougher time at higher speeds compared to a typical car rated the same cd and mpg.

 

What you want for optimal MPG is a speed that gets you comfortably into top gear, and not lugging. Not so hard into it you are fighting the aerodynamics of the lovable brick that is the 3rd gen nose, though.

It's deceptive because with cars that have a deep overdrive (6TH GEAR IS 0.69!!) you actually want to hit a slightly higher speed than you might expect to stretch the legs of that overdrive and not have the engine chugging trying to maintain that speed over the natural undulations of the road. At 55 you will probably see a hard mileage hit because 6th should have you at about 1400 RPM and that simply isn't enough to contend with any sort of grade, so it pops into 5th or even 4th to maintain your speed which kills mileage.

For my 4 banger the optimum is about 65 or so, but in the range of 60-70 doesn't hardly make a lot of difference. Go over 70 and MPG start to fall rapidly.

 

65mph. Averaged 25.5mpg on my last 80 mile freeway trip.

 

55 mph in a gear that is around 2300 rpm is absolutely optimal mpg. If you have an auto, use the S mode to get the proper gear. If manual, 4th or 5th is optimal, depending on terrain. Anything over 55 mph, you lose no matter what you do.
I actually just got back from the beach, 450 miles and averaged 72 mph moving. I got 20.9 mpg with a stock Tacoma with about a 500 lb total load.

 

65-70 I have seen pretty good mpg. Console says around 26 on an 80 mile trip Freeways. Gassing on the downhills (nothing past 75-80) and cruise 65-70 on flat to uphills.

 

Under 70.

 

Just drove 2,000 miles in 3 days . 0n flat lands Wisconsin, Illinois,Iowa, and Nebraska. At 76 mph it was 26-27 mpg Wyoming, Utah,and Nevada Mountain ranges I got 16-18mpg. But as a side note it couldn’t find a gear or it was reving high to climb steep hills. 80 mph speed limit in Wyoming and Utah I sat a 84-85mph and it barely got 15mpg. It made it. Also just recently purchased a trifold soft tonneau . It seemed to help

 

My experience has been similar. The best mileage I've ever been able to get (unloaded, completely stock) is 8.2L/ 100 km over a distance of 70km. Any time my speed exceeded 80 km/h the efficiency started to fall off according to the cluster calculations. Now, this is not real world driving - I was seriously grannying on back roads with very little traffic. I kept the truck in 6th as much as possible.
Full disclaimer - I've not tried to run my whole mileage loop without using 6th to see what would happen.
I can tell you that pokey highway driving in 6th at 60-65 mph (2000-2200RPM) tops out at 10.5 L/100km under ideal conditions for me and my truck.

 

You can. But you're not going to get good MPG.

The best tool to help with MPG you'll ever use is a mechanical vacuum gauge.

Drive to keep the needle in the green. Much more viable that any 'instantaneous' readings from in dash or other OBDII based units.

 

2018 DCSB TRD Offroad with an auto transmission and about 2800 miles on the odometer. Currently, 65mph-70mph will get me 22mpg-25mpg. I recently made a trip from Detroit to Nashville running 80mph for most of the trip. At the speed, I averaged 19.3mpg there and back.

 

Curious.

Are those hand calcuations, or what the dash tells you?

If dash, have you had the chance to validate the numbers?

 

My hand calculation have all been no more than +/-0.3mpg difference from the displayed value since I purchased the truck with 3 miles on it. That being said, the value from my recent trip to TN was the displayed MPG.

 

I made a thread on that not too long back, on my truck the hand calculated is pretty close to the display.

 

On flat/level roads 65-70mph seems to provide the highest numbers. Small drop from 70-75mph, and then more significant drops for anything above 75mph. 76-78 is what I tolerate on most trips, still 18mpg and only some risk of getting rear-ended by everybody doing 85.

It's a truck -> large frontal area and poor coefficient of drag, so as the speed increases, aerodynamics are not your friend

 

I read in Car and Driver one time that a vehicles speedometer was designed to show that the best driving envelope was when the needle was pointing straight up.

I don’t know if it is true, but it was an interesting comment from one of the writers.