Better Gas Milage at 80 MPH?

[Dansnare]

So a friend of mine was telling me that he normally gets about 14-15 mpg in his 04 V8, and tops out at 17 mpg with all highway driving. This is regardless of what octane gas he uses.

Then he tells me that the other day he was driving long distance, all highway miles, averaging between 75 and 80 mph the whole way (faster than he normally drives), filled up after using about 3/4 of the tank and calculated a whopping 19 mpg (way higher than he has ever calculated before)!

I always thought that going that fast was less efficient not more efficient. How is that possible when his highway milage has never before exceeded 17 mpg?

Dan

 

[khaug]

If I recall my engineering text correctly, power required (and, therefore, fuel consumption) increases as the square of the speed.

My '03 V8 gets a calculated 20 mpg at 70 mph. This drops to about 18.5 at 80. So many factors come into play, however, that it's hard to compare one speed to another and get accurate results.

I'd guess his apparent increase at 80 mph was likely due to some extraneous factor like a tailwind, incompletely filling the tank, math error, etc.

 

[dcampen]

Last weekend, just after filling my tank and zeroing the MPG readout, I drove several miles with cruise control at 55 mph on a level to slightly downsloping highway - my mpg readout hit an all time record high (for me) of 24.7 and it was many miles more before it dropped below 21. Now I am back to my usual 15-17.

 

[mike_d]

If I recall my engineering text correctly, power required (and, therefore, fuel consumption) increases as the square of the speed.

funny you mention this. i have been discussing this with my brother in terms of cycling and trying to use as little power as you can. it turns out that the power required is proportional to the cube of speed, if you approximate the air drag by a function that's quadratic in speed (which is what everyone does). if you want proof, i'm happy to cut and paste the email i just wrote to my brother.

anyway, wind drag is probably has the most effect on gas milage when cruising on the highway, but there are a number of other factors. one, is the efficiency of the engine at the crusing rpm. granted, today's engines have a wider efficiency band than cars from the days of yore, but still, there's one best rpm to be cruising in for efficiency. in the days of yore, that rpm was designed to be when you were going at 55 in top gear. now it may be higher (and with vvti on the new cars, there could be a number of efficient rpms).

so back to the original posters comment. it's entirely possible that increasing speed to 80 put the engine at a more efficent that was able to make up for the extra drag. or it could be as simple as a tail wind, mostly down hill travel, or mis-calculation of the milage.

 

[EDGE]

I can get 19mpg at 80-90, thats 100% highway soon as I mix stop and go into that it drops

 

[Northof49]

I got a solid 19 mpg in my 03 V8 4x4 on a trip from Vancouver (Whistler actually) to basically the Alberta border. This included 3 people on board, trunk and 1/2 the backseat FULL of stuffand 70lbs worth of MTB's on the hitch rack. This route is mostly highway, although there is a lot of braking/speeding up for corners, and it includes 3 mountain passes with 30+minutes each of solid uphill. I drive at least 20km/h over the speed limit at all times and pass whenever there's a car in front of me and have the tickets to prove it.....Considering those conditions, I see no reason 19 mpg isn't attainable at 80mph.

 

[bulldog-yota]

i find that my V8 is much happier at speeds above 75MPH while cruising. if i cruise at 70 my gas mileage is similar to when I cruise at 80, only if i try to restrain my self to 60 is when it increase and above 90 it drops off prety steep. but between 65-80 i have not noticed a difference in real mileage on my V8 while cruising. Have doen the calculations over 35K miles now. I also noticed that mileage improved after 10k miles by 1-2 mpg on average, and then after the lift and Revos I have lost like approx 2-3 mpg (includes corrected for tire size). The biggest culprate was probably the tires, but the lift doesn't help gas mielage.

 

[jeober]

The 4runner has the aerodynamics of a brick. Highway mpg will be highly dependent on cruising speed in top gear [and if you can get a slight draft from a truck in front of you]. I would expect to see top mileage in 5th gear at about 45-50 and for it to fall quite steadily as speed increases. I get about 23-24mpg at 65mph with my v-6 2wd. and 1-2 of mpg's less at 70mph.

 

[rossmon1]

The 4runner has the aerodynamics of a brick. Highway mpg will be highly dependent on cruising speed in top gear [and if you can get a slight draft from a truck in front of you]. I would expect to see top mileage in 5th gear at about 45-50 and for it to fall quite steadily as speed increases. I get about 23-24mpg at 65mph with my v-6 2wd. and 1-2 of mpg's less at 70mph.

Do u know what the drag coefficient is? The t4r sure looks slippery compared to my earlier suv's like wranglers cherokee's and early bronco's!! Hey it has a curved windshield, thats gotta be good for some mpg

 

[boston_kevin]

Do u know what the drag coefficient is? The t4r sure looks slippery compared to my earlier suv's like wranglers cherokee's and early bronco's!! Hey it has a curved windshield, thats gotta be good for some mpg

That's why we see mileage in the high teens, not single digits. Fact still remains that sitting high off the ground and those big mirrors (not to mention that this thing weighs 3 tons), are all factors that reduce the gas mileage.

 

[EDGE]

Thing gets good mileage but let off the gas pedal and it slows down like you just released a parachute:

 

[Tleahcim]

Wow, and I thought I was the only nerd on the boards. Notice I used the term nerd and not dork...

Seriously though, I wonder where we could find the drag coeff. for the 4 runner. Can't you make a rough estimation using surface area and angle from the norm? I really wish I had paid more attention in class...

 

[Brewster]

Last fall in Colorado I consistently achieved 20 mpg at 75 mph. At 75 mph at lower altitudes here in Texas, I can't beat 17 mpg without a tailwind.

This is especially disappointing, since my wife's Suburban gets better than 16 mpg under the same local circumstances. It has greater frontal area, more weight, and a bigger engine. Its aerodynamics also look worse, but looks can be deceiving w.r.t. drag coeffecients. The fact that it performs so well with a pushrod, 2-valve head is annoying, also.

 

[jeober]

As Boston Kevin says, the 4runners do pretty good considering the weight and the engine horsepower.

It's neat how the engine and trans work together to keep the rpm so low during normal driving. The variable valve timing helps to increase torque down at these lower rpms and make the vehicle more drivable yet still have the high power available.

The electronic throttle control makes the frequent shifting of a 5 speed so smooth.