air drag and mileage

[rickerman]

what is the minimum speed that we need to concern about air drag on our vehicles? I know large trucks and buses don't need an aerodynamic design as they are not speeding up that much to a get a considerable air drag.

 

[Binksman]

According to this fuel economy paper by Cummins, 50 mpg.

https://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf

 

[jeep-N-montero]

Depends on the vehicle itself and how much extra crap is attached to the exterior.

 

[Buliwyf]

Anything over 55mph and wind drag makes a difference.

And a Kenworth T2000 (aero bodywork) does get better mileage than an equally equipped Kenworth W900 (which still ain't that boxy). Maybe only 0.4mpg, but those trucks only get 6mpg. If I owned a truck, I'd try to get a cabover (pure box), for less weight, not better aero.

 

[PhulesAU]

Ummm, have you ever tried to explain Aerodynamics to a Jeep?

 

[rayra]

Just read some of the topics on RTTs and loads of external junk where folks confess their mileage impacts. It's as much as 10% reduction in their highway MPG. Aerodynamics matter, cruising speed in any gear. Even to those driving bricks.

 

[workerdrone]

I got about 25% better mileage at 45mph vs 55-60mph in our F550 dually with truck camper on it - this was the ultimate sweet spot of low air resistance plus ideally efficient engine rpm's.

A lot of people are used to driving 75mph or more on the highway and would go crazy driving 55 - but it does make a real difference in fuel consumption, especially with boxy vehicles and drag inducing stuff like RTT's, aux lighting, Hilift jacks, etc bolted on. Also, bigger tires with offroad tread vs. aired up street tires optimized for low rolling resistance and long wear make quite a difference.

If you've never checked out the results people get from 'hypermiling', it's pretty amazing - they can double or triple the mpg with their tweaks to the cars and driving patterns.

 

[Comanche Scott]

With my '14 JKU running 4.10 gears, auto and 35" tires, I pulled a 4K lbs + Scout towed 4 down at 50-55mph, and got more than 1mpg better than I did pulling an unloaded U-Haul car trailer at 60-65.
So in this specific situation weight had less effect than air. This was on a 1,000 mile up (Scout), and 1000 mile back (U-haul trailer) trip.

 

[PhulesAU]

I pull a pretty low profile trailer, so I stay down about 60 for trips. It ain't pretty, but it works.

 

[1stDeuce]

Actually, wind drag becomes significant enough to make a difference above about 45. There's a cool chart on this page showing how it's not linear... Above 45, the wind drag becomes more significant than rolling resistance, and the faster you go the more drag there is, and it's climbing faster as you go faster... (Rises with the square of speed, not linearly.)

I've heard people say 65 or 68 is the cutoff, and that's misleading, or even just wrong. The question is where the point of diminishing returns lies for you and your vehicle... If you're willing to lose a half a mpg to go 65 instead of 60, but not willing to eat 1mpg to go 70 instead of 65, then that's where your cutoff is. That cutoff is different for every person and ever vehicle, but the more aerodynamic the vehicle, the higher that point will probably be for any given person.

And your theory on Semi trucks and buses is flat out wrong. They have HUGE aero drag, even at 55mph. That's why you'll see skirts and those crazy back door aero panels on semi trailers, wheel covers on buses and semi trucks, etc. Many companies have mandated max speeds for fuel economy, not safety. School buses perhaps not as much a concern, since they're not going so fast, but OTR trucks and buses do everything they can to gain MPG.

 

[java]

It makes a huge difference. On my RV (a rolling brick) If I stay 60-62 and under, I get 8-8.5 mpg, do 65 and it drops to 7.

 

[Eaglefreek]

Just read some of the topics on RTTs and loads of external junk where folks confess their mileage impacts. It's as much as 10% reduction in their highway MPG. Aerodynamics matter, cruising speed in any gear. Even to those driving bricks.

I added all terrain tires and a roof rack to my 86 Eagle I used to own and it cost me about 20% in MPG. Rack alone cost me about 11%.

 

[jeep-N-montero]

I can only hope that those claiming they find their best mpg while driving 45 or 55 are not attempting to drive such slow speeds on the freeway/highway, as you will be the cause of a potentially unsafe situation.

 

[danfromsyr]

in our eastern (New England) highways the speed limit is only 65mph with a minimum of 45mph.. and yes on occasion you come upon a car with the driver barely seeing over the steering wheel at 45-50mph.

that's not saying that there are people rolling at 80+mph. unabated by the highway patrol.

 

[anickode]

My 4x4 Colorado used to do 18-20 highway at 60-70mph. I added an aluminum cap with a rack and it went down to 17-18 Then I lifted 2" and put on a tube front bumper that exposes the full front profile of the tires, suspension, and skid plate, and it went down to 16. Then I put on a set of 32" mud tires and that went to 14 and it will drop out of overdrive on even a slight Hill at 70mph, but that's rolling drag, not aero.

I don't start to see the numbers come back up till I'm down in the 45-50mph range. Aerodynamics make a huge difference.