Fuel economy experiment

[c00nhunterjoe]

took a vacation this past week to Tennessee for some time on the mountain with the bears. To make the 10 hour drive go by i conducted an experiment. Test mule is my 2005 chevy tahoe. 5.3, 4l60e with 3.55s. This is a non mds engine so v8 all the time. I tested 55mph, 65, 75, and 80. Each test was with cruise on for roughly 100 miles. I conducted the same test on the way home opposite the trip down to rule out road hills and changes even though it was mostly flat ground and no traffic. Differences in the trips were negligible.
  At 65mph the tach read 1450 rpm with minimal increases up to 80mph at 2000. To my surprise the old girl loaded down with the wife, dogs, kids and luggage, at 55 mph it clocked 24.5 mpg. 65 decreased to 20, 75 to 18.5, 80 to 17. I suspect based on the throttle needed to sustain the speeds that the brick-like aerodynamics is the root cause for the susbstantial drop given only a few hundred rpm. At 55 barely any throttle was needed to maintain speed while at 80 it was nearing 1/4 throttle at all times even on downgrades.
   Not super scientific but passed the time and was within .2 mpg down vs back on each test.

[Kern Dog]

Where in Tennessee ? The Wife and I are considering a move there.

[c00nhunterjoe]

Where in Tennessee ? The Wife and I are considering a move there.

Gatlinburg.  On the mtn just outside of downtown.

[Swampwing2]

I have done similar tests since my job involves a fair amount of driving. One surprising test I did was compare the mileage difference between dry roads and heavy rain. My old 2000 Grand Caravan lost 4 mpg when driving in heavy rain, at the same highway speed. I was not expecting that big of a drop from rain and standing water, but it creates a lot of drag rolling through it. My current Taurus does not seem to lose nearly as much in the rain, but I have been less scientific with monitoring this one. Most of the roads here are flat and straight so its easy to test.  

[Kern Dog]

I have wondered about the effect of rain on fuel economy.

[70 sublime]

I tried something similar many years ago
But what I did was try regular gas vs the high octane stuff
I had a one year old 1985 Dodge Daytona at the time
Went to visit my girl friend ( turned into my wife) at her summer job way up north
The gas tank on the car was not big enough to hold enough fuel for one way on the trip so had to stop for gas
I do not remember the exact numbers but when I used the low vs high octane fuel the tank of high octane made the car go farther but when I figured out the $ per mile it was the same as the low octane fuel

[Kern Dog]

That theory has been proven again and again.
Late model RWD performance cars show the same results. The LX and LC cars, Chrysler 300, Dodge Charger, Magnum and Challenger 5.7 Hemi calls for 89 octane recommended, 87 octane okay. I have 350,000 miles on my 5.7 Ram 1500. It can run on the cheap 87 gas but WOT power is lower. I have not checked MPGs in years though.

[c00nhunterjoe]

Interestingly enough, the truck had 93 in it on the 1st leg down and it rained multiple times, 87 on the whole way home and dry, the difference down to back was negligible.

[Mike DC]

      
 

If you drive the same speed in heavy rain that you go in dry weather, with a loaded truck . . . you're asking for bigger problems than gas mileage.  

[Todd Wilson]

I ran an experiment with my 2005 Dodge 2500 Cummins. 65mph got 22.5mpg. 70mph got 21.5mpg and 75mph got 20.5mpg and running at 55 also got 20.5mpg.


Todd

[Swampwing2]

     
 

If you drive the same speed in heavy rain that you go in dry weather, with a loaded truck . . . you're asking for bigger problems than gas mileage.  

Agreed.

With lighter vehicles, the effect can be even worse. Back when I was racing in the SCCA, the fastest I could get my BMW 325 (with slicks) up to, was 110 during unexpected rain on track. Otherwise I would be over 140 in the same areas before running out of track. Those speeds will highlight vehicle dynamics, wet and dry. In heavy rain, the "edge" of control becomes a deep cliff instead of a small hill. Tracks like Daytona and Sebring can have heavy rain in some turns and sun on the rest.

IIRC my van rain test speeds were in the 60-65 range. Most highway speed limits in Florida are 70 with really good roads, compared to the other 46 states I have traveled. 

[stripedelete]

Is your Tahoe four 2wd or 4wd?

Tested my 01 Silverado 4x4.  5.3.   Slightly over inflated tires vs normal - about a + 1 mile/gallon.  Keeping it under 70 - +2 mpg.   Tailgate vs no tailgate - same.

1999 Suburban with 255/75-16 tires with me driving 18mpg.  Wife driving 14.  275/70-17 16mpg.  I don't let her drive anymore.

[c00nhunterjoe]

4 wheel drive, stock tires at 40 psi.

[John_Kunkel]

Each test was with cruise on for roughly 100 miles.

Didn't know there was anywhere you could do 100 miles on cruise. Even on the interstates around here you have to continually change speed because of slow semis.

My 5.7 powered LX has EVIC (Electronic Vehicle Information Center) and can display real-time fuel economy.

[Kern Dog]

My truck does too but I don't trust it for accuracy.

[c00nhunterjoe]

Each test was with cruise on for roughly 100 miles.

Didn't know there was anywhere you could do 100 miles on cruise. Even on the interstates around here you have to continually change speed because of slow semis.

My 5.7 powered LX has EVIC (Electronic Vehicle Information Center) and can display real-time fuel economy.

Driving at the time of morning i left there was no traffic and not hard to ride the left lane for the majority of the trip. Also, note i said roughly 100 mile intervals. My heads up display also provides realtime data. Are you suggesting i made it up?

[GreenMachine]

 At 8,000 ft elevation I got 31 MPG, and at 1500 ft elevation I got 26 MPG (also 20 degrees hotter). All in the same day, according to the dash display. Something the airlines have known since the early days, better MPG at higher elevation.

The reason for the national 55 MPH speed limit in 1974 was for fuel economy due to the oil embargo, I'm sure there were lots of studies done back then.

[John_Kunkel]

My heads up display also provides realtime data. Are you suggesting i made it up?

What gave you that idea? In the OP you didn't state if your numbers were math or from a readout and your numbers are believable.

If I play with the throttle on flat ground, I can get an impressive readout for that instant in time but it isn't a true average.

[c00nhunterjoe]

My heads up display also provides realtime data. Are you suggesting i made it up?

What gave you that idea? In the OP you didn't state if your numbers were math or from a readout and your numbers are believable.

If I play with the throttle on flat ground, I can get an impressive readout for that instant in time but it isn't a true average.

Your reply implied that it is not possible to drive with the cruise on. And as you said, the instant econony reading available on display can easily be skewed, much like dyno numbers. Thus why i did the test twice on opposite stretches of the trip and used a long mileage between tests. I used the computer calculated number and old school arithmetic at the gas station. As i said in the origonal post. The differences were negligible.

[67440chrg]

I have a 1991 s10 4x4 v6 auto with overdrive. It spent most of its life in a carport. It has 60,000 miles on it. If I drive it 65 to 70 it gets 18 mpg. If I drive it 75 it will get 20 mpg. It is under more of a load going slower in OD and will even run hotter going slower. On real hot days I have to run in drive around town to keep the temp down.

[HeavyFuel]

Each test was with cruise on for roughly 100 miles.

Didn't know there was anywhere you could do 100 miles on cruise. Even on the interstates around here you have to continually change speed because of slow semis.

My 5.7 powered LX has EVIC (Electronic Vehicle Information Center) and can display real-time fuel economy.

C'mon out to I94 running through ND.   

Fargo to Beach...350 miles.   Might have to tap the brakes a few times when passing Bismarck.

Lots of this...

[Kern Dog]

I read once that the Europeans purposely designed their highways to have curves as a means to keep drivers awake and engaged to the driving experience. Their cars were built to handle better as a result.
All the while, our freeways are often straight like a long drag strip and for many years, our cars were fast but handled poorly in the turns!

[odcics2]

I remember, back in the 70s, taking my Charger around cloverleafs FAST...
The goal was to not brake at all.

[Mike DC]

I read once that the Europeans purposely designed their highways to have curves as a means to keep drivers awake and engaged to the driving experience. Their cars were built to handle better as a result.
All the while, our freeways are often straight like a long drag strip and for many years, our cars were fast but handled poorly in the turns!

That's how the German autobahn was done.  Keep the road fitted to the landscape and avoid long straight sections.  They also built the road bed about twice as deep/thick as US highways and it has stayed smoother over time.  Just a better build quality all around. 

It influenced German cars to be built for higher speeds.  Better brakes, stiffer suspension, aero, etc.   


All these interstates (including the USA in the 1950s) were basically military infrastructure operations.  A nice fat interstate was the best way to move a lot of manpower & resources in a pinch.   And you could land planes (at least the smaller/lighter planes of the 1940s-50s) on the highways pretty easily.       

We take the interstates for granted because they have been here for most/all of our lifetimes.  But imagine trying to drive across the country on windy little 2-lane roads that go through every little town - totally different experience.  It would take a lot longer.  On the other hand you would see a lot more of the country. 

[XH29N0G]

There is an highway connecting Pasadena to LA that passes the dodger stadium that I was told was made with curves to keep people on their toes.  The information comes from my mother in law, but she was born in pasadena and loves cars so if anyone would know it'd be her..