in the case of my '99 4.6 F150 - I've debunked the gas mileage myth of driving slower saves gas. I drove 189 (all interstate) miles this weekend to Houston, then back to San Antonio. I set the cruise on 62 mph and got 17.24 mpg. the return trip I set the cruise on 75 mph and got 18.74 mpg.

I ran this as a test. I filled the tank both times from a station on the access of the interstate. drove. then refilled from another station on the interstate. no in city driving, no stop and go, no slowing down for traffic... just drove at set speed.

interesting, huh?

 

I do believe there is a sweet spot when cruising for maximum mpg's....mine did better at 110km/hr compared to 120 or over. I have the scangaugeII which gives me live data on mpg's so i can make my adjustments as I'm driving.

 

That's odd! Houston is about 50' above sea level, San Antonio is about 800' above sea level. If anything, your numbers should have been opposite of what you found.
ERIC

 

Head or tail winds?

 

that's not much difference.... only about 1/4 ft change per mile. I wouldn't think that small of a change would matter. but, then again I'm not an engineer - I just drive a truck.... what do I know??

 

the engine is most effiecient in the 1800-2200 rpm range.

 

ok... so a 4.6 with 3.08 gears turns how many RPMS at 75 MPH?

 

Taking 2 drives doesn't debunk any "myth's" My scan gauge gives me live mpg and I get better mileage at 55mph than I do at 65mph in my truck. It has never been the other way around and the scangauge is spot on on my avg. mpg compared to my mileage/gallons. So I guess my drive just debunked your myth.

 

I didn't use a gauge, I'm not sure how accurate they really are, do you?

my gas consumption was based on mathmatics - not a gauge that probably has lag in it.

so, I'm not sure you've proved anything - have you? unless the manufacturer of your gauge will provide pertinent data that backs up you claim that their gauge is 100% accurate, 100% of the time. please post their cut sheet here..... thanks.

 

you only used 10 gallons of gas per trip. if you missed the fill up by 1/2 a gallon, your numbers are bad. you need to take an average over hundreds of miles. if you do that, you will prover that 55 mph gets better milage than 65 mph and 65 is better than 75 mph, etc. its the wind drag that kills you, and that is mathmatics and can be scientifically proven.

 

oh - so you're saying if I would have burned another 20 gallons of gas, my mileage would have come down?

ok - that sounds scientific.

 

I have driven 75 MPH and have gotten 20 MPG doing it, once. One or two trips doesn't prove or disprove anything.

It's a scientific fact that as you go faster, drag from wind takes more power to overcome. That power comes from gasoline.

I'd say that at a certain RPM point for a specific engine given the right conditions that you might get more MPG by going faster, but overall slower is more fuel efficient.

Mike

 

LOL!

I'm not sure you've proved anything - have you?

 

My truck is more efficient at higher RPM's than at lower RPM's. Same thing applies to my motorcycle. When an engine is running at a higher RPM, it takes less fuel to keep it there.

I still can't afford to fill up my truck and drive it to figure it out, but my motorcycle is a different story. If I keep it around 3k-4K RPM's at 55-60mph, I average about 135 miles before the gas light comes on. At 5k-6k RPM's and 75-80mph, I average about 150 miles until the gas light comes on.

 

Not sure how he's measuring it but dad claims 24 mpg out of his 99' F150 4.6/auto/3.08/2wd with 235/75/16 All-seasons. He has absolutely no acceleration but the truck is capable of much better fuel economy than my 99' 5.4/auto/3.55LS/2wd with 33x12.50 Super Swamper TSL's and 6" of lift, which still gets a respectable 17mpg on the highway cruising at around 75 most of the time.