quest for 30 mpg

[widman]

Altitude reduces power (3% per 1000 ft), with the first 6000 ft compensated for by a turbo. But mileage seems to be a different story. I get much better mileage crusing the altiplano (12,000 ft) than crusing the tropics (1000 ft). That is not a scientific study, just 9 years of history.

I don't think many cars reach the 54 mpg I used to get on my Bug-eye Sprite in the 60's. I used to drive from DC to New York and back easily on one 10 gallon tank. Cost $1.05 each way in gas and $3.95 for tolls. It was only 948 cc, but with dual carbs, low to the ground, and 1200 lobs, it was a winner.

 

[Shannow]

wulimaster,
you'll feel the difference in acceleration with manual hubs, and will most certainly notice mileage.

Every 4WD I've owned has/had manual hubs, and you have to lock them in every month or so to run the diff/bearings/oil/CVs etc.

You certainly don't forget that they are in.

 

[Fuelrod]

I recently drove a 1989 20R carbureted, 4-speed standard pick-up, camper shell, 205/75R/14 tires from Boston to Denver. 175,000 miles on the meter. The speed driven was between 70-80 mph, which was pushing it a little. The best mileage was 25mpg and the worst was 22.....Which is exactly the same mileage my '98 Crown Victoria (4.6 ohc V-8 P-71 package) will deliver.

The Toyota P/U was given a complete tune-up before the trip and ran flawlessly. It consumed no oil. The Vic likes a quart every 1500 miles.

 

[John K]

Every study I've seen on tailgates has also come to the conclusion that taking off the tailgate doesn't help. In my experience the biggest impact on mileage is usually from tires, overinflating them helps. Anytime my mileage drops the first thing I check is tire pressure. Smaller narrower tires help too.

 

[LVHospiceRN]

Also, a washed and waxed vehicle provides 1.863% LESS drag than a unwashed/unwaxed one...YMMV...

darrell
sin city

 

[Jimbo]

quote:

---

Originally posted by wulimaster:



I don't believe the larger tires helped me any and in fact I believe they hurt my mileage. (did compensate for the 31" versus the 28.2" in my mpg calculation) I will make this run again on the original rims and tire size at a later date when my 31's wear out in 10k more miles and I get some miles on the next set of tires (13k should do it). The 31's definitely hurt the city mileage with all the stop and go driving.

---

31 inch tires can help highway mileage because they act as an overdrive. Your truck has somethng like a 4.1 final drive ratio, which is pretty aggressive for 28 inch tires. The extra inertial mass of bigger tires can hurt city mileage, along with possibly delaying upshifts.

 

[wulimaster]

quote:

---

Originally posted by loudist:
Windows up or down?
That has an appreciable effect on aerodynamic drag, and could be the factor in you mileage quest.

---

With 70 degree weather the windows were up. I always keep the windows up.

 

[wulimaster]

quote:

---

Originally posted by Jimbo:

quote:

---

Originally posted by wulimaster:

---

31 inch tires can help highway mileage because they act as an overdrive. Your truck has somethng like a 4.1 final drive ratio, which is pretty aggressive for 28 inch tires. The extra inertial mass of bigger tires can hurt city mileage, along with possibly delaying upshifts.

---

At the speeds I was ging I don't think the overdrive effect was enough to overcome the larger contact footprint size of the 10.5" tires. The engine peak torque happens at 56 mph with the stock size tires.

This being an automatic, it has 4.56 gearing.

 

[loudist]

Windows up or down?
That has an appreciable effect on aerodynamic drag, and could be the factor in you mileage quest.

 

[darkdan]

the reasoning behind the tailgate off increasing drag is pretty simple.

With the tailgate up the air hits the back of the tailgate and fills the bed with a pool of air basically.

Air hitting a bed full of air and spilling out and flowing out creates less drag than air slamming into the bed of the truck.

air to air resistance < air to steel resistance

 

[John K]

It just doesn't work the way you think it does... tailgates down increases drag!
http://www.cartalk.com/content/columns/Archive/1997/October/05.html

 

[John K]

Here is another link -

http://truck-bed-covers.com/Tonneau_covers_save_fuel.htm

 

[wulimaster]

quote:

---

Originally posted by John K:
Every study I've seen on tailgates has also come to the conclusion that taking off the tailgate doesn't help.

---

The studies you quoted were wind tunnel studies. Do you have any where they actually took a truck out and confirmed their wind tunnel results with actual driving tests?

 

[ediamiam]

I concur w/ darkdan and JohnK. Leaving tailgate up does decrease drag. I've seen some of the reports, don't remember whether it was wind-tunnel tested or not. But I remember from being in the back of a pickup, that the wind currents virtually die down.

 

[John K]

I've tried it myself when I had a Ranger and saw no difference. Problem is there are too many variables to detect in day to day driving. Why don't you believe wind tunnel testing? If there is more drag, then it takes more horsepower to power it, thus more fuel...... Makes sense to me. I seem to remember Consumer Reports trying it and it didn't help, but I can't find the reference.

 

[John K]

This link stirs things up a bit, but does point out that it is againt the law in some places to drive with it down and it weakens the structure...
http://www.sanfrangasprices.com/Forum_MSG.aspx?master=1&category=1054&topic=50975&page_no=1&FAV=N

 

[Jimbo]

quote:

---

Originally posted by wulimaster:

...At the speeds I was ging I don't think the overdrive effect was enough to overcome the larger contact footprint size of the 10.5" tires. The engine peak torque happens at 56 mph with the stock size tires.

This being an automatic, it has 4.56 gearing.

---

Yikes! That is a very aggresive ratio, I think helping to prove my point.

Tire contact patch is only a function of air pressure and axle load, idependent of tire size. 1500 lbs of tire load / 30 lbs per sq in = 50 sq inches.

 

[John K]

Here is another wind tunnel site someone else posted that says a hard cover improves drag by 22%!
http://sitemaker.umich.edu/um.gm.truck

 

[XS650]

quote:

---

Originally posted by John K:
Here is another wind tunnel site someone else posted that says a hard cover improves drag by 22%!
http://sitemaker.umich.edu/um.gm.truck

---

The base model pickup they used had a Cd of 0.311, which would be good for amodern car. Had they used a real world pickup instead of the model they used, reluts might have been a bit different.

Or they might not have.

 

[oilyriser]

Reducing drag also frees up horsepower, so you can get up to speed faster, or reach a higher top speed.