Fuel milage and final drive ratio
#1
12-18-2003, 11:01 AM
Fuel milage and final drive ratio
I have a 2000 Expy XLT 5.4 A4WD w/3.31 gears and 255/70/16's with about 40k on them. I get an average of about 13mpg per tank, a mix of about 70% highway, 30% city, with about 15% of the highway time stuck in traffic.
I use synthetic, in everything, bosch +4s, Brute Force intake, and gibson SIDO exhaust.
I'm thinking of changing gears to 3.73's for towing, and also, because I thik it might improve my milage, although I know that sounds counter-intuative (sp?).
Its strikes me that my overall final drive ratio may not be efficient. That although I'm proably spinng a lower RPM than most at a givin speed, my engine may not be efficient at that RPM. Also, that driving in the city, my engine would have to "work" less to get me to a given speed, even if it reved more, and perhaps this is good for milage.
If you read through the posts in the Expedition milage thread, it seems like thouse reporting even remotly decent milage are running 3.55's or 3.73's.
The absolute best milage I've ever gotten is 14.0, running all- highway at ~67mph & 1950rpm.
What do you think? Is my theory possibly valid?
I use synthetic, in everything, bosch +4s, Brute Force intake, and gibson SIDO exhaust.
I'm thinking of changing gears to 3.73's for towing, and also, because I thik it might improve my milage, although I know that sounds counter-intuative (sp?).
Its strikes me that my overall final drive ratio may not be efficient. That although I'm proably spinng a lower RPM than most at a givin speed, my engine may not be efficient at that RPM. Also, that driving in the city, my engine would have to "work" less to get me to a given speed, even if it reved more, and perhaps this is good for milage.
If you read through the posts in the Expedition milage thread, it seems like thouse reporting even remotly decent milage are running 3.55's or 3.73's.
The absolute best milage I've ever gotten is 14.0, running all- highway at ~67mph & 1950rpm.
What do you think? Is my theory possibly valid?
#2
12-18-2003, 01:23 PM
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Larry -
I agree with you. The best mileage will be acheived when your truck exerts the least amount of effort to maintain its speed. The biggest factor is probably the wind resistance on the truck.
In my truck, 65-68 mph is the optimal speed to maximize fuel efficiency. With the 3.31 gear, my 4.2 needs to downshift to overcome the slightest grade - so I think a few more rpms would allow the engine to work more efficiently. Speeding up, though, will increase wind resistance - making the body less efficient. The trick would be to increase the rpms at the optimal speed - which can be done by changing the gears, or turning off the O/D.
I agree with you. The best mileage will be acheived when your truck exerts the least amount of effort to maintain its speed. The biggest factor is probably the wind resistance on the truck.
In my truck, 65-68 mph is the optimal speed to maximize fuel efficiency. With the 3.31 gear, my 4.2 needs to downshift to overcome the slightest grade - so I think a few more rpms would allow the engine to work more efficiently. Speeding up, though, will increase wind resistance - making the body less efficient. The trick would be to increase the rpms at the optimal speed - which can be done by changing the gears, or turning off the O/D.