Help me understand this....
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From: Raleigh
Help me understand this....
99 F150 2WD replaced the rear end, old one was a 3:55 replacement is a 3:08 at least that is what the guys at the salvage yard told me. I figured it would get a little better gas mileage and the guys that did the swap agreed. But my son says he is getting about 40 miles per tank less than with the old rear end, how can this be? Or did the salvage yard get the ratio wrong? What other rear end gearing did Ford put in 97 - 03 F150's and how do I determine what this one is?
Thanks in advance for your help!
Bobby
Thanks in advance for your help!
Bobby
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From: Louisville,Ky.
Because with the 3.08, the truck has to work much harder to accelerate due to the lack of torque multiplication of the 3.08 vs 3.55. On a long highway trip you may gain a little mpg. But in city driving, it will suffer.
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On most FORD rear ends there is a tag attached by a cover bolt that has the ratio and year and where built. On ours it clearly is 3.31 or the 3.31 ratio. An alternative would be 3L31 which would be a 3.31 with limited slip.
Actually on out 98 there two tags, one says synthetic fluid only.
If the tags gone the best way is to remove the cover and count the gear teeth that always works on any rear end.
It is possible to get worse mileage with taller gears. But I have never observed that.
I went from 3.08 to 3.54 and lost at least 1 mpg on the highway which is less than I expected.
I have also gone from 2.76 to 3.08 and observed no difference.
I tend to run large cube engines the little 4.6 engine could be working too hard and requiring more fuel with the taller gears.
You can also jack up one wheel put the tranny in neutral and turn and count the drive shaft turns until the tire makes one complete turn. Remember that the tire is turning 2 X because of the differential so multiply the ds turns by 2 and average to the closest expected ford ratio.
** my feeling is the ratio is not what you thought you were buying **
Actually on out 98 there two tags, one says synthetic fluid only.
If the tags gone the best way is to remove the cover and count the gear teeth that always works on any rear end.
It is possible to get worse mileage with taller gears. But I have never observed that.
I went from 3.08 to 3.54 and lost at least 1 mpg on the highway which is less than I expected.
I have also gone from 2.76 to 3.08 and observed no difference.
I tend to run large cube engines the little 4.6 engine could be working too hard and requiring more fuel with the taller gears.
You can also jack up one wheel put the tranny in neutral and turn and count the drive shaft turns until the tire makes one complete turn. Remember that the tire is turning 2 X because of the differential so multiply the ds turns by 2 and average to the closest expected ford ratio.
** my feeling is the ratio is not what you thought you were buying **
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