gear ratios

1971ford · Post by **1971ford** » Mon Mar 03, 2008 10:25 pm

i dont know anything about gear ratios in the rear end, but in my f100, what would be the best gear ratio or gears for milage? also i dont want the truck to be slow or anything.

maybe someone could teach me something about these things?

fordman · Post by **fordman** » Mon Mar 03, 2008 11:16 pm

what do you have in it now. most of the trucks around here that i have seen have had 3.25 in them. and they are automatic transmission trucks.

1971ford · Post by **1971ford** » Mon Mar 03, 2008 11:18 pm

i have no clue what i have.
how do i find out?

my truck is a f100, auto, 2wd lwb.

fordman · Post by **fordman** » Mon Mar 03, 2008 11:20 pm

the code is on the door vin its the last set of numbers in the top row under the vin number. it is also suppose to be on the tag bolted to the third member of the rearend.

1971ford · Post by **1971ford** » Mon Mar 03, 2008 11:26 pm

ill check that next time im working on the bump

thanks

what do you guys think about the best ratio for what i mentioned though?
i may have it , may not.

fordman · Post by **fordman** » Tue Mar 04, 2008 12:03 am

for mileage the smller the number the better the gas milage.

cdeal28078 · Post by **cdeal28078** » Tue Mar 04, 2008 12:35 pm

fordman wrote:for mileage the smller the number the better the gas milage.

Up to a point is my thinking. Go too low a ratio and I feel like the truck is lugging too much and takes too much gas peddle to get up to speed. Then the first hill you hit it lugs down and takes more gas to keep up to speed.
I'm not any engineer so that is just my shade tree thinking.
I also think which engine and set up you have makes a big difference.
I always liked the 3.25 when I had a 351w with a c6. Left the lights good and didn't rev too high on the road.
Now that I have a 460 in the truck I like the 3.0 better. Plenty of torque to leave the lights and get up the hills without adding more gas.
If I had a center chunk with some 2 somethings in it I would try those in my truck.

clint

basketcase0302 · Post by **basketcase0302** » Tue Mar 04, 2008 6:27 pm

Ryan,

If it appears that the original rearend has been changed or serviced...
You can jack up one rear wheel, (if it's not positive traction).
Put the tranny in nuetral.
Spin the wheel one complete revolution, while counting the driveshaft revolutions.

Do the math and...

You'll know close to what's inside the chunk.

Jeff

390Nut · Post by **390Nut** » Tue Mar 04, 2008 7:22 pm

3.25's are about as low as you'll likely want to go. They wont increase your mileage substantially, but it will be noticeable, and you wont lose too much take off power.

427blackpearl · Post by **427blackpearl** » Wed Mar 05, 2008 3:59 pm

also by changing the tire size you can mess with the ratio. all gears,trans,engine,speedo cable,and tire sizes are a match set. for diesel trucks you want a cruse rpm of 2000 to 2400 for best fuel usage and the cruse speed is around 65 to 72. i am sure that a gas truck has a cruse rpm too around 3000 to 3500 not sure sombody else take it from here.

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