So I'm looking into changing the gear ratio in my rear end instead of doing the AOD swap (it was giving me a head ache and I really want my truck ha). I was told that he (mechanic) was going to "add" a gear to it. What gear do I need so my truck isn't at a high RPM on the interstate (55-70 mph)?

 

If your truck is an original 302, it's likely a 3.50 ratio(could be more, 3.70/4.10 depending on how it was ordered).

You could put a 3.00 in it, but remember that will remove some of it's pulling power. Ya, if you just want to drive down the highway, fine, but it will get real gutless as it lugs that little motor down. They also make a 3.50 ratio.

Going from 3.50 down to 3.00 will result in about 450 rpm drop in RPM. So if you are cruising at 3000 rpm at 70 mph, it will be 2550 now.

You need to first find out what's back there. If no one has touched it, there will be a metal tag attached to one of the bolts to the center section with the ratio stamped on it.

 

Most common rear axle ratio used in 1965/72 F100's w/a V8, A/T & 9" rear axle = 3.25-1.

The AXLE code is stamped on the Warranty Plate, 1965/72 F100 w/o Limited Slip:

08 = 3.50-1 / 07 (1965 only) = 3.25-1 / 17 = (1966/72) 3.25-1.

 

An easy way to get gear ratio if it is unknown or questioned, is to put truck on a flat surface so it wont roll by itself, with the front wheels straight. Perhaps have a spotter for safety, get under truck and at hog-head, rotate drive shaft so the one of the universal joint knuckles are pointed straight down in the 6 o'clock position and mark it with white chalk. mark bottom most portion of tire facing the ground. rotate drive shaft moving truck forward, count the number of times the drive shaft rotates. Do so until the tire has made a complete revolution and the tire is facing down where you marked it. Note how many times the drive shaft rotated and you will have gear ratio. For example, if you started out with drive shaft knuckle marked in the 6 o'clock position and you rotated it 3 times and the forth time the marked knuckle stopped in the say the 2 o'clock position when the wheel has made one revolution then you most likely have about a 3.70 to 1 gear ratio. If you make 4 full turns of the drive shaft and it is on the say 6:30-7:00 o'clock position then you have a 4:10 gear ratio... It does not matter what size tire you have as a rotation is a rotation... smaller tires the truck will travel a shorter distance, larger tires the truck will move forward a further distance.

Cheers Jay

 

generally when your mechanic says he is gonna add a gear or make the gear taller, means that he will decrease the gear ratio.