1979 F-150
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Freshman User
Joined: Oct 2001
Posts: 48
Likes: 0
From: Midwest
1979 F-150
I have a 1979 F-150 pictures are on here under user gallery boone6978. Im having trouble figuring my gear ratios. someone ripped the little tag of the housing on the 9 inch axle. and the calculator says i have 4.10 gears, cause i have 31 inch tires and at 60 mph i run about 2800 rpm's. funny thing is that the door tag says i have something like 2.75 gears (does this sound right?). is there any other way to find out for sure. Could this be a cab and box on a different frame? any help is appreciated greatly. Thanks Dan 
New User
Joined: Aug 2002
Posts: 9
Likes: 0
From: HERKIMER USA
1979 F-150
sOMEONE SOMEWHERE PUT ANPOTHER DOOR ON YOUR TRUCK, AND IT SOUNDS LIKE THAT DOOR CAME FROM A 2WD, AS THEY WERE THE ONLY ONES THAT HAD 2.75 GEARS IN THEM. cHECK YOUR FRONT TAG ON THE AXLE BY YHT DIFF COVER, THESE USUALLY DON'T GET TORN OFF, THAT SHOULD LET YOU KNOW WHAT GEARS THAT YOU HAVE. eric d>
Senior User
Joined: Sep 2002
Posts: 321
Likes: 0
From: kansas
1979 F-150
Just to let you know, if you've got 31-10.50-15 tires, they probably aren't 31" tall. That's what's referred to as a nominal size. You'll have to physically measure them if you want to know how big they are. Even a little bit can throw you're calculations off. I would only consider anything calculated off of you're tire diameter an estimate. Even putting it on a wider or narrower wheel will affect it's diameter.
FTE Leadership Emeritus
Joined: May 2001
Posts: 13,653
Likes: 6
From: Cabot, Arkansas
1979 F-150
Agood way to estimate your gear ratio:
1.Jack up the rear of the truck and place on jack stands.
2.Place a chalk mark on the tire top center, and another on the driveshaft where you can see it.
3.Place truck in neutral
4.Rotate tire one revoloution, and count the number of driveshaft revoloutions.
Example: The driveshaft turns 3.5 times per tire rev.= 3.5*1, or 3.50 gears.
1.Jack up the rear of the truck and place on jack stands.
2.Place a chalk mark on the tire top center, and another on the driveshaft where you can see it.
3.Place truck in neutral
4.Rotate tire one revoloution, and count the number of driveshaft revoloutions.
Example: The driveshaft turns 3.5 times per tire rev.= 3.5*1, or 3.50 gears.
