Checking Rear gear ratio
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Cargo Master
Joined: Oct 2005
Posts: 2,084
Likes: 94
From: Stanton, Michigan, USA
Checking Rear gear ratio
Will someone please confirm whether or not I'm recalling correctly the technique to determine the rearend gear ratio - without disassembling anything. We're talking about a '65 F100 with a T-18 and NON-Limited slip differential.
1) Put the 4-spd in netural.
2) Block the front wheels to keep the truck from rolling forward/back.
3) Jack up the rear to raise one rear tire off the ground.
4) Position myself under the truck so I can slowly turn the raised tire while counting the revolutions that the driveshaft turns.
5) Turn the tire until the driveshaft makes full rev.
The ratio the tire turns vs. the driveshaft revs is the rear gear ratio....
Example: Tire turns 3.5 times while the driveshaft turns 1 time = 3.50 gears.
Will that method give me a correct result?
If not, how do I do it?
Thanks for your assistance, Fellas!!
BarnieTrk
1) Put the 4-spd in netural.
2) Block the front wheels to keep the truck from rolling forward/back.
3) Jack up the rear to raise one rear tire off the ground.
4) Position myself under the truck so I can slowly turn the raised tire while counting the revolutions that the driveshaft turns.
5) Turn the tire until the driveshaft makes full rev.
The ratio the tire turns vs. the driveshaft revs is the rear gear ratio....
Example: Tire turns 3.5 times while the driveshaft turns 1 time = 3.50 gears.
Will that method give me a correct result?
If not, how do I do it?
Thanks for your assistance, Fellas!!
BarnieTrk
Posting Guru
Joined: Nov 2004
Posts: 1,111
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From: Turlock CA
Ok a couple of corrections here.
First if it is limited slip you may have to lift both tires off of the ground or it may be real hard to turn the tire.
Second you need to turn the drive line and count that for one full revolution of the tire. The driveline turns faster than the tires. My rearend is a 3.00 there for my drive line turns three times for every tire rotation.
And don't forget to put the truck on jack stands we don't need anybody to get hurt.
jd
First if it is limited slip you may have to lift both tires off of the ground or it may be real hard to turn the tire.
Second you need to turn the drive line and count that for one full revolution of the tire. The driveline turns faster than the tires. My rearend is a 3.00 there for my drive line turns three times for every tire rotation.
And don't forget to put the truck on jack stands we don't need anybody to get hurt.
jd
Last edited by jd_sylvia; Dec 18, 2005 at 10:47 AM.
Senior User
Joined: Jun 2002
Posts: 177
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From: Red Deer Canada
Nice catch JD. Safety is important. The other points you made would probably been quite obvious when performing the work. Lets see, what would the ratio be using the wrong method? Details man, details.....like you said "if it's worth doing......"
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Cargo Master
Joined: Oct 2005
Posts: 2,084
Likes: 94
From: Stanton, Michigan, USA
Thanks for the replys, phat56 & jd !
jd,
As I mentioned earlier, it's a NON limited slip differential, so only one tire up should be fine then.....
I also appreciate the concern over safety. However, with only one tire off the ground, even if my floorjack was to somehow blow up and the one tire up was to drop back to the ground, I suspect my hat wouldn't even be rattled since there's soo much ground clearance under the ol' boy anyway!
Lastly, thanks for correcting me on the math..... As phat56 suggested, I probably would of been suspect of my math (wrong method) had I counted one revolution on the driveshaft and 3 revolutions at the tire, resulting in a calculated 0.33 rear gear ratio! LOL!
Thanks for all your help, Fellas!
BarnieTrk
jd,
As I mentioned earlier, it's a NON limited slip differential, so only one tire up should be fine then.....
I also appreciate the concern over safety. However, with only one tire off the ground, even if my floorjack was to somehow blow up and the one tire up was to drop back to the ground, I suspect my hat wouldn't even be rattled since there's soo much ground clearance under the ol' boy anyway!
Lastly, thanks for correcting me on the math..... As phat56 suggested, I probably would of been suspect of my math (wrong method) had I counted one revolution on the driveshaft and 3 revolutions at the tire, resulting in a calculated 0.33 rear gear ratio! LOL!
Thanks for all your help, Fellas!
BarnieTrk
Posting Guru
Joined: Nov 2004
Posts: 1,111
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From: Turlock CA
I am a safety nut. I always end up taking care of all the safety issues were I work.
My father once thought he was safe and when the floor jack blew out from under the truck and landed on his chest and ruptured his spline I gues it kind of woke me up. I always tell people at work there is no such thing as to safe.
You guys take it easy and work safe.
jd
My father once thought he was safe and when the floor jack blew out from under the truck and landed on his chest and ruptured his spline I gues it kind of woke me up. I always tell people at work there is no such thing as to safe.
You guys take it easy and work safe.
jd
Senior User
Joined: Jun 2002
Posts: 177
Likes: 0
From: Red Deer Canada
10/4 -With the health care system we now have up here, there's no way I want to end up in the waiting room. I always use all the proper PPE when I work at home. It's probably the brain washing I get at work. later dudes
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Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
There is a method, that I like better. Grab a bud (for safety) put a mark on a tire out near the edge. With the truck on level ground roll it one tire revolution, while counting the driveshaft revolutions. You can tape a string to the d.s. if you like.
That way the only thing you have to keep up with is the revs. not whether it's ls or open. I always forget how to calculate. You will get an acurate ratio with any size tire.
John
That way the only thing you have to keep up with is the revs. not whether it's ls or open. I always forget how to calculate. You will get an acurate ratio with any size tire.
John
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