Rear Gearing?????
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Senior User
Joined: Oct 2004
Posts: 100
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From: Maggie Valley, NC
Rear Gearing?????
All right guys...I have decided to keep the truck and finish building it, forget the Jeep. Maybe Later...anyways. It has a 302 with a 3 speed. It has a 9" rear. Does anyone know what rearend ratio I have? When I am going down the interstate, it runs 80mph at about 3000 rpms. Im thinking its a 3.89. This summer I am putting a locker and new ring and pinion so I was wondering what ratio I was getting so I can still have the off the line power, but be able to lower the rpms while going 80mph.
This summer I am putting aluminum heads and a bigger intake, 3" exhaust, and maybe a cam with a little higher lift. I am also lowering it 4 more inches, putting a fiberglass front clip on and redoing the interior. It going to be almost finished after that.
This summer I am putting aluminum heads and a bigger intake, 3" exhaust, and maybe a cam with a little higher lift. I am also lowering it 4 more inches, putting a fiberglass front clip on and redoing the interior. It going to be almost finished after that.
Fleet Mechanic
Joined: Jul 1999
Posts: 1,360
Likes: 1
From: Iowa
If we knew the tire diameter, rpm, and speed, we could figure the rear ratio. It sounds to me as if you've got a 3.00 or a 3.50 rear to be spinning only 3000 at 80mph. Like fordxxx said, the codes on the door tag will help identify the ratio.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Door tags will tell you how it left the factory.
IMHO, Ya gotta roll the truck one tire revolution while counting drive shaft revs. to get an idea of your real rear ratio.
John
IMHO, Ya gotta roll the truck one tire revolution while counting drive shaft revs. to get an idea of your real rear ratio.
John
Senior User
Joined: Jan 2003
Posts: 387
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From: Calgary
The factory gearing is a good place to start and how long does it take to go outside and look at the door tag. Then, as 390FE suggests, calculate your expected engine speed with that rear end ratio and see if it matches up with what you have observed. It will take 10 minutes and you won't have to get your hands dirty.
Thread Starter
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Senior User
Joined: Oct 2004
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From: Maggie Valley, NC
The truck is 212 miles away so it would be kind of hard to go outside and look at it...I didnt know that the tag on the door had the ratio on it so now I can figure it out.
Senior User
Joined: Jan 2003
Posts: 387
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From: Calgary
Ok we don't know the tire size but let's say that they are 235/75 R15 like John has which are 27.5" in diameter. Each rotation will cover 7.1995'. At 80mph your are covering 7040' per minute which will equal 978 tire revolutions. For you to be turning 3000rpm then you would need a rear end gear of 3000/978= 3.06.
If your tires are bigger then the gear ratio will be numerically higher but I doubt that you have 3.89 unless you have huge tires.
I guess you need to look at the truck to know for sure.
If your tires are bigger then the gear ratio will be numerically higher but I doubt that you have 3.89 unless you have huge tires.
I guess you need to look at the truck to know for sure.
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Lead Driver
Joined: Jan 2003
Posts: 9,893
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From: Ohio
My 302/3 spd truck had 3.50 gears from the factory. Seems it is 3.00 by the calculations, but that seems pretty light for a 302/3 spd truck. Never know, until you actually check the numbers with known details.
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Joined: Apr 2005
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From: San Rafael, Californy
Check the door tag. Write down the last two characters on the second line. For a '71, the 9" ratios are as follows:
Tag# Ratio
02 3.00
H1 or 17 3.25
H2 or 08 3.50
09 3.70
H3 4.09
05 4.11
Here is a cool link to use:
http://www.csgnetwork.com/multirpmcalc.html
Tag# Ratio
02 3.00
H1 or 17 3.25
H2 or 08 3.50
09 3.70
H3 4.09
05 4.11
Here is a cool link to use:
http://www.csgnetwork.com/multirpmcalc.html
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