what gear ratio?
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Senior User
Joined: Jan 2003
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From: Florida
what gear ratio?
Is there any way to figure out what gear ratio I've got? I know what the label inside the door jam says (3.55) but I want to know if this is true or not. The person I bought my Bronco from said that the previous owner had replaced the rear end and possibly some other stuff but he wasn't sure. It's a 302 with a c-6 and 31's. At 55mph it's running around 2700 - 2800 RPM. At 70 mph it's around 3700 - 3900 RPM. I was just hoping there might be some sort of calculation I could figure out what the gear ratio is without taking anything apart. I'd like to lift it and put 35's on and I know 3.55 isn't an optimal gear ratio for this size tire. It seems like the RPM's are pretty high for a 3.55 gear ratio, but this being my first truck with no OD perhaps this is normal for a c-6. Thanks, Eric
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Joined: Jan 2003
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From: Memphis, TN
what gear ratio?
You can pull the fill plug from the front of the diff housing and rotate the driveshaft until the stamped ID on the ring gear comes into view, but go slowly because the thick oil will have to drain off it for you to read it. It could either be the actual ratio (3.55), or it could be the numbers of teeth on the 2 gears (13 40).
Alternatively, you can lift one tire, mark the tire&ground and the yoke&diff housing, spin the yoke (d'shaft) until the wheel makes exactly 2 full revolutions (because the diff is dividing the speed), and count EXACTLY how many turns of the yoke it took. That's the ratio number.
Alternatively, you can lift one tire, mark the tire&ground and the yoke&diff housing, spin the yoke (d'shaft) until the wheel makes exactly 2 full revolutions (because the diff is dividing the speed), and count EXACTLY how many turns of the yoke it took. That's the ratio number.
Senior User
Joined: Nov 2001
Posts: 295
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From: Brandon, Mississippi
what gear ratio?
Try this
http://www.dirtroad.com/gears.htm
http://www.ramblerman.com/blazer/gearcalc.htm
Click the calculators icon at top
http://www.4lo.com/
http://www.dirtroad.com/gears.htm
http://www.ramblerman.com/blazer/gearcalc.htm
Click the calculators icon at top
http://www.4lo.com/
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Senior User
Joined: Jan 2003
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From: Florida
what gear ratio?
Originally posted by mrwizard
Sounds awful high for 3.55's. It sounds like the previous owner changed them to 4.10's. If he did, I sure hope for your sake that he changed the front ones to match.
Sounds awful high for 3.55's. It sounds like the previous owner changed them to 4.10's. If he did, I sure hope for your sake that he changed the front ones to match.
Senior User
Joined: Dec 2002
Posts: 118
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From: Dublin Ca
what gear ratio?
take off the cover and count the teeth! then divide it by the teeth on the pinion if you want to know 100%.
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Senior User
Joined: Jan 2003
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From: Florida
what gear ratio?
Originally posted by dblue351
take off the cover and count the teeth! then divide it by the teeth on the pinion if you want to know 100%.
take off the cover and count the teeth! then divide it by the teeth on the pinion if you want to know 100%.
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Joined: Jan 2003
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what gear ratio?
Originally posted by ecsmith0624
Guess that seems to be the only way to know for sure. I was hoping there was some lazy way to do it but I guess not. Not knowing for sure anyway. Thanks!
Guess that seems to be the only way to know for sure. I was hoping there was some lazy way to do it but I guess not. Not knowing for sure anyway. Thanks!
Post Fiend
Joined: Nov 2002
Posts: 9,987
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From: Memphis, TN
what gear ratio?
speed
For that to work, you either have to have a tight limited slip, or make SURE that both wheels have each made EXACTLY one full turn. Otherwise, the diff will split the rotation between the other tire & the d'shaft and you won't get an accurate measurement. That's why I said 2 turns with one wheel off the ground in my post. That way, there's no chance for error.
For that to work, you either have to have a tight limited slip, or make SURE that both wheels have each made EXACTLY one full turn. Otherwise, the diff will split the rotation between the other tire & the d'shaft and you won't get an accurate measurement. That's why I said 2 turns with one wheel off the ground in my post. That way, there's no chance for error.
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what gear ratio?
Steve,
agreed...sorry i missed that part about the wheel spin in your post. I just couldn't see the point of someone pulling the cover off the rear end just to count teeth, its much easier to jack it up..... by the way, is there a site that gives the breakdown of codes..for instance, my door sticker says the rear is an H9 I believe. Guess maybe I'll have to take some of my own advice and crawl under there but I do know this thing is pretty much all stock.....Larry
agreed...sorry i missed that part about the wheel spin in your post. I just couldn't see the point of someone pulling the cover off the rear end just to count teeth, its much easier to jack it up..... by the way, is there a site that gives the breakdown of codes..for instance, my door sticker says the rear is an H9 I believe. Guess maybe I'll have to take some of my own advice and crawl under there but I do know this thing is pretty much all stock.....Larry
what gear ratio?
Post Fiend
Joined: Nov 2002
Posts: 9,987
Likes: 7
From: Memphis, TN
what gear ratio?
H6 is Ford 9" limited slip 3.50. The H always means limited slip.
H9 is Ford 8.8" L/S 3.55. Also note in my original post that you can probably read the ring gear stamping thru the fill plug.
H9 is Ford 8.8" L/S 3.55. Also note in my original post that you can probably read the ring gear stamping thru the fill plug.