OE tire size?
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Lead Driver
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From: On the Edge of the Desert
OE tire size?
what was the OE tire size for a 2wd 71 f100?
im trying to figure out what speedo gear I need now that its got 31x10.50s... the truck has 3.25 gears.
as it is now, the speedo says 50 and im going about 40...
im trying to figure out what speedo gear I need now that its got 31x10.50s... the truck has 3.25 gears.
as it is now, the speedo says 50 and im going about 40...
Fleet Owner
Joined: Dec 2009
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From: NorCal
I used an online calculator using the stock size and new tire size and the difference should only be 4.4 mph.
Fleet Owner
Joined: Dec 2009
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From: NorCal
Yeah, like 3.70s. I think you may have minsunderstood what I meant by final drive ratio.. it's acting like a 2.89 due to the larger diameter tires... long legs, less revs to maintain the same velocity.
Dunno about doing an AOD. Someone will chime in soon... wait for it.
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Lead Driver
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From: On the Edge of the Desert
but it has 3.25s now????
heres my train of thought,
3.25s with 215-75-15s would be correct speedo, but bigger tires = less rpm, the same thing as higher gearing. so a speedo drive gear that was for a 2.89 gear would be correct??
as far as the difference in speedo reading vs actual, yes it is only about 5mph, I confused myself. (again)
heres my train of thought,
3.25s with 215-75-15s would be correct speedo, but bigger tires = less rpm, the same thing as higher gearing. so a speedo drive gear that was for a 2.89 gear would be correct??
as far as the difference in speedo reading vs actual, yes it is only about 5mph, I confused myself. (again)
Fleet Owner
Joined: Dec 2009
Posts: 20,678
Likes: 82
From: NorCal
but it has 3.25s now????
heres my train of thought,
3.25s with 215-75-15s would be correct speedo, but bigger tires = less rpm, the same thing as higher gearing. so a speedo drive gear that was for a 2.89 gear would be correct??
as far as the difference in speedo reading vs actual, yes it is only about 5mph, I confused myself. (again)
heres my train of thought,
3.25s with 215-75-15s would be correct speedo, but bigger tires = less rpm, the same thing as higher gearing. so a speedo drive gear that was for a 2.89 gear would be correct??
as far as the difference in speedo reading vs actual, yes it is only about 5mph, I confused myself. (again)
"Higher gearing" means lower numerically. Hence the final drive ratio acting numerically lower because of the increased tire size resulting in lower RPMs to maintain the same velocity (speed).
You need to speed up the speedometer, right?
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Lead Driver
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From: On the Edge of the Desert
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Fleet Mechanic
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From: Gainesville, GA
351, check how many 1/10s it's off in a mile and go to your friendly Ford dealer and tell him. Take your present gear with you. The freeways have mile posts.
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Lead Driver
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From: On the Edge of the Desert
haha, yea I need to do that too, but it doesn't quite run yet hahaha
I just needed the OE tire size to figure out my diameters. I can do the math and figure it out...
OLD TIRE DIAMETER X OLD SPEEDO GEAR / NEW TIRE DIAMETER =NEW SPEEDO GEAR
now I just need to know what my old speedo gear is, and I can find that out when I pull it loose...
this is just another drop in my glass of troubles...
I just needed the OE tire size to figure out my diameters. I can do the math and figure it out...
OLD TIRE DIAMETER X OLD SPEEDO GEAR / NEW TIRE DIAMETER =NEW SPEEDO GEAR
now I just need to know what my old speedo gear is, and I can find that out when I pull it loose...
this is just another drop in my glass of troubles...
Fleet Owner
Joined: Dec 2009
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From: NorCal
Old Tire Diameter: 27.6 inches
New Tire Diameter: 31 Inches
Indicated Speed 40 mph
Actual Speed: 44.92 mph
That means that the speedometer needs spin faster to state the actual speed.
Terminology is important. Let's speak the same language.
A "low" gear is numerically high like 6.69 first gear for the NP435
A "high" gear is numerically low like 1:1 for a four-speed manual.
Your truck thinks its final drive is 2.89, not a 3.25 anymore. Therefore, it needs to be tricked into thinking it's back to a 3.25 by using a speedometer drive gear that is for a numerically higher geared truck like a 3.50 or a 3.73.
Am I losing my mind on this?
Ford Parts Specialist
Joined: Nov 2006
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From: Simi Valley, CA
DRIVE gear located on output shaft of trans / DRIVEN gear located at end of speedo cable.
Both are 'figured out' by trans type/axle ratio and tire size.
F100's of this era came with either 7 or 8 tooth DRIVE gears (obsolete) and 16 thru 21 tooth nylon DRIVEN gears (available from Ford).
To slow speedo down, add additional teeth. To speed it up, deduct teeth. Both apply to DRIVEN gear.
OP's user name contains C4, so if his F100 has this trans:
Both are 'figured out' by trans type/axle ratio and tire size.
F100's of this era came with either 7 or 8 tooth DRIVE gears (obsolete) and 16 thru 21 tooth nylon DRIVEN gears (available from Ford).
To slow speedo down, add additional teeth. To speed it up, deduct teeth. Both apply to DRIVEN gear.
OP's user name contains C4, so if his F100 has this trans:
Hotshot
Joined: Mar 2013
Posts: 14,255
Likes: 199
From: Phoenix, Az.
DRIVE gear located on output shaft of trans / DRIVEN gear located at end of speedo cable.
Both are 'figured out' by trans type/axle ratio and tire size.
F100's of this era came with either 7 or 8 tooth DRIVE gears (obsolete) and 16 thru 21 tooth nylon DRIVEN gears (available from Ford).
To slow speedo down, add additional teeth. To speed it up, deduct teeth. Both apply to DRIVEN gear.
OP's user name contains C4, so if his F100 has this trans:
Both are 'figured out' by trans type/axle ratio and tire size.
F100's of this era came with either 7 or 8 tooth DRIVE gears (obsolete) and 16 thru 21 tooth nylon DRIVEN gears (available from Ford).
To slow speedo down, add additional teeth. To speed it up, deduct teeth. Both apply to DRIVEN gear.
OP's user name contains C4, so if his F100 has this trans:
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Lead Driver
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From: On the Edge of the Desert
i do have the C4 in it now. but plans are for an AOD in the future.
Hio, yes that is correct, but my guage is reading fast now, I need to slow it down. I think you might be getting caught in my brain loop
I pulled mine loose and I have a 16T gear.
so according to jeffs 4-6 mph per tooth, to slow the speed meter aprox 5mph id need one with 17teeth right?
another wrench in the works may be that this tranny came from a torino... and the AOD that is planned from a 'stang
and where the heck can I find one of these? I seem to be having a hard time tracking one down...
Hio, yes that is correct, but my guage is reading fast now, I need to slow it down. I think you might be getting caught in my brain loop
I pulled mine loose and I have a 16T gear.
so according to jeffs 4-6 mph per tooth, to slow the speed meter aprox 5mph id need one with 17teeth right?
another wrench in the works may be that this tranny came from a torino... and the AOD that is planned from a 'stang
and where the heck can I find one of these? I seem to be having a hard time tracking one down...
Hotshot
Joined: Mar 2013
Posts: 14,255
Likes: 199
From: Phoenix, Az.
i do have the C4 in it now. but plans are for an AOD in the future.
Hio, yes that is correct, but my guage is reading fast now, I need to slow it down. I think you might be getting caught in my brain loop
I pulled mine loose and I have a 16T gear.
so according to jeffs 4-6 mph per tooth, to slow the speed meter aprox 5mph id need one with 17teeth right?
another wrench in the works may be that this tranny came from a torino... and the AOD that is planned from a 'stang
and where the heck can I find one of these? I seem to be having a hard time tracking one down...
Hio, yes that is correct, but my guage is reading fast now, I need to slow it down. I think you might be getting caught in my brain loop
I pulled mine loose and I have a 16T gear.
so according to jeffs 4-6 mph per tooth, to slow the speed meter aprox 5mph id need one with 17teeth right?
another wrench in the works may be that this tranny came from a torino... and the AOD that is planned from a 'stang
and where the heck can I find one of these? I seem to be having a hard time tracking one down...
sell it to you at a wholesale price. PM me if you want it.
BTW, AOD's had 7 or 8 tooth Drive gears also.
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