speedometer calibaration...
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Senior User
Joined: Jun 2003
Posts: 203
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From: Lancaster, SC
speedometer calibaration...
I have a 3spd manual '79 F100 with 2.75 gears. Currently it reads about 5 mph under when driving, ie running at 60 mph on the speedo gps' at 65mph.
I have taller tires than the stock 205/75R15, using 235/75R15.
I'm regearing the rearend to 3.50 gears and want the speedo to be accurate when finished.
Do I change a gear in the tranny or speedometer?
How do I calculate what gear I need?
Is is hard to change?
I have taller tires than the stock 205/75R15, using 235/75R15.
I'm regearing the rearend to 3.50 gears and want the speedo to be accurate when finished.
Do I change a gear in the tranny or speedometer?
How do I calculate what gear I need?
Is is hard to change?
New User
Joined: Jan 2008
Posts: 0
Likes: 24
In the transmission.
All speedometers are the calibrated thesame.
You need a driven gear that turns slower.
They are colour coded for ease of recognition.
You can do the check here.
https://www.ford-trucks.com/calculat.../p-tire_sizes/
All speedometers are the calibrated thesame.
You need a driven gear that turns slower.
They are colour coded for ease of recognition.
You can do the check here.
https://www.ford-trucks.com/calculat.../p-tire_sizes/
New User
Joined: Jan 2008
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P235 R78-15" = L78-15
3 spd MT w/3.50 axle ratio = driven gear C1DD-17271-A
driven gear = Type 2A- 20 teeth - Orange in colour
You should be removing a TAN ( with oil dis-colourment) coloured one
3 spd MT w/3.50 axle ratio = driven gear C1DD-17271-A
driven gear = Type 2A- 20 teeth - Orange in colour
You should be removing a TAN ( with oil dis-colourment) coloured one
Last edited by Mil1ion; Dec 4, 2007 at 03:10 PM.
Laughing Gas
Joined: Feb 2007
Posts: 821
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From: hohenwald, tn
hey dennis, my door doesn't have a tag and the tag on the rear end is rusted off. my speedo is off. the gear is red now. duh? i need to get my wifes gps working.
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Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
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From: Simi Valley, CA
Originally Posted by Mil1ion
P235 R78-15" = L78-15
3 spd MT w/3.50 axle ratio = driven gear C1DD-17271-A
driven gear = Type 2A- 20 teeth - Orange in colour
You should be removing a TAN ( with oil dis-colourment) coloured one
3 spd MT w/3.50 axle ratio = driven gear C1DD-17271-A
driven gear = Type 2A- 20 teeth - Orange in colour
You should be removing a TAN ( with oil dis-colourment) coloured one
The gears (15 thru 21 teeth) are still available from Ford.
C8SZ-17271-B .. 20T Gear-Orange ~ Ford list price $8.42.
Last edited by NumberDummy; Dec 5, 2007 at 08:56 PM.
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Ok, maybe not a question with an easy answer, but...
my truck with a C6 and 3.00 rear end and 235/75R15 tires the speedo was dead nuts on. Now with 31" (30.5 actual) tires and 3.50 rear gears the speedo reads way fast. What gear would I possibly move to? I believe the gear is red (it's been a while and it's 10 degrees outside at the moment)
Josh
my truck with a C6 and 3.00 rear end and 235/75R15 tires the speedo was dead nuts on. Now with 31" (30.5 actual) tires and 3.50 rear gears the speedo reads way fast. What gear would I possibly move to? I believe the gear is red (it's been a while and it's 10 degrees outside at the moment)
Josh
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Joined: Jun 2004
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From: Dracut, Massachusetts
notice his rear gear ratio was changed from 3.00 to 3.50...
im having a similar problem, i have 32/12.5 r16 4.11 gears and my speedo is off by about 5 mph to fast, it says im going 65 and i'm only going 60. the current gear was white maybee tan? which once should i try now?
im having a similar problem, i have 32/12.5 r16 4.11 gears and my speedo is off by about 5 mph to fast, it says im going 65 and i'm only going 60. the current gear was white maybee tan? which once should i try now?
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Well 235 series tires are 28.8" and what is the ACTUAL measurement? Maybe 28? My 31" BFG TA/KO tires measure 30.5.
Also the gear change from 3.00 to 3.50. So I think my speedometer reads 5 MPH fast at indicated 40 MPH according to the Police speed signs around town (in other words I actually going 35, but the speedometer says 40).
Josh
Also the gear change from 3.00 to 3.50. So I think my speedometer reads 5 MPH fast at indicated 40 MPH according to the Police speed signs around town (in other words I actually going 35, but the speedometer says 40).
Josh