Speed - O - Gearing
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Junior Member
Joined: Apr 2001
Posts: 1
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From: Scottsdale, AZ., USA
Speed - O - Gearing
I have a 98 F150 with a 4.2 V6, 5 speed manual transmission, 8.8 3.55 rearend . Last weekend I changed tires from 235/70R16 (28.9528" dia.) to 265/75R16 (31.6495 dia.). This change helped my gas mileage but set my speedo and tripmeter off by -10%. I called a speedometer shop and they quoted $65.00 to change the speedo drive gear if possible and $450.00 if not, to correct this problem. Can anyone tell me if a drive gear change is possible? Why would it cost $450.00 if the gear can not be changed?
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Senior Member
Joined: Feb 2001
Posts: 641
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From: Calgary, Oilberta
You CAN change the gear on a '98. It is 99 and up where reprogramming is required. They are still charging too much, though. You might want to change the gear yourself.
The Vehicle Speed Sensor (VSS) is located on the top of the tailshaft of either the transmission (4x2) or transfer case (4x4). It is held in by one bolt. After you remove it, count the number of teeth you have multiply that by (28.9528/31.6495) to get the new number of teeth required. The gears are under $20 from Ford.
Ian
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99 Expedition EB
5.4l, 4x4, 4.10 LS
WMS Tube, Bassani Cat Back Exhaust, Superchip Flipchip
The Vehicle Speed Sensor (VSS) is located on the top of the tailshaft of either the transmission (4x2) or transfer case (4x4). It is held in by one bolt. After you remove it, count the number of teeth you have multiply that by (28.9528/31.6495) to get the new number of teeth required. The gears are under $20 from Ford.
Ian
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99 Expedition EB
5.4l, 4x4, 4.10 LS
WMS Tube, Bassani Cat Back Exhaust, Superchip Flipchip
Junior Member
Joined: Apr 2005
Posts: 9
Likes: 0
From: Greeley, CO
Help with Speedo gear selection!!
I hope you can help me with a problem. I have a 91 Ford F150 XLT Lariat Supercab (5.8L) I recently upgraded the factory optional 235/75-15 tires to 31/10.50's and now of course my speedo is off. Can someone help me do the math to figure out which speedo gear I need to order from Ford to get the speedo accurate again? I believe the tranny is the E4OD if that helps. Thanks!!!!
John Chatman
John Chatman
Senior Member
Joined: Jul 2004
Posts: 1,380
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From: cincinnati
Okay some changes can be accounted for in speedo gear on these older trucks. If the changes are beyond that range you will have to install a speedcal or find another way to correct the ratio. Gear changes are not infinitely variable.
Here is what I use to calculate the ratio.
http://home.att.net/~jroal/drive.htm
You will need to count the teeth on your ouput shaft to get all the information. The only one I am sure of is that trucks with 4r70w transmissions have an 8 tooth output shaft. I don't know what the count is on the manuals. You can count your existing gears and figure the stock tire size and gearing to figure it out.
Alan
Here is what I use to calculate the ratio.
http://home.att.net/~jroal/drive.htm
You will need to count the teeth on your ouput shaft to get all the information. The only one I am sure of is that trucks with 4r70w transmissions have an 8 tooth output shaft. I don't know what the count is on the manuals. You can count your existing gears and figure the stock tire size and gearing to figure it out.
Alan
Last edited by dirtyd0g; Apr 10, 2005 at 01:55 PM.