Bigger Tires
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Cargo Master
Joined: Sep 2004
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Bigger Tires
I just bought my brother's 92 F-150. A few years back he installed a 3 inch lift and put monster 33 in tires. I remember it ran funny (didn't shift at the right times) after the big tires until he re-geared the rear-end. I bought it with 31 in tires. Here's my question. When you put bigger tires on the truck, is there more you have to do than just re-gear the rear? What I was wondering is what do you do to make the speedometer read correctly?
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Cargo Master
Joined: Sep 2004
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andym, I don't think my brother ever did this. Was re-gearing the rear-end unnecessary? Would this speedo reprogramming have handled everything? What I am getting at is that right after he initially put on the huge tires, the truck didn't shift correctly. It makes sense that it wouldn't because the car thinks its moving slower than it really is. So he went and re-geared it. Now if I re-program the speedo, will the truck still think it has the old rear-end? Or does it even matter? Thanks for your help.
Senior User
Joined: Sep 2003
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everything can be cut down to an equation. if you factor in gears, tire size, and engine rpm, you can get speed. If it is balanced by changing one of the factors you can get the equation to work. Your brother may have offset the tiresize by changing the gear. By recalibrating the speedometer, you can easily balance the equation. That's the way i see it, and the way it was explained to me.
Post Fiend
Joined: Mar 2002
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From: Bonita Springs FL
On the 1992-96 F-150s, the speedo is electronic, and reads off a vehicle speed sensor on the rear diff.
When you put bigger tires on, the truck thinks it's going slower than it is, and you will have a noticable loss in power. You can correct the speed issue by reprogramming the speedo. You'll still have the power loss, so you need to re-gear to get your power back.
They're not really related to each other. Just re-gearing the truck won't fix the speedo issue and won't correct the tranny shifting at the wrong time, so he had to have both done. Re-gearing doesn't affect how fast the truck thinks it's going. It only affects power.
When you put bigger tires on, the truck thinks it's going slower than it is, and you will have a noticable loss in power. You can correct the speed issue by reprogramming the speedo. You'll still have the power loss, so you need to re-gear to get your power back.
They're not really related to each other. Just re-gearing the truck won't fix the speedo issue and won't correct the tranny shifting at the wrong time, so he had to have both done. Re-gearing doesn't affect how fast the truck thinks it's going. It only affects power.
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Cargo Master
Joined: Sep 2004
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thanks guys. He said that he never reprogrammed the speedo. I remember that it ran jerky after he put the bigger tires on it. It is better since having been re-geared but still doesn't feel right. He changed the gearing from 3.73 to 4.10. I'm gonna put 33" tires back on it and reprogram the speedo. There was another thread in the last couple days about re-gearing. The advice there was that 3.73 was correct for 31" tires and 4.10 for 33". I'm gonna put 33's on it since the gearing is 4.10. Do you agree that will balance the equation?
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Joined: Jun 2003
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From: southern cal
speedo not reading accurately
Originally Posted by qman
thanks guys. He said that he never reprogrammed the speedo. I remember that it ran jerky after he put the bigger tires on it. It is better since having been re-geared but still doesn't feel right. He changed the gearing from 3.73 to 4.10. I'm gonna put 33" tires back on it and reprogram the speedo. There was another thread in the last couple days about re-gearing. The advice there was that 3.73 was correct for 31" tires and 4.10 for 33". I'm gonna put 33's on it since the gearing is 4.10. Do you agree that will balance the equation?
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