Rear Diff gear swap
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Rear Diff gear swap
I have a 02, F250 4-4 PSD with 38's &3.73's. I was wanting to know if anybody has swap the diff. gears out. If so, did this bring back the lost power & correct the speedometer? I've done some calculating & think if i go from 3.73 to 4.56 this will be close to what i had before.
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From: mid michigan
I went with 4.56 gears. I have 35" tires. My goal was to go lower than stock tire and 3.73 gear. I did get my performance back. Highway driving is just fine also.
With your tire size I would look at 4.56 min. maybe 4.88 - 5.13 if you plan to tow much or go off road.
For the speedo you'll still need to correct. Gear ratio has no bearing on the speedo readings. Have a dealer tech set your tire size to lowest of the tire sizes available(biggest tire). There will be a 3 digit mumber in () along with the tire size. It's for tire revolutions per mile.
With your tire size I would look at 4.56 min. maybe 4.88 - 5.13 if you plan to tow much or go off road.
For the speedo you'll still need to correct. Gear ratio has no bearing on the speedo readings. Have a dealer tech set your tire size to lowest of the tire sizes available(biggest tire). There will be a 3 digit mumber in () along with the tire size. It's for tire revolutions per mile.
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Joined: Apr 2002
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From: IN FRONT OF YOU
As far as your speedo goes, I don't beleive the dealer can re calibrate for a 38" tire. I heard that even 35's were hit and miss.
You could look into the TRUESPEED, everybody who runs one seems to really like it.
You could look into the TRUESPEED, everybody who runs one seems to really like it.
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Joined: Oct 2002
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From: Del Rio, Texas USA
Originally posted by 99 sd v10
I went with 4.56 gears. I have 35" tires. My goal was to go lower than stock tire and 3.73 gear. I did get my performance back. Highway driving is just fine also.
With your tire size I would look at 4.56 min. maybe 4.88 - 5.13 if you plan to tow much or go off road.
For the speedo you'll still need to correct. Gear ratio has no bearing on the speedo readings. Have a dealer tech set your tire size to lowest of the tire sizes available(biggest tire). There will be a 3 digit mumber in () along with the tire size. It's for tire revolutions per mile.
I went with 4.56 gears. I have 35" tires. My goal was to go lower than stock tire and 3.73 gear. I did get my performance back. Highway driving is just fine also.
With your tire size I would look at 4.56 min. maybe 4.88 - 5.13 if you plan to tow much or go off road.
For the speedo you'll still need to correct. Gear ratio has no bearing on the speedo readings. Have a dealer tech set your tire size to lowest of the tire sizes available(biggest tire). There will be a 3 digit mumber in () along with the tire size. It's for tire revolutions per mile.
Great White.. for your original question.. there is a chart DCSpecial knows the link that shows the comparison.. but 4.56's with 36's(my setup) was equal to 4.10 gear ratio with stock tires.. so with 38's id guess youd be around 3.73 which is where you want to be......
JJ
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Thanks for the input, see if ya'll can understand this one. After the install of the 38's the trans. still has it's factory shift points & still thinks it's rolling on factory tires (265/75/R16) because of the speed sensor on the rear diff ,right. Do or does any body make a reprogramer (superchips) that can reconfigure for tire size, rear gear swaps, & boost power all in one hand held tool. The main problem i'm have right now is the truck lugs & on the slightest incline the trans wants to downshift.Trans is constantly hunting & i'm afraid this will mess it up if i dont do something.
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Joined: Mar 2002
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From: mid michigan
Originally posted by Yeti
As far as your speedo goes, I don't beleive the dealer can re calibrate for a 38" tire. I heard that even 35's were hit and miss.
You could look into the TRUESPEED, everybody who runs one seems to really like it.
As far as your speedo goes, I don't beleive the dealer can re calibrate for a 38" tire. I heard that even 35's were hit and miss.
You could look into the TRUESPEED, everybody who runs one seems to really like it.
This true. For me it turned out well. Going to the dealer that is. At 60 mph I read between 59.5 and 60. I'd say pretty close. The charge for the recal. at the dealer I went to was 20.00. Worth a shot if you ask me. No sense spending more than you have to. You might end up close enough on your speed.
My speed was over-stated stock, by about 9%. That also contributed to it coming out as well as it did for me.
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