3.31 to 4.10 Gear Upgrade
12-07-2018, 08:39 AM
#11
Senior Member
I guess I should have been more specific rather that simply answering the question as asked. The speedo is set based on tire revs - at least it is in my SCT handheld. You ALSO have an input for final drive gearing. Point being, if you just change the gears and make that input then it does the math for you (you didn't change tire size). If you change tire size, you have to manually do the rev calc and make that input so the speedo is correct. Mine was set per an online calc but my speed was off based on GPS so I played with the revs until it was correct.
12-07-2018, 10:41 AM
#12
Grumpy Old Man
Unless Ford changed it recently, Ford's speedo calibration is based on tire revs/mile. Revs/mile is based on the rolling radius of the tire, and thus loosely based on the diameter of the tire. Axle ratio doesn't matter. Changing axle ratio shouldn't affect the speedo, but changing tire size does.
The number of tire revs/mile is programmed into the ABS computer (module). The ABS module counts the number of tire revolutions as you drive, using the ABS sensor on the axle shaft. The ABS module then communicates with the powertrain control module (PCM, or "the computer") which computes time, distance. and speed. So for example, if the ABS module is programmed with 624 revs/mile, when the count reaches 624, the odometer and trip meter are updated with one more mile traveled. With the clock in the PCM and the miles traveled from the ABS module, the PCM computes time/distance, then updates the speedo with the current speed.
Axle ratio effects engine RPM. But the speedo doesn't care how many engine revs it takes to result in one tire rev. It only cares how many tire revs you have over how much time. So if your speedo was calibrated to an accurate speed per GPS with the old axle ratio, it should still be calibrated to an accurate speed per GPS with the new axle ratio.
The number of tire revs/mile is programmed into the ABS computer (module). The ABS module counts the number of tire revolutions as you drive, using the ABS sensor on the axle shaft. The ABS module then communicates with the powertrain control module (PCM, or "the computer") which computes time, distance. and speed. So for example, if the ABS module is programmed with 624 revs/mile, when the count reaches 624, the odometer and trip meter are updated with one more mile traveled. With the clock in the PCM and the miles traveled from the ABS module, the PCM computes time/distance, then updates the speedo with the current speed.
Axle ratio effects engine RPM. But the speedo doesn't care how many engine revs it takes to result in one tire rev. It only cares how many tire revs you have over how much time. So if your speedo was calibrated to an accurate speed per GPS with the old axle ratio, it should still be calibrated to an accurate speed per GPS with the new axle ratio.
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chimmike (12-20-2018)
12-11-2018, 08:43 AM
#13
Warranty Issues?
Hello all. This is my first post but I've been reading these for ever, thank you all for the help in the past. I have had my 2017 F150 XLT 4x4 truck in the differential shop this week and should get it back today with an 4.10 upgrade from my 3.31's. I'm looking forward to seeing the performance difference especially while towing. My trailer is 32 feet long and fully loaded I weigh around 14000 lbs. It's always been tough going up and down the hills to the places I like to camp. Has anyone else done this upgrade recently? I think i'll post some before and after RPM pics when I get my truck back to help compare the differences. I asked the folks at the garage I left it at if my Speedometer will need recalibrated and he said probably not, because the speed sensor is on the wheels. Does this sound about right to those of you who know? I appreciate the help!
12-11-2018, 10:32 AM
#14
Grumpy Old Man
12-11-2018, 10:46 AM
#15
Grumpy Old Man
There should be no warranty issues if you properly install the ring gear and pinion from Ford parts, and you choose a ratio that was optional in your F-150. Ratios available for 2015 F-150 include 3.15, 3.31, 3.55, and 3.73. Sorry, Charlie, but 4.10 or 4.30 was not available in an F-150.
Last edited by smokeywren; 12-11-2018 at 10:48 AM.
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chimmike (12-20-2018)
12-20-2018, 07:56 AM
#16
Flaccid Member
12-20-2018, 08:21 AM
#17
Senior Member
12-29-2018, 11:32 PM
#18
Senior Member
I did this a few years ago on my 2013. RPM's up about 500 and took a ~1/2 MPG hit, but tows sooooo much better. Speedo stayed the same.
Last edited by SpeedViktm; 12-29-2018 at 11:39 PM.
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Twin snail putput (04-07-2020)
05-31-2019, 03:31 PM
#20
Speed, the 5.0/six speed 4x4 truck I'm considering has the 8.8", 3.55 open diff. If/when we get an Airstream and towing regularly, I'm considering your upgrade to the Ford Performance 4.10 gears, which I understand are what populates Raptors. Was yours a 4x4, and an 8.8", requiring the upgrade on both axles? labor cost? One criticism I've seen is increased noise from the back end. What is your experience? Do you have an LS, open, or aftermarket diff? I had good experience with the Detroit geared LS in my first Superduty [2000, 7.3L,, bigger pumpkin there ....] and would consider that. Any thoughts?