tmcolegr

As my signature states I have a 2015 F150 with LT325/60R20 tires with 5.0L & 3.31 gears. Performance isn't all that bad regardless of what others may lead you to believe. Nonetheless in an effort to return performance to "stock like acceleration" and possibly even improve MPG, I was considering changing to 3.73 gears as that was a factory available option.

I went to my dealer of choice and had a conversation with the Service Manager whom I have come to trust over the many years I have been dealing with him. He said he more than likely would be a willing participant but wanted to run this change by Service Engineering to get their input. Later that same day he sent me their response to which I was somewhat surprised. I won't share the entire correspondence - just the part that is relevant:

The Ford Technical Assistance Center cannot support an axle ratio change modification.
The vehicle was certified for sale with a 3.31 final drive ratio, modifications can affect emissions, fuel consumption and maybe considered tampering.

The PCM is hard coded for the 3.31 gear ratio based on the VIN#, in most cases the PCM will not accept the gear ratio change and a P1635 DTC will result for tire size or axle ratio out of range.
Load calculations will be off.
Speedometer speed will be off.
Transmission shift scheduling, harsh shifts and erratic TCC (torque converter clutch) concerns occur.

The axle ratio change is not recommended.

Since IDS no longer supports gear ratio and tire size changes, these parameters cannot be changed by the dealer. Therefore my dealer is unable to reprogram the PCM for the new gear ratio and will not perform this gear change.

Additional information: The Hypertech Speedometer Calibrator reprograms the BCM (Body Control Module) - not the PCM (Powertrain Control Module). Therefore changing the gear ratio and tire size with the Hypertech device merely makes the speedometer reading accurate - which is exactly what it professes to do.

Conclusion: at this time I will not be changing gear ratios based on Ford's recommendation

jcat

But a custom tune calibrates the PCM, not the BCM, so you'd be all set.

Additionally, by swapping the gears and correcting the ECU with a tune, any failures would have to be proven beyond a shadow of a doubt to be directly related to the gears themselves or the tune in order for them to not honor the warranty.

Of course Ford themselves won't support the change, they don't want the liability associated. Done aftermarket there is literally zero issue so long as you do it right and get a custom tune written for the truck.

tmcolegr

I was expecting Ford to support the gear change as the 3.73 gear ratio was available as a factory option.

At this time I am not willing to use an aftermarket tune. Been down that road before on my GT 500.

Although the Hypertech Speedometer Calibrator does alter the calibration in the BCM, the device was supplied by the dealer when I purchased the truck. Kind of hard for them to deny warranty when they sold the vehicle with the 6" lift kit, 35" tires and Hypertech device.

Recalibrating the speedometer with the Hypertech Speedometer Calibrator is very different than installing a custom tune.

2011XLT5.0

It's not recommended but I'm sure they'll install the gears if you pay them to. Sounds like they're just trying to cover their butt.

tmcolegr

No they won't since they cannot adjust the gear ratio in the PCM.

On earlier model years this wasn't an issue as IDS would allow them to make this change as long as it was originally available as a factory option.

bigandtall

Well crap! Wish I had known this before buying my truck!

robbieg

I call BS on fords response. They just don't won't you to mess with it and they are covering themselves, sounds like a response from the legal team and not engineering. Not to mention that several aftermarket tuners (X4 for example) specifically state for 2015-2016 F150 "User Adjustable Tune Options: Adjust for common changes such as tire size and gear changes." and " Axle Ratio: Allow you to set the axle ratio (gear ratio) to correct shift patterns and the speedometer on your vehicle."

T_R

So have your dealer install the gears and buy yourself an aftermarket tuning device. Change nothing in the PCM calibration but the gear ratio and flash it. Done. It's that simple.

jkenprice1

The dealer where I get my truck service has a couple of techs who do all kinds of mods for Mustangs and Pickups, including gear changes. Some dealers even do lifts on new trucks and factory warranty them. It all depends on the dealer. I don't know about the '15+ trucks, but a lot of guys have changed gears on pre '15s without problems. Someone here should know the answers to your questions for a '15

tmcolegr

The dealer won't change to the 3.73 gears because they fully support the repair by changing the gear ratio through IDS.

I have spoken with 3 other dealers and all of them are very firm on this issue.

I do not want to install a Non-OEM Tune in the PCM. With today's PCM technology it is impossible to reflash the vehicle back to stock without the dealer being aware that it was reflashed. The ignition counter will clearly show the dealer the PCM has been reflashed