Why would a transmission shift better if the speedometer is calibrated to a different tire size?

[jsalbre]

I know it's an "everyone knows it" thing, but I can't figure out why a transmission would shift better if the speedometer is calibrated to a different tire size. It's not like you've changed the gear ratio in the tranny. It's still hitting the same power at the same output shaft speed.

 

[G Mcpherson]

I believe that they are programmed for the Diameter of tire . Speed sensor. Changing that to a larger size will change the Calibration.

 

[jsalbre]

I believe that they are programmed for the Diameter of tire . Speed sensor. Changing that to a larger size will change the Calibration.

Yes, that’s what you’re changing is the tire size setting, but how would that affect the transmission?

 

[CdnHO]

Yes, that’s what you’re changing is the tire size setting, but how would that affect the transmission?

Just a guess but a larger tire would put more torque/load on the transmission for a given throttle setting. Be like putting lower gears in the rear end. Example going from 4.10 to 3.73 ratio.

 

[rampulse]

I didn’t think it would matter much personally until I saw a few comments and videos mentioning the transmission. Since there’s no tuning for the AISIN I figured it was worth a shot. It’s noticeably smoother especially on grades and from 1st to 2nd.


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[jsalbre]

Just a guess but a larger tire would put more torque/load on the transmission for a given throttle setting. Be like putting lower gears in the rear end. Example going from 4.10 to 3.73 ratio.

Yeah, that’s definitely the affect the tire change itself has, but why would the vehicle speed reading affect where the transmission shifts? Its shifting (simplified of course) based on load, engine RPM, and output shaft speed. Where does indicated speed across the ground matter?

 

[CFE]

I didn’t think it would matter much personally until I saw a few comments and videos mentioning the transmission. Since there’s no tuning for the AISIN I figured it was worth a shot. It’s noticeably smoother especially on grades and from 1st to 2nd.


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PPEI does have tuning out for early Aisin models and is in beta testing stages for 2019+ Aisin models.

 

[G Mcpherson]

Does not Diameter Of the Tire effect Rotation.

 

[jsalbre]

Does not Diameter Of the Tire effect Rotation.

No. At a given transmission input shaft speed the output shaft speed will be the same, no matter how fast over the ground you’re going.

This discussion probably should be its own thread instead of cluttering up the AlfaOBD thread. @Brutal_HO, could you move it?

 

[Jimmy07]

I know it's an "everyone knows it" thing, but I can't figure out why a transmission would shift better if the speedometer is calibrated to a different tire size. It's not like you've changed the gear ratio in the tranny. It's still hitting the same power at the same output shaft speed.

Because the wheel speed/revolutions taken from the wheel speed sensors, among other factors, is taken into account for the shift points.

 

[Enve46]

What Jimmy said. Thats why dyno's tend to mess with newer vehicles and they need to put in a dyno mode or whatever its called.

 

[phatboy64]

I don’t know the answer but I am running 37s and due to other reasons it took me awhile before I could change my tire size. During that time in certain situations mostly slow speeds (around 25 - 35 mph) it felt like it was in too high of a gear for the RPMs, almost like it was lugging a bit. After setting my tire size correctly that issue seems to have gone away and it feels smoother overall.

 

[joshuaeb09]

Every electronically controlled slushbox I've ever had has been happier with the correct tire size parameters in the computer and/or gear ratio if changed. There's a bit of data that goes into the shift tables as well as the torque management calculations that are predicated on having the correct data to calculate speed and final drive ratio.

Definitely not like the old days of just changing the speedo gear and/or axle ratio and adjusting shift points by playing with the TV cable and/or hydraulic modifications.

 

[Crusty old shellback]

Changing tire size will change the engine/transmission RPM for a given speed. Since it is all computer controlled, having the correct tire size in the computer will allow it to select the correct gear in the tranny for the given speed/RPM. It does more than just change your speedometer.

 

[ramblinChet]

The transmission gear ratios and output shaft speeds remain unchanged when altering tire size, but calibrating the speedometer for a different tire diameter - such as upgrading from the OEM 32-inch tires to 37-inch BFG All-Terrains on my 2021 RAM 3500 with the 6.4L HEMI and ZF 8-speed - improves shift quality because it corrects the Transmission Control Module's (TCM) interpretation of vehicle speed, a primary input for shift timing. In my setup (AEV Prospector build calibrated via the AEV ProCal), the larger tires increase rolling circumference, resulting in fewer wheel revolutions per mile. Without adjustment, the TCM receives inaccurate speed data from the wheel speed sensors, perceiving the vehicle as traveling slower than actual road speed. This misalignment disrupts the adaptive shift maps, which are designed to optimize shifts based on speed to maintain the engine in its efficient torque band. Consequently, upshifts may be delayed, causing the transmission to hold lower gears longer and leading to a sensation of lugging or sluggishness, while downshifts can occur prematurely under load, increasing thermal stress and clutch wear.

The ZF 8HP transmission in our heavy-duty Rams relies on a hierarchy of inputs for optimal operation: vehicle speed as the foremost factor, followed by throttle position and engine load to balance power demand, and engine RPM to prevent over-revving or under-speed operation. Larger tires effectively alter the final drive ratio - similar to installing taller axle gears which reduces engine RPM at highway speeds for potential fuel efficiency gains but imposes higher torque requirements during acceleration. By using the AEV ProCal to update the tire specifications (revolutions per mile), the system recalibrates the Body Control Module and TCM, ensuring shift points align with actual dynamics. This restores precise timing, yielding smoother engagements and reduced gear hunting, particularly beneficial for towing or varied terrain.

Key improvements observed in my configuration:

• Enhanced low-speed shift transitions, eliminating lugging in gears 1-3 by aligning with true vehicle speed
• Optimized highway performance through timely upshifts to overdrive, maintaining efficiency without power loss
• Decreased transmission strain, as proper calibration minimizes unnecessary downshifts and extends component longevity

For those performing similar modifications, allowing 50 miles of driving post-calibration enables the adaptive algorithms to refine based on usage patterns.