How Does Automatic Know When to Shift?

[Mzekew]

I put 32.7 inch tires on my Base (which comes with 30s) about a week after I got it.

Now it feels like I’m always in a too high of a gear when driving around town. Maybe this is in my head, maybe not.

How does the auto transmission know when to shift? Does the trans software differ across models, depending on assumed tire size? Could the 3” larger tires be putting me in a higher gear than I should otherwise be in? What are possible solutions to address this if it is an actual concern?

I haven’t yet had the speedometer adjusted for the larger tires, but my understanding is the spedo software doesn’t share info with the transmission software, anyway.

Thanks in advance for all the input. Grateful for all the knowledgeable members on B6G!

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[Bmadda]

I put 32.7 inch tires on my Base (which comes with 30s) about a week after I got it.

Now it feels like I’m always in a too high of a gear when driving around town. Maybe this is in my head, maybe not.

How does the auto transmission know when to shift? Does the trans software differ across models, depending on assumed tire size? Could the 3” larger tires be putting me in a higher gear than I should otherwise be in? What are possible solutions to address this if it is an actual concern?

I haven’t yet had the speedometer adjusted for the larger tires, but my understanding is the spedo software doesn’t share info with the transmission software, anyway.

Thanks in advance for all the input. Grateful for all the knowledgeable members on B6G!

Goes by the RPM of the driveshaft, not actual vehicle speed. Changing tire size will change shift points relative to vehicle speed, however speed of the driveshaft should be the same. More drag will require more throttle, and the computer SHOULD make the shifts later in response to throttle input, but, not always perfectly. Adding more meat under a 4x4 always makes it feel like more of a dog. The PCM has adaptive strategy too, and may not have changed trans adapts...it could be worth disconnecting the battery for a few min (which should reset all adapts), then try driving it for a few days

 

[Bongo Fury]

Thanks in advance for all the input. Grateful for all the knowledgeable members on B6G!

Check in on this post
https://www.bronco6g.com/forum/threads/rear-axle-ratio-questions.3251/
Veritable Encyclopedia Broconica
My first guess is rear gearing is matched with 30" with a slim margin.

 

[Mzekew]

Goes by the RPM of the driveshaft, not actual vehicle speed. Changing tire size will change shift points relative to vehicle speed, however speed of the driveshaft should be the same. More drag will require more throttle, and the computer SHOULD make the shifts later in response to throttle input, but, not always perfectly. Adding more meat under a 4x4 always makes it feel like more of a dog. The PCM has adaptive strategy too, and may not have changed trans adapts...it could be worth disconnecting the battery for a few min (which should reset all adapts), then try driving it for a few days

Thanks in advance for all the input. Grateful for all the knowledgeable members on B6G!

Check in on this post
https://www.bronco6g.com/forum/threads/rear-axle-ratio-questions.3251/
Veritable Encyclopedia Broconica
My first guess is rear gearing is matched with 30" with a slim margin.

good info. Looks like base comes with 3.73 gearing, as does the BB and OBX. Thinking BB and OBX on 32.1” stock tires vs. base on 32.7” tires with the same 3.73 gearing would be fairly similar. I definitely won’t go any taller than 33s with the 3.73 gearing, but it should be acceptable for daily driving and light(ish) off-road use. Figure it’s not worth regearing to 3.91 (if it’s even available) for the minimal difference. If I ever do regear, it’ll be to 4.27, but that’s not in the cards for a while. That being said, the Base definitely has less pep that it did on 30s, and the gear ratio certainly sounds like it’s contributing to that.

stock BB vs Base on 32.7s gearing compare

 

[SwissBronco]

torque sensor and RPM

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