Auto tranny shifting early

Hello guys. Taco newbie.... So here's my problem maybe you can shed some light to my situation. So I bought a 2012 Tacoma Trd sport with a 3"lift and it has stock wheels but tires are 285's... So anyhow my speedometer is off by a lot!! Like 15 mph, the previous owner customized the interior dash lights and in the process of doing so he managed to mess up some sensor or electronic piece that regulates the speedometer. The truck, in my opinion shifts early every time and on every single gear... The part that controls the speedometer was placed on order and supposed be here in about a week or so. I asked the technician if correcting the speedometer would correct the transmission shift points and he basically in his own way said he didn't know. I will eventually find out I guess but the mystery is killing me. What do you think? Thanks in advance

 

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I will start by asking a question .
This transmission uses adaptive learning or learns how you drive the truck . If you have a heavy foot , the transmission should shift late and more aggressive . Light footed accelerator gives you early smooth shifting .
Have you disconnected the battery terminals and tried a re-learn since the original owner ?
I beleive the abs system is somehow affiliated by measuring each wheel speed to control shift timing. I dont beleive the speedometer is used in any transmission control functions like it was in the past . As I understand it , the speedometer is part of the cruise control system now and is no longer used for shift timing / converter lock up . A good question mind you !!
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EDIT : forgot to ask if you had the ecm scanned .

 

as a matter of fact, yes the ecm was scanned and apparently everything is fine.... so i will definately take your advice and see what's my outcome. post results asap