Interesting Observation

[TrainMan]

Just a month after owning my JT I used the Rough Country calibrator to set my 35s. I choose 33.50 and the speedometer was correct at speeds up to 80mph. Fast forward to yesterday, (9 months later) I used my JScan App to make some changes with seatbelt chime and changing my lights back to legal status. This is something I've done before. Anyway, I decided to see what my ECU was reading for tire size just out of curiosity. To my surprise the ECU reading was listed as 34.98". What's the deal with that? I know the computer probably modifies itself after driving, but this seems odd, especially since the true tire size is 34 inches. I didn't change anything because my speed is still correct. Not sure how that can be.

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[tysongladiator]

Just a month after owning my JT I used the Rough Country calibrator to set my 35s. I choose 33.75 and the speedometer was correct at speeds up to 80mph. Fast forward to yesterday, (9 months later) I used my JScan App to make some changes with seatbelt chime and changing my lights back to legal status. This is something I've done before. Anyway, I decided to see what my ECU was reading for tire size just out of curiosity. To my surprise the ECU reading was listed as 34.98". What's the deal with that? I know the computer probably modifies itself after driving, but this seems odd, especially since the true tire size is 34 inches. I didn't change anything because my speed is still correct. Not sure how that can be.

It could be (I may be wrong) because you switched to Jscan from something else. I have been using Jscan for over a year and a half. I traveled home to Florida (6hrs) to get regeared. I left my cables back up in SC. I had to purchase a Flashcal to get back up to SC. Once I got back, I unmarried the flashcal and went back to Jscan. I noticed that some of my settings that I had previously done with jscan before flashcal had changed and I had to change them back to where I wanted them with the Jscan.

 

[TrainMan]

Interesting. I just changed back to stock with the Rough Country calibrator and re-calibrated with the JScan. I needed to program 34.25 with JScan, and it's still about 1/2 a mph off after 50mph. Actual tire measurement is 33.75.

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