Question on how the odometer works

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Sep 26, 2024 | 09:18 AM

daktah

Thread Starter

Mountain Pass

Joined: Feb 2020

Posts: 107

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Question on how the odometer works

Is mileage accrued based on what the PCM thinks the vehicle speed is or is it based on what the actual needle in the cluster shows? Yes, I know not LITERALLY what the needle physically shows, but how the cluster interprets what the PCM is telling it. I got some new tires that are approximately 2.5% bigger than stock and according to Torque the PCM (which I have not calibrated) thinks I'm going 2 to 3 MPH slower than GPS at basically all speed ranges, which oddly is closer to a 3 to 4 percent difference. However, the needle itself is very close, usually within a mile of GPS. I would just like to know if I'm actually under reporting my mileage To the odometer or if it's just fine. Not for any legal reason, I would just like it to be accurate.

I actually tried to calibrate it after coming home using HPtuners to put in the new revs per mile but while that made the PCM read correctly vs GPS, the needle was way off at over 5 mph fast vs actual, so I reverted to the stock values for the time being.

Sep 26, 2024 | 12:17 PM

Johnny Paycheck

Fleet Mechanic

Joined: Jul 2019

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For what it’s worth, my ‘03 reads 3 MPH fast no matter what. Straight across the dial, 3 MPH fast no matter what roadside radar I pass.

It has the bigger tire size option from the factory (window sticker and door jamb sticker both agree), so that’s not an issue.

Do with that what would will.

Sep 26, 2024 | 12:59 PM

RLXXI

Lead Driver

Joined: Nov 2020

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From: Sportsman's Paradise

Need to reprogram the pcm to account for the tire size change. Forscan may do it.

Sep 26, 2024 | 05:59 PM

daktah

Thread Starter

Mountain Pass

Joined: Feb 2020

Posts: 107

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Quote:

Originally Posted by Johnny Paycheck

Your cluster reads 3 over or your PCM reads 3 over? Because like I said, my cluster is just about as accurate as you can expect it to be. It's the actual PCM itself that is reading slow via whatever generic PID torque uses to read vehicle speed. My truck came with the standard 235 tires as well.

Quote:

Originally Posted by RLXXI

Need to reprogram the pcm to account for the tire size change. Forscan may do it.

Forcecan cannot on these old of trucks, But like I said I already attempted to do this using HP Tuners which does have the tables for revs per mile, axle and transmission ratios, etc, But then like I said, it made the PCM accurate to GPS, but then through the instrument cluster way off by 5 MPH.