Calibration backwards, please advise

[2nd 392]

I got 35’s, speedy pro properly showed ~ 3 mph faster than speedo at 65 mph. 4.56’s just installed and speedy pro now shows ~ 4 mph faster. This is backwards, it should be showing slower as I am about 300 rpm higher at 65. Ideas, experience? Please advise.

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[BDinTX]

I think the speedo is driven from the transmission so unless you tell the computer the diff gears changed that sounds right. The driveshaft is turning faster now for a given speed.

 

[2nd 392]

I think the speedo is driven from the transmission so unless you tell the computer the diff gears changed that sounds right. The driveshaft is turning faster now for a given speed.

Makes sense for the speedo, but shouldn’t the GPS be showing the actual slower speed due to gears?

 

[BDinTX]

Oh yeah, good point. Man you got me.

 

[Some Random Guy]

I believe speedo is off wheel speed sensors, not transmission. I can’t explain the change though unless a significant change in tire pressure or removing and reinstalling the wheel speed sensor was enough to nudge the reading by 1-2%.

 

[2nd 392]

I believe speedo is off wheel speed sensors, not transmission. I can’t explain the change though unless a significant change in tire pressure or removing and reinstalling the wheel speed sensor was enough to nudge the reading by 1-2%.

They did increase tires 6psi, So if the speedometer is off wheel speed why get a Tazer to change gears and tires? I am truly scratching my head.

 

[Some Random Guy]

They did increase tires 6psi, So if the speedometer is off wheel speed why get a Tazer to change gears and tires? I am truly scratching my head.

Wheel speed and tire size combined determine speedo, I believe. I’m fairly confident I did not have to switch my gear ratio to make my speedo accurate. I do think the gear ratio impacts shift points for the transmission though. I have an M/T so I have no first hand experience with that.
I regeared 3.45 to 4.88 and my speedo was still where it was pre-regear.

 

[2nd 392]

Wheel speed and tire size combined determine speedo, I believe. I’m fairly confident I did not have to switch my gear ratio to make my speedo accurate. I do think the gear ratio impacts shift points for the transmission though. I have an M/T so I have no first hand experience with that.
I regeared 3.45 to 4.88 and my speedo was still where it was pre-regear.

The math was 3.5 mph higher at 65 mph going from spec 32.8” to 34.5” tires and is about right. The shift points feel a bit higher but could “feel” higher due to gearing as I wasn’t smart enough to note numbers so not sure. I am trying to decide on getting a Tazer and it now appears the only “need” for it is to lower the TPMS warning, new tires have max 35 psi, set at 32, cool night, 28 and warning in the morning, increase to 37 to shut warning off, try 34psi, warning, this is becoming a PITA, 35psi today- to be determined.

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