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Discussion Starter · #21 ·
If the speedometer is not correct, that affects several things. Timing of shifts, fuel mileage being the main 2. My '16 with 4" lift and 35x12.50 tires dropped from 24-26 to 19. Recalibrated speedo and it came back to 20-22. Use a GPS speedo on phone to check speedo accuracy.
I checked speed using gps and found 2 mph difference, is this a considerable drift that will require calibration ?
 

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2015 Trademan 4x4, 3.92, 6.4' bed air lift 1000 bags, bilstien shocks, 34" nitto g2s, Unknown tune.
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I checked speed using gps and found 2 mph difference, is this a considerable drift that will require calibration ?
So the speed will be off different amounts depending on the speed it's not a consistent amount.
What size tires does the truck have now? Did the dealer install those tires? They may have attempted to adjust the size already.
 

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2015 Trademan 4x4, 3.92, 6.4' bed air lift 1000 bags, bilstien shocks, 34" nitto g2s, Unknown tune.
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I've never owned a vehicle or motorcycle that was withing 1% in 50 years 🤔. More like 2 to 3% in my experience
I think the point was if it's off by 2mph at 20 it's off by more at highway speeds and that makes a real difference.
 

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I have a stock 21 Rebel w/3.92. My best highway only mileage is 29 and normally will average 26 - 27 mpg. These results are at highway speeds of 55 - 65 mph. On the interstate at speeds of 70 mph plus, my mileage will average 24 - 26 mpg.
 

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2019 RAM 1500 Classic Tradesman 4X2 EcoDiesel
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I checked speed using gps and found 2 mph difference, is this a considerable drift that will require calibration ?
You haven't told us which way the speedo was off, as well as what speed you checked it at.
 
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Discussion Starter · #31 ·
[QUOTE="SlowRoller"
Depends 2 mph off at 25 or at 80? Its best to get the speedo within 1% accuracy so at 100 mph that would be a 1 mph variance.
[/QUOTE]
Truck is under reporting speed, below is a summary. I read that speedometer error tolerance can be up to 10% but should overreport speed and should never under-report.
Dash SpeedGPS speedError %
40437.5
50548
60658.3
70768.6
 

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2019 RAM 1500 Classic Tradesman 4X2 EcoDiesel
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Truck is under reporting speed, below is a summary. I read that speedometer error tolerance can be up to 10% but should overreport speed and should never under-report.
Dash SpeedGPS speedError %
40437.5
50548
60658.3
70768.6
That sounds like speedometer was not reset for the larger tires. You could calculate if the error matches the tire size difference:
Speedometer Calibration (tiresize.com)

Your MPG on the EVIC is off by about 8% plus the usual error.
 

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Discussion Starter · #34 ·
That sounds like speedometer was not reset for the larger tires. You could calculate if the error matches the tire size difference:
Speedometer Calibration (tiresize.com)

Your MPG on the EVIC is off by about 8% plus the usual error.
Thanks. I'll ask the dealership to get it calibrated. MPG actually improved to 19.5-20.5 after filling from a different gas station. With the 8% difference that would make it 21-22 which makes sense in comparison to original MPG with stock tires.
 
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