Jeep MPH off by 2 MPH v. GPS MPH

[HorneyBadger]

GPS has always been a mile or 2 different. Been that way for years on the Harleys and different Garmin Zumo's. This can be further varied if different tires were put on the vehicle and calibration not changed to reflect the correct tire size.

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[kennyglm]

Mine reads spot on i compared it with an app i downloaded. I have 4.88 gears with 37s. Everything was set with the tazer also

 

[Sting-Gray Neutral Pres.]

FWIW, ford has this stupid "bias" built in to their speedo that shows 2 mph over your actual speed. So you either adjust so that your odometer is accurate, or your speedo is accurate. For someone pyscho like me about accurate mileage tracking that makes my head spin. I don't even want to know if Jeep does that!

It probably does, speedometers are always set at least a smidge high at the factory. Reading lower than actual would be a serious problem for OEMs. The worst offender I know of is BMW, they read about 7% high when compared to GPS. Here is one of many articles on the subject.

https://www.roadandtrack.com/car-culture/car-accessories/a32026968/are-speedometers-accurate/

 

[Josh00333]

It took some trial and error but I got mine 99.98% on.

Don’t just look at the Speedo. Do a trip set your odometer and then do a trail on your gps.

On a longer drive 100+ miles you can see what the real accuracy is.

 

[ShadowsPapa]

You just may be right....My tire still measured at 31.7" at 33 PSI just now, but, in reading more into tire sizes and speedometer, the tazer may need to be bumped up to be more accurate. I'll report back here when done.

Simple - adjust your tire size until it's correct. Screw a number given - adjust until it's correct. Rolling diameter can be different than measured.
I've never been able to go by the unloaded height - it's always somewhere in between because there's a rolling diameter than is different than the measured number of the tire.
In fact, if you look at the setting in JSCAN or AlfaOBD, it even says something like rolling circumference. The software and tazer do math for you and plug in the circumference based on the diameter you plug in (it knows pi )
If someone says "use xxx" - that's a starting point.
I kept adjusting (3 times if I recall) and got it spot-on using the odometer. 17.1 miles on the odo was 17.1 miles traveled on the GPS.

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