Jeep MPH off by 2 MPH v. GPS MPH

[ATL_Rubi]

I may be going crazy, maybe not. No matter what I do, it seems that drive to drive, my MPH displayed by the jeep and measured by Three GPS devices (phone, radar detector, and Garmin GPS) are off by 2 MPH consistently, i.e. 65 displayed by jeep, 67-67.8 measured by GPS. Sometimes, it's spot on, no issues, next drive, off 2mph.

I have a tazer, and stock Rubicon tires that measure 31.7 at current pressures, that's set as such in the system with the Tazer. I have done a full unmarry, remarry, new settings, reboot and sleep cycles, to no avail, is anyone else having a similar issue? Picture attached for fun

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[jay21mojave]

Yes, I have noticed same; except it is in the company car, the GPS is 1-2 mph slower than the speedo shows.. (it has both digital and analog displays); I think it's GPS Satellite link variances causing the difference.

 

[bleda2002]

I may be going crazy, maybe not. No matter what I do, it seems that drive to drive, my MPH displayed by the jeep and measured by Three GPS devices (phone, radar detector, and Garmin GPS) are off by 2 MPH consistently, i.e. 65 displayed by jeep, 67-67.8 measured by GPS. Sometimes, it's spot on, no issues, next drive, off 2mph.

I have a tazer, and stock Rubicon tires that measure 31.7 at current pressures, that's set as such in the system with the Tazer. I have done a full unmarry, remarry, new settings, reboot and sleep cycles, to no avail, is anyone else having a similar issue? Picture attached for fun

Your tire size is too small, rubicon tires are 32.8 unladed and thats what you should use as the starting point, not the squished height. Mounted height != actual tire diameter which is what needs to be entered to most accurately compute the circumfrence of the tire.

 

[bleda2002]

Quick calculator math checks out, 31.7 at 65 is actually 67 if you have 32.8 inch tires.

 

[ATL_Rubi]

Your tire size is too small, rubicon tires are 32.8 unladed and thats what you should use as the starting point, not the squished height. Mounted height != actual tire diameter which is what needs to be entered to most accurately compute the circumfrence of the tire.

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.

 

[jav_eee]

I'd adjust tire size until you get the speedo correct, doesn't matter what you measured. Just as long as the speedo is correct.

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!

 

[ATL_Rubi]

I'd adjust tire size until you get the speedo correct, doesn't matter what you measured. Just as long as the speedo is correct.

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!

I 100% agree with you, my mind needs it to be accurate. I track fuelups with fuelly and I am annoyed when fuelly says "13.6 MPG and the jeep says 15 mpg." I get that they may be a few percentage points off, but to be 1.5+ mpg off is not acceptable to my pea sized brain.

 

[bleda2002]

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.

Are you measuring under the weight of the truck? The squish makes that inaccurate, stock rubi tires are metric 285/70r17 so that's 32.8 for an unladen tire. That's the size you want.

 

[ATL_Rubi]

Are you measuring under the weight of the truck? The squish makes that inaccurate, stock rubi tires are metric 285/70r17 so that's 32.8 for an unladen tire. That's the size you want.

My understanding is that is how you are supposed to measure tire height, inflated, and weighed down under the vehicle as that is the accurate representation of actual tire diameter.

If we went off of pure tire size unladen, everyone who has RTT's, Campers, bumpers, etc would be very far off on tire size as the unladen tire doesn't account for the added weight. 37s quickly turn to 35s weighed down and inflated to their proper pressures. This surely would lead to a large discrepancy if you input 36.8 in the system when in reality, due to pressure and weight, the tire is closer to ~35.5.

We need discount tire in here to help lol

 

[sharpsicle]

My understanding is that is how you are supposed to measure tire height, inflated, and weighed down under the vehicle as that is the accurate representation of actual tire diameter.

If we went off of pure tire size unladen, everyone who has RTT's, Campers, bumpers, etc would be very far off on tire size as the unladen tire doesn't account for the added weight. 37s quickly turn to 35s weighed down and inflated to their proper pressures. This surely would lead to a large discrepancy if you input 36.8 in the system when in reality, due to pressure and weight, the tire is closer to ~35.5.

We need discount tire in here to help lol

It doesn't really matter how you're 'supposed' to measure it if the end result is wrong anyway. You need to adjust it to the size that makes the speedo/odo accurate, even if that conflicts with what someone tells you 'should' be the size.

You could get ten answers on what you 'should' do, the only one that matters is the one that gets the right output.

 

[ATL_Rubi]

It doesn't really matter how you're 'supposed' to measure it if the end result is wrong anyway. You need to adjust it to the size that makes the speedo/odo accurate, even if that conflicts with what someone tells you 'should' be the size.

You could get ten answers on what you 'should' do, the only one that matters is the one that gets the right output.

Right, which brings us back full circle to the original issue, in that my speedo is accurate, but them at times, its off 2mph.

 

[sharpsicle]

What GPS are you using? Many do have variable accuracy which could impact a speed readout trip-to-trip based on a variety of factors. Could be that the Jeep is right, you're just seeing a variance in the GPS like @jay21mojave mentioned.

Could be chasing a ghost.

 

[ATL_Rubi]

What GPS are you using? Many do have variable accuracy which could impact a speed readout trip-to-trip based on a variety of factors.

Using 3 different devices, my phone (disconnected from the Jeep), my radar detector (escort redline 360) and my Garmin inreach. Each of those three devices are usually wihtin .1/.2 mph of each other.

I need to find a radar camera near me and run by that to confirm the speeds are accurate as measured on the GPS devices.

Further, when running speeds to test the difference, it's on a flat bit of highway, maybe a mile in length. Set cruise at 65, 60% of the time, it's spot on with the GPS devices, the other times its 2 MPH lower than GPS speed.

 

[bleda2002]

My understanding is that is how you are supposed to measure tire height, inflated, and weighed down under the vehicle as that is the accurate representation of actual tire diameter.

If we went off of pure tire size unladen, everyone who has RTT's, Campers, bumpers, etc would be very far off on tire size as the unladen tire doesn't account for the added weight. 37s quickly turn to 35s weighed down and inflated to their proper pressures. This surely would lead to a large discrepancy if you input 36.8 in the system when in reality, due to pressure and weight, the tire is closer to ~35.5.

We need discount tire in here to help lol

That is incorrect. The speedometer takes the tire height multiplies it by pie and gets a circumference. if I take a 37 inch circle and squish it to a 35 inch oval it still has the same circumference as a 37 inch circle. Airing tires up and down doesn't change your speedo because the circumference is still the same.

 

[ATL_Rubi]

That is incorrect. The speedometer takes the tire height multiplies it by pie and gets a circumference. if I take a 37 inch circle and squish it to a 35 inch oval it still has the same circumference as a 37 inch circle. Airing tires up and down doesn't change your speedo because the circumference is still the same.

Well ... I learned something new today, that's a good Friday in my book. Time to go experiment!

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