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[briscoelab]

If his speedometer is reading 45 instead of 40, it thinks he’s going faster and further than he is.

You're reading his comment wrong.

The speedo in his jeep is never changing. The reading on Waze is changing, depending if it is plugged into the head unit or not. The Jeep is always reading the incorrect (lower) speed.

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[WILDHOBO]

You're reading his comment wrong.

The speedo in his jeep is never changing. The reading on Waze is changing, depending if it is plugged into the head unit or not. The Jeep is always reading the incorrect (lower) speed.

You’re absolutely right. Need more coffee. I obviously understood that lower speed equals lower odometer increases, but yes, read it wrong.

 

[AustinKalb]

I have the tire size set to twice the measured radius, which is exactly the same as what AEV had set. 35.2 on 37 KO2’s.

At 60mph my speedo is still low about 2 mph compared to GPS.

Ideas?

 

[briscoelab]

2mph slow at what speed?

Just do the math, figure out for percentage that you are off… and increase the set tire size by that amount.

 

[RRP]

If your phone is plugged in, your phone will pull the MPH reading from the Jeep, not the GPS. Unplug and you’ll get the correct MPH from GPS on your phone.

 

[AustinKalb]

2mph slow at what speed?

Just do the math, figure out for percentage that you are off… and increase the set tire size by that amount.

60 mph (I added that to my post, thanks). I can change the tire size, but since I measured it, why?

 

[briscoelab]

Because it isn’t giving you an accurate speed.

increase your size 3.33%.

~36.3” should get you dialed in.

Yes, you measured. But what really matters is rotations per mile (or x unit of distance). That relates to circumference.

In an ideal world, your measurement would get you there. But a tire under load is actually not the best thing to measure. Yes, height of a tires changes when on a vehicle, but it just squishes down.

Circumference doesn’t change. So the tire is effectivly a bit “taller” as far as the speedo is concerned… when rolling down the road at speed.

 

[AustinKalb]

Because it isn’t giving you an accurate speed.

increase your size 3.33%.

~36.3” should get you dialed in.

Yes, you measured. But what really matters is rotations per mile (or x unit of distance). That relates to circumference.

In an ideal world, your measurement would get you there. But a tire under load is actually not the best thing to measure. Yes, height of a tires changes when on a vehicle, but it just squishes down.

Circumference doesn’t change. So the tire is effectivly a bit “taller” as far as the speedo is concerned… when rolling down the road at speed.

Great explanation! I’m a physicist. The part about the circumference and the tire being oval made exquisite sense.

I won’t try and convince anyone else. I’m not a great physics teacher?

 

[briscoelab]

Lol. Glad to connect on some level.

I’m a biochemist and cancer researcher… quite the pair here, acting like we know about vehicles

 

[jonsheritahe04]

Get the ZAutomotive extension cable set so that you only have to do it one time.

So, got my extension extension cable today and I will be damned if I can get the connectors back into the factor ports. I have large hands so most like the first problem. Any tricks on getting this installed without taking the dash off? Maybe a long thing needle nose?

 

[ShadowsPapa]

Your analog gauge is actually displaying about +2 in both pics. Switch your center digital to display MPH for pinpoint accuracy.

Both of my JTs showed about 2 mph over reality. The analog gauges in the first and now this one have been high since day 1.
I found the digital readout to be accurate.
I use the odometer to calibrate things............. it's a far better tell. Speedometers round, so do GPS devices.

I rely on a true GPS device that uses satellites and I use the odometer.
If the phone says 55 it could be 54.6 and if the truck says 55 it could be 55.5 so you may still be a mile off.
But the trip meter part of the odometer will measure in tenths of a mile.

I set the trip tracker on my Garmin - and set my Trip A to 0 miles.
I drove 17 miles and my truck's odo/trip meter read to the tenth of a mile with the Garmin GPS device.

 

[Vtur]

The analog always 1-2 mi higher is due to the needle spacing being too far from the dial face. Lean your head over a little.

Tires circumstances expand at higher speed and once heated up. Therefore, you can only calibrated the accuracy at either lower or higher speed, not both. If calibrated between 0-50 mph, it will show a couple mph higher when doing 80 mph.

 

[Gvsukids]

Both of my JTs showed about 2 mph over reality.

Next class action lawsuit for Jeep.

Maybe a long thing needle nose?

Yes.

 

[ShadowsPapa]

The analog always 1-2 mi higher is due to the needle spacing being too far from the dial face. Lean your head over a little.

Tires circumstances expand at higher speed and once heated up. Therefore, you can only calibrated the accuracy at either lower or higher speed, not both. If calibrated between 0-50 mph, it will show a couple mph higher when doing 80 mph.

That's called parallax error - and that's not it with these. Even looking straight-on at it, it's off. The needle's end is what I look at, and it's off.
Besides, there's really hardly any gap between the needle and the face of the gauge so the difference in the two planes is minimal.
Been using analog gauges since I was 14, back in the 1970s and this one is just plain off.
Until these JTs, I've never had a speedometer pointer that was literally off as far as these. Even when the odometer is calibrated and the digital speedo is accurate, the needle is wrong.
It's not the parallax error thing where the needle or pointer are on a different plane.
Use the end of the needle or pointer - it's wrong.

 

[ShadowsPapa]

Next class action lawsuit for Jeep.


Yes.

I don't think I'll be suing and end up with $1.54 for my troubles.
Besides, their attorneys would simply blame it on my not using the digital speedometer they supply.

When you own and drive cars with the gear ratios changed, tire changed, other mods, you get used to knowing how to compensate.

Oddly, I found the speedometer and odometer of my wife's 4xe to be as exact as anything I've ever worked on or driven. Yes, it's digital only, no analog gauge to it (because there's so many other gauges and displays crowded into the cluster, I assume) but it's extremely accurate.
Took me a while to get used to having digital only, no analog needle to look at. But the other gauges and indicators were cool to monitor over the more than 3,000 miles we put on it last week.

Oh, back on another topic from earlier - the change in tire circumference due to speed isn't going to change it THAT much..... I've proven that with mine, and done comparing using various tire sizes when choosing the tires I put on mine. Some change, but it's fractional. You can be pretty accurate. On the odometer, it averages out extremely well and the speedometer isn't off enough to throw you into the next mph.
If these were wide racing slicks, yeah, those babies grow a lot and you can see the back of the car jump up when they spin up in the water before the tree. Those tires flex a heck of a lot, these - not so much. I had a car with tires that barely cleared the rear wheel opening lips when I was younger - they all but rubbed when sitting. (L 50s, I think) and they didn't touch much at speed.

Speedometer accuracy is a percentage - when someone says "it's off 3 mph" - at what speed?
It will only be off 3 mph at a given speed- faster and it will be off more mph, slower and it will be off fewer mph. OTOH, the analog gauge speedometer in both of mine always seem to read about 2 mph higher. It's as if the thing is assembled with the needle not at true 0 so it seems to always be that much off - not a percentage but actually always off that much. That tells me it's a gauge issue, not a calibration or gear ratio thing. Those are percentages.

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