Speedometer Help

[NateKY]

Part of my deal for the 2021 Rubicon was that I got to keep the 35s that were on my 2020 Rubicon. They switched them and changed the size to 35”. On the way home, I noticed the speedometer was 3 mph than the GPS which was expected. When I got home, I hooked up the Jscan app and changed the tire size from 35” to 33” (same as the 2020) along with other things. I tested the speedometer the next 2 days and it was .5-1 mph faster then GPS. That was the same as the 2020, so I was happy.

Yesterday morning, I set the cruise like have done every morning, but noticed that I was passing cars that I normally follow. Pulled up the GPS and I was 3 mph faster again. Thought okay, maybe the change didn’t take or switched back. Before I left work yesterday, I hooked up the JScan in the parking lot. My tire size was still set at 33”. I changed it to 32.75”. Last night and this morning, my speedometer was back to .5-1 mph faster than GPS.

On my way home a hour ago, my speedometer is back to 3 mph slower than my GPS. I checked my speed on 3 different apps with the same results on each one. I get home, hook up the JScan, and my tires are set at 32.75!?!?!?!

WTH is going on. Honestly I never had a reason to recheck the 2020, so I don’t know for sure that it never changed. Anyone else have this issue??

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[HorneyBadger]

Why are you comparing to GPS. Its always off from the GPS due to delay. GPS on the harleys was always 2-3MPH slower than actual. Set your wheel size properly and run with it!

 

[NateKY]

Why are you comparing to GPS. Its always off from the GPS due to delay. GPS on the harleys was always 2-3MPH slower than actual. Set your wheel size properly and run with it!

Not sure how to reply to this. Should I laugh or just shake my head.

 

[MrZappo]

Why are you comparing to GPS. Its always off from the GPS due to delay. GPS on the harleys was always 2-3MPH slower than actual. Set your wheel size properly and run with it!

This makes no sense ... No ... Must be kidding ?

 

[jay21mojave]

Think of it this way.. if it's 3 mph fast than REAL You might not be chatting with "the Man" on the side of the road. 3mph fast.. I would not concern myself.

 

[Lateralus]

I spent an hour driving and changing my tire size on jscan for my tires. I used a GPS app as well to tune mine. You have to mount your phone on the outside of your front windshield so you can still see it but the roof material won't slow down the GPS beams.

 

[ShadowsPapa]

Why are you comparing to GPS. Its always off from the GPS due to delay. GPS on the harleys was always 2-3MPH slower than actual. Set your wheel size properly and run with it!

Kidding, right? GPS isn't off that far. Set the cruise and watch the speedometer and within a half second MY GPS is there. If you watch the two for a few miles you'll see how fast the GPS truly changes. At least mine. It was less than a second behind the actual speed and if you can't hold your speed steady for more than one second - maybe you have a foot problem.
I compared to GPS - but further, I checked mine against my Garmin GPS, my phone, at the same time - and did it comparing miles driven. I set the JT trip meter to 0, set my GPS and phone to track distance and drove 17 miles. When I got back they all matched to the tenth of a mile and my GPS tracked pretty darned close to my speedometer.

 

[ShadowsPapa]

I spent an hour driving and changing my tire size on jscan for my tires. I used a GPS app as well to tune mine. You have to mount your phone on the outside of your front windshield so you can still see it but the roof material won't slow down the GPS beams.

My GPS was on the windshield - no discernable lag. I'd check the JT speed and look over to the GPS and it had changed to match.

 

[jjdustr340]

It’s possible you didn’t/don’t have a lot of satellites when you checked.
I know from some of my drones, certain days or times I can barely lock in 5 satellites other times I get 12.
The fewer the satellites the less accurate the gps obviously.
Generally what I do after tire recalibration is get it to where it matches the gps, then drive by the local school where they have one of those permanent radar “your speed” things to compare.

Also, I haven’t verified this myself, but I have read that if the phone you have the gps app on is plugged in, it will give a false reading.

 

[NateKY]

I just checked my app settings in my phone and there was an update yesterday to the JScan app. I remeasured my tires at 33.25” and reset the JScan. I actually did it twice. Lol. Thanks for all the helpful replies. I just don’t understand how the speedometer would change on its own. I take the same route everyday day to work on cruise which is 64 miles one way. GPS satellites won’t change position that much to get 2 different speeds with in the same week.

 

[Kevin_D]

The fewer the satellites the less accurate the gps obviously.

Not necessarily…
One satellite will provide a frequency reference, two satellites will give longitude/latitude, three satellites will add altitude. More satellites just provides more options for your receiver.

Kevin

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