JScan tire size is... Right?

[JTNARJ]

Not sure if this is even the right section, but here goes.

I ditched my stock 245 75R17 tires for 285 70R17 KO2s. Love 'em. Hooked up JScan and followed the directions about tire measurements. Measuring from the ground to top through the center my tires are 32.25". Load up JScan, connect and it's already set to 32.25.

Huh?

I know the sizes aren't that far apart diameter wise, but I didn't expect to see 32.25. I even measured the old tires and they are under 32". I also understand different manufacturers are slightly different sizes. Should I just leave it? Even if it's off, I think at 60 mph it's only like a 2 mph difference. Am I just being too anal?

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[rm1515]

Use GPS and put them to whatever sizes makes the speedo the same as the GPS.

 

[JTNARJ]

Use GPS and put them to whatever sizes makes the speedo the same as the GPS.

Is a phone GPS that accurate? I have no idea.

 

[rm1515]

Is a phone GPS that accurate? I have no idea.

I used Waze. I am sure others are close enough.

 

[Mr._Bill]

When you do the GPS testing, do not connect the phone to the truck. The most accurate method is comparing actual distance traveled. The longer the test run, the better the accuracy.

When measuring height, go from the center to ground and then double it. That gives a good starting point. It will take several tries, and the number you end up with won't exactly match the measured number.

 

[Hootbro]

JSCAN also has a speedometer error % adjustment feature. If you have accurately measure and inputted tire size into JSCAN and are having to grossly adjust your tire size to have the speedometer match GPS speed, you need dial it back to actual tire size and then input a error % in the speedometer setting.

I had to do this on my WILLYS when going up a tire size.

 

[AmosMoses]

Use GPS and put them to whatever sizes makes the speedo the same as the GPS.

This is what I did. Ignore actual size of tire and calibrate with GPS mph app.

 

[redfish]

If understanding your initial question about that size tire already been in the computer, yes that could happen from the factory as it is stock for a Rubicon, you may not have a Rubicon but the factory has a 5% fudge factor on the speedo and probably does not adjust for every tire size

 

[ShadowsPapa]

Don't use the MPH method, use the miles traveled.
Only miles traveled will be accurate
Speedometers round numbers, GPS rounds your speed. You can still be off by quite a bit.
I used my trip meter on the truck and a dedicated Garmin GPS and used miles and tenths.
You may be going 55.6 but your truck will say 56 and your GPS may see it as 55.4 and round down to 55 - so it looks like you are doing 1mph difference when in fact it's .2 mph difference.
Or it can be worse and not look so bad due to rounding of speed.
Miles driven aren't rounded, though.
When I did mine, the odo/trip meter on my truck and the GPS matched exactly to the tenth of a mile after driving over 17 miles. When taking long trips the truck and GPS are generally very close.
I've never found using tire measurements to be particularly accurate.
Maybe for some it's "close enough" but for me "close enough" has never been good enough.

And a phone GPS should be very accurate as well, but again, I don't use speed in any case. Speed can also be delayed a bit as the phone or GPS updates information.
You'd have to set the truck on cruise and be on flat ground for the speeds to be even sort of close, IMO.
But distance driven will be very accurate.

but the factory has a 5% fudge factor on the speedo and probably does not adjust for every tire size

Who says? Source?
They only come with a limited tire size selection so plugging in the number for a given tire they come with is easy for them. They don't adjust, they know the rolling diameter of a tire for a level or model of Jeep and it's what is put in. They have templates.
5% is a heck of a lot and will matter in times of warranty - or speeding tickets.

 

[Brahmajoe]

Not sure if this is even the right section, but here goes.

I ditched my stock 245 75R17 tires for 285 70R17 KO2s. Love 'em. Hooked up JScan and followed the directions about tire measurements. Measuring from the ground to top through the center my tires are 32.25". Load up JScan, connect and it's already set to 32.25.

Huh?

I know the sizes aren't that far apart diameter wise, but I didn't expect to see 32.25. I even measured the old tires and they are under 32". I also understand different manufacturers are slightly different sizes. Should I just leave it? Even if it's off, I think at 60 mph it's only like a 2 mph difference. Am I just being too anal?

Mine said 32.25 as well. Did you wind up leaving it or is there a sweet spot you found

 

[just_another_guy]

Don't use the MPH method, use the miles traveled.
Only miles traveled will be accurate
Speedometers round numbers, GPS rounds your speed. You can still be off by quite a bit.
I used my trip meter on the truck and a dedicated Garmin GPS and used miles and tenths.
You may be going 55.6 but your truck will say 56 and your GPS may see it as 55.4 and round down to 55 - so it looks like you are doing 1mph difference when in fact it's .2 mph difference.
Or it can be worse and not look so bad due to rounding of speed.
Miles driven aren't rounded, though.
When I did mine, the odo/trip meter on my truck and the GPS matched exactly to the tenth of a mile after driving over 17 miles. When taking long trips the truck and GPS are generally very close.
I've never found using tire measurements to be particularly accurate.
Maybe for some it's "close enough" but for me "close enough" has never been good enough.

And a phone GPS should be very accurate as well, but again, I don't use speed in any case. Speed can also be delayed a bit as the phone or GPS updates information.
You'd have to set the truck on cruise and be on flat ground for the speeds to be even sort of close, IMO.
But distance driven will be very accurate.


Who says? Source?
They only come with a limited tire size selection so plugging in the number for a given tire they come with is easy for them. They don't adjust, they know the rolling diameter of a tire for a level or model of Jeep and it's what is put in. They have templates.
5% is a heck of a lot and will matter in times of warranty - or speeding tickets.

It's funny that you will trust the distance from your gps but not the speed when they both use the same location information. Using mile markers on the highway would make more sense, but the best answer is to just use your gps speed and cruise control.

 

[ShadowsPapa]

It's funny that you will trust the distance from your gps but not the speed when they both use the same location information. Using mile markers on the highway would make more sense, but the best answer is to just use your gps speed and cruise control.

You totally missed my point. The truck speedometer rounds, the GPS speed is always a bit behind and it rounds as well.
Cruise control ? Why? Even it varies. Is it going 55 or 54 or 56?

I have explained in detail here and other places how the speedometers can be off -
Say the truck says you are going 55 - is it 55, or is it 55.4? And the GPS - which lags behind at times, it's typically not giving you tenths of a mile an hour but will say 55 when you are going 54.7
Right there you are off some.
Odometer is the only true way. Mile markers are sort of ok, but you'd have to cover several miles.
I've tested all of this more than once.
I set things by speedometer and GPS speed and found them to be off. And cruise control itself has some variation - it has to. I've watched cruise control and found it to vary a couple mph even on the flats because of passing traffic, wind and so on.

Speedometers and GPS rounds to the nearest mph. Odometers don't.
They are also off by a percentage so the difference will be worse the faster you go.

 

[just_another_guy]

You totally missed my point. The truck speedometer rounds, the GPS speed is always a bit behind and it rounds as well.
Cruise control ? Why? Even it varies. Is it going 55 or 54 or 56?

I have explained in detail here and other places how the speedometers can be off -
Say the truck says you are going 55 - is it 55, or is it 55.4? And the GPS - which lags behind at times, it's typically not giving you tenths of a mile an hour but will say 55 when you are going 54.7
Right there you are off some.
Odometer is the only true way. Mile markers are sort of ok, but you'd have to cover several miles.
I've tested all of this more than once.
I set things by speedometer and GPS speed and found them to be off. And cruise control itself has some variation - it has to. I've watched cruise control and found it to vary a couple mph even on the flats because of passing traffic, wind and so on.

Speedometers and GPS rounds to the nearest mph. Odometers don't.
They are also off by a percentage so the difference will be worse the faster you go.

I didn't miss your point, I just disagree with you. Typical phone GPS is 1hz and I use a Bluetooth gps that is 10hz with no delay. GPS speed isn't inherently a rounded number, any rounding would be done in the app you are using to display it so use a better one instead. The main goal for most people is to have an accurate speedometer reading, and matching your speedometer to a good GPS reading accomplishes that very well.

 

[ShadowsPapa]

I didn't miss your point, I just disagree with you. Typical phone GPS is 1hz and I use a Bluetooth gps that is 10hz with no delay. GPS speed isn't inherently a rounded number, any rounding would be done in the app you are using to display it so use a better one instead. The main goal for most people is to have an accurate speedometer reading, and matching your speedometer to a good GPS reading accomplishes that very well.

Then do it however you want. I prefer to use my tried and tested method that showed the speedometer method to be off.
GPS does lag - watch the screen as you accelerate. Your truck is going faster then a second later the GPS displays the new speed. There can't help but be a delay due to latency of satellite communication.
How can GPS speed not be rounded - it's displaying mph in whole numbers, not 65.7639 mph.
Does yours show down to the tenths or hundredths of a mile per hour?
Bluetooth has nothing to do with the latency of satellite communications - you don't BT to a satellite. I'm not talking display, I'm talking the distance from satellite to GPS. (BTW - GPS doesn't communicate at 1hz - There's L1, L2 and now L5 frequencies.
It all takes time - the software of your NAV is aware of this and uses predictive methods to display your location. Speed takes a bit to catch up due to the latency. Miss and exit and you may be a block away before it realizes hey you missed your turn - recalculating.
Same happens for speed. It's not instantaneous.
Your speed change from 65 to 67 doesn't show up instantly. Cruise control helps minimize speed changes introduced by the human factory, but cruise also has a range.

Do whatever you wish - but to say that GPS is exact and up-to-date isn't correct either.
You use your method, I'll use mine. I've found it to be more accurate.
Driving 20 miles and comparing odometers is as accurate as it gets.
If the speedometer is fine with you, then use it.
I'm more picky. "Close enough" isn't generally something I say.

 

[just_another_guy]

Then do it however you want. I prefer to use my tried and tested method that showed the speedometer method to be off.
GPS does lag - watch the screen as you accelerate. Your truck is going faster then a second later the GPS displays the new speed. There can't help but be a delay due to latency of satellite communication.
How can GPS speed not be rounded - it's displaying mph in whole numbers, not 65.7639 mph.
Does yours show down to the tenths or hundredths of a mile per hour?
Bluetooth has nothing to do with the latency of satellite communications - you don't BT to a satellite. I'm not talking display, I'm talking the distance from satellite to GPS. (BTW - GPS doesn't communicate at 1hz - There's L1, L2 and now L5 frequencies.
It all takes time - the software of your NAV is aware of this and uses predictive methods to display your location. Speed takes a bit to catch up due to the latency. Miss and exit and you may be a block away before it realizes hey you missed your turn - recalculating.
Same happens for speed. It's not instantaneous.
Your speed change from 65 to 67 doesn't show up instantly. Cruise control helps minimize speed changes introduced by the human factory, but cruise also has a range.

Do whatever you wish - but to say that GPS is exact and up-to-date isn't correct either.
You use your method, I'll use mine. I've found it to be more accurate.
Driving 20 miles and comparing odometers is as accurate as it gets.
If the speedometer is fine with you, then use it.
I'm more picky. "Close enough" isn't generally something I say.

The refresh rate of a phone's position is typically 1hz, as in once per second. You can buy a GPS module such as the Garmin Glo that will pair with your phone/tablet via Bluetooth and give you higher accuracy and refresh rates. I'm going to assume maybe you aren't a huge mobile tech guy here.

I can appreciate that a gps speed will lag when you are accelerating/decelerating, but that's a pretty bad way to compare an instantaneous speed isn't it? Your air of superiority in methodology is pretty ironic given that you are using gps location for your odometer comparison.

Feel free to do whatever makes you feel good because the result is very similar and of no real consequence anyway.

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