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Discussion Starter #1
Ok, I put on the Jeep 2-inch lift on my 2015 Rubicon Unlimited automatic trans and changed my tires from stock size to Toyo LT285/75-17. Since the tire change, the speedometer is spot on with the GPS. Either my speedometer was off some with my stock tire size or am I missing something here? My throttle response seems sluggish and I dropped about 3-4 mpg. I know the new tires have more meat on the ground than stock but that's a big change in MPG. How are the shift points computed?. Gearing has the stock 4:10 which I know have changed the ratio some with the tire change. Do I need to recalibrate because of the tire change even though the speedometer is the same as my GPS?
Thanks
 

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Did you use gps from another source other than the jeeps? If you didn't try using your phone. The stock GPS and speedo gets the same signal.
 

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Calibrate against highway mile markers. I do it over a 10 mile stretch, if the error is more than +- 0.1 mile I adjust the calibration.
 

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Did you use GPS from another source other than the jeeps? If you didn't try using your phone. The stock GPS and speedo gets the same signal.
I did not use a second GPS. Thought the GPS went by satellite signal? I will use my Garmin handheld to verify
 

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430N uses jeeps speedo which is nuts. use a stand alone gps or phone speedo app. I have run those same tires and difinitely need reprogramed. if you have an AEV procal use 32.5" to start.
 

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430N uses jeeps speedo which is nuts. use a stand alone gps or phone speedo app. I have run those same tires and difinitely need reprogramed. if you have an AEV procal use 32.5" to start.
Thanks...I don't have a programmer but will get one. Which do you recommend? Not looking to reprogram computer(I have a lifetime bumper to bumper warranty on the Jeep)
 

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AEV procal will not go far enough into your computer to leave a fingerprint and will not mess anything up.
 

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OK, went out and checked against my handheld. Speedometer at 60, GPS handheld 64mph. Speedometer 70, Handheld 75.
Guess I will be ordering a re-cal tool. Now does the tach work from the engine or is it also tied to the speedometer?
 

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Superchips also makes a recalibration tool that allows to you change a few other features on you JK. You can also upgrade it to a tuner (via internet) at a later time for a fee if you choose.
 

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Did you use gps from another source other than the jeeps? If you didn't try using your phone. The stock GPS and speedo gets the same signal.
The speedometer and odometer doesnt use the GPS, it uses wheel ticks (revolutions)
if it used GPS for your speedometer, it would lose signal too often to create a viable gauge.
 

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Discussion Starter #12
The speedometer and odometer doesnt use the GPS, it uses wheel ticks (revolutions)
if it used GPS for your speedometer, it would lose signal too often to create a viable gauge.
There was a difference in speed readings between the factory GPS in the nav system and my Garmin handheld. The factory GPS was spot on with the speedometer but not my Garmin handheld.
 

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There was a difference in speed readings between the factory GPS in the nav system and my Garmin handheld. The factory GPS was spot on with the speedometer but not my Garmin handheld.
correct. when you input the correct tire size and change your speedo they should all match. it takes some time tho as might have to go up or down on tire size until the speedo and your Garmin handheld agree.
 

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There was a difference in speed readings between the factory GPS in the nav system and my Garmin handheld. The factory GPS was spot on with the speedometer but not my Garmin handheld.
Fun fact: vehicle engineering requirements for the speedometer usually permit an accuracy of only 5%. This accounts for things like tire wear and PSI. Police radar guns are only accurate to 1% of your speed.
The fact that the two GPSs were different could be any number of things; GPSr accuracy settings, sample rate, etc. GPS really only allots for location; the speed is a derived value of the change in distance over a set sample rate. Lets say that the speedometer in the vehicle was based solely on the GPS signal. How would you know how fast you were going when the signal is lost? Dead reckoning is the only way that it would work and that is based on the assumption that your velocity and heading has not changed since the signal was last acquired. Also, it is inherently inaccurate in cities with large buildings as the signal is prone to bouncing off of the buildings and you lose a good number of satellites as the sky is blocked. Youre only going to get the most accurate signal in an open area, with no overhead obstructions (including clouds and trees) and even only in specific hours of the day.
Basically, GPS signalling is not as accurate over the life of a vehicle and prone to wild inaccuracies. Cars use the wheel revolutions based on a known diameter. The only way to change that is to change the tire diameter the vehicle expects.
 

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I bought an AEV ProCal when I bought my lift kit, even though it was another two months before I upgraded my tire size. I keep it in the glove box for those instances when I need to make more changes, such getting the TPMS to shut up when I air down. Used it Memorial weekend when I changed the tires to a different size. It will also allow me to account for changes in diff ratios if I ever regear, plus other odds and ends.

It's one of several options available. Works on dip switches. I keep the instructions with it so I know what settings to use.
 

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Fun fact: vehicle engineering requirements for the speedometer usually permit an accuracy of only 5%. This accounts for things like tire wear and PSI. Police radar guns are only accurate to 1% of your speed.
The fact that the two GPSs were different could be any number of things; GPSr accuracy settings, sample rate, etc. GPS really only allots for location; the speed is a derived value of the change in distance over a set sample rate. Lets say that the speedometer in the vehicle was based solely on the GPS signal. How would you know how fast you were going when the signal is lost? Dead reckoning is the only way that it would work and that is based on the assumption that your velocity and heading has not changed since the signal was last acquired. Also, it is inherently inaccurate in cities with large buildings as the signal is prone to bouncing off of the buildings and you lose a good number of satellites as the sky is blocked. Youre only going to get the most accurate signal in an open area, with no overhead obstructions (including clouds and trees) and even only in specific hours of the day.
Basically, GPS signalling is not as accurate over the life of a vehicle and prone to wild inaccuracies. Cars use the wheel revolutions based on a known diameter. The only way to change that is to change the tire diameter the vehicle expects.
The reason the built in GPS in the Jeep and the handheld GPS he compared it to were different is because the built in GPS that Jeep comes with stock does not actually use GPS to determine speed. The stock built in GPS uses the same exact speed signal the speedo uses, and if you change tire sizes you alter that signal (as you allude to) and make it inaccurate. So, while you would think a GPS would display speed based on the GPS, the stock GPS does not. So you cannot use the stock GPS in a Jeep to compare to see if your speedo is off after changing tire sizes. The two will always agree, as they use the same speed signal and if that signal is off they will both be off.

GPS speedo's work well in my experience. They do drop out in certain situations, but it is rare and they seem to come back quickly. For custom motorcycles (and cars) there are companies that make speedos that use GPS. That way you don't have to figure out how to send them a speed signal. And they do not need calibrating.
I suspect Jeep went the way they went to make sure that both speedo and GPS always showed the same speed. Exactly what their motivation there was I am not sure, but I suspect that it would reduce the number of complaints they would get from people crying that their speedo wasn't reading right.
 

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Discussion Starter #18
Fun fact: vehicle engineering requirements for the speedometer usually permit an accuracy of only 5%. This accounts for things like tire wear and PSI. Police radar guns are only accurate to 1% of your speed.
The fact that the two GPSs were different could be any number of things; GPSr accuracy settings, sample rate, etc. GPS really only allots for location; the speed is a derived value of the change in distance over a set sample rate. Lets say that the speedometer in the vehicle was based solely on the GPS signal. How would you know how fast you were going when the signal is lost? Dead reckoning is the only way that it would work and that is based on the assumption that your velocity and heading has not changed since the signal was last acquired. Also, it is inherently inaccurate in cities with large buildings as the signal is prone to bouncing off of the buildings and you lose a good number of satellites as the sky is blocked. Youre only going to get the most accurate signal in an open area, with no overhead obstructions (including clouds and trees) and even only in specific hours of the day.
Basically, GPS signalling is not as accurate over the life of a vehicle and prone to wild inaccuracies. Cars use the wheel revolutions based on a known diameter. The only way to change that is to change the tire diameter the vehicle expects.
Ok, I think we both can agree that changing tire diameter is going to have an effect on true ground speed Correct? I went from stock 32" to 34". I know why the speedometer is reading the same as it was before the tire change. If the GPS isn't reading off some same signal input, why isn't it reading the correct increased ground speed? Just for the hell of it I went back out and took 3 GPS units (the handheld Gramin and two other Garmin units from my motorcycles) to compare with the factory unit. I am in Central Calif. No clouds, no trees etc to interrupt GPS signals. With my cruise control set on 70mph on the speedometer, the factory GPS navigation read 70mph. The other 3 Garmin units consistently read 75mph.
 

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The factory display doesn't calculate speed based on GPS data. It just takes the vehicle speed from the wheel sensors, just like the speedometer.

Your handheld GPS' are all correct and you need to adjust tire size with the unit you buy to get your on-board speed (both speedo and gps screen) to read right.

Tip: Take the tire size that's in there already, multiply it by 75 and divide by 70. (The two speeds reported). That will give you the tire size you need to program in to get your speedometer accurate.
 

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Heres another way for the most accurate setting.
Disclaimer: I dont know the exact format that the procal uses, but you can use this to get a very accurate measurement

With two people, do the following
1) Find a driveway or parking lot that lets you drive very slowly for a good 10-15 yards
2) Take a piece of chalk and mark your tire at the exact point it meets the ground, the 6:00 position. Also mark the point on the ground where the tire meets. This is your start point
3) Drive slowly forward and have a friend count how many times the wheel makes a complete revolution 5 or 10 times (or whatever space you have, more than 3 is best)
4) Stop the Jeep at the point where the mark you made on the tire is again pointing straight down at the 6:00 position. Mark this point on the ground, your end point
5) Take a tape measure and measure between the two points you made, start and end. Divide that by the number of revolutions. This is your overall wheel circumference and probably the number you want to input into the procal. Ideally, you should get the same, or very similar number from the post above
 
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