Shift Points painful after 3 inch tire size increase.
05-04-2014, 07:03 AM
#1
JK Newbie
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Shift Points painful after 3 inch tire size increase.
Searched every keyword on every site I can find and can’t seem to find another user with the same issue - or can only find conflicting views: 2012 Jeep Wrangler Sport stock is 225/75R16 which I upgraded using OEM for the Sport S: 255/75R17.
Specs: 225/75R16 - Diameter 29.3 with 736 revs per mile.
255/75R17 - Diameter 32.1 with 649 revs per mile.
Depending on which post I read the outcome of the change should vary. In my case my speed (and odometer) were both 10% higher than three gps based checks and one ODB-II test. Logic shows in the numbers above that the 225 turns 10% more times in a mile than the 255 so the speedo/odo should be showing the opposite yes???
Because I also had a different shock per Jeep’s technical spec I walked through that with the service manager who told me there was no factor there.
Next, PSI. The only point is this would affect the overall diameter of the tire if very slightly. I found the balance between ride and handling I wanted at 36 psi, one pound over factory spec. It chalk tests out to a little over 90% tread contact (compensated for test tire temp increase to 38 PSI) thought this is my first try at chalk testing… so I know that PSI is not too much over stock. Those readings were taken with my $30 Milton pressure gauge which I cross-checked. The only point is this would affect the overall diameter of the tire if very slightly.
OK - so at this point you are thinking “get a life” but at this point this Jeep pretty much IS my life.
I fixed the speedometer and by reprogramming with the 3570 to a tire height of 30.25 even though it kills my shift points and drops me from 19 mpg city (adjusted for error to 17mpg) to 13 city because if I set it to 33 which it measures to using a level the speedometer and odometer return to 10% above true. I played with the diameter setting adjusting by quarter inch until it was nearly dead on. Great… the speedo/odo is right and the jeep is more than 10% less fun and 30% more expensive to drive.
Finally: Is my only option to choose between the 100k mark 10k miles early to enjoy driving the jeep or have the odometer/Speedometer correct and drive downshifting an automatic? Something doesn’t add up. Thoughts/Suggestions/Help (other than get a life… that part I already know). I want the speedo/odo to be correct, the odo especially, and the trans shift point to act like the settings it had at 33” tires. In fact everything is fixed but the auto transmission shift points.
Yes I have read hundreds of pages, multiple sites and endless threads but flame anyway. I know this is my first post here. I just need the collective knowledge of your years of Jeep experience.
Specs: 225/75R16 - Diameter 29.3 with 736 revs per mile.
255/75R17 - Diameter 32.1 with 649 revs per mile.
Depending on which post I read the outcome of the change should vary. In my case my speed (and odometer) were both 10% higher than three gps based checks and one ODB-II test. Logic shows in the numbers above that the 225 turns 10% more times in a mile than the 255 so the speedo/odo should be showing the opposite yes???
Because I also had a different shock per Jeep’s technical spec I walked through that with the service manager who told me there was no factor there.
Next, PSI. The only point is this would affect the overall diameter of the tire if very slightly. I found the balance between ride and handling I wanted at 36 psi, one pound over factory spec. It chalk tests out to a little over 90% tread contact (compensated for test tire temp increase to 38 PSI) thought this is my first try at chalk testing… so I know that PSI is not too much over stock. Those readings were taken with my $30 Milton pressure gauge which I cross-checked. The only point is this would affect the overall diameter of the tire if very slightly.
OK - so at this point you are thinking “get a life” but at this point this Jeep pretty much IS my life.
I fixed the speedometer and by reprogramming with the 3570 to a tire height of 30.25 even though it kills my shift points and drops me from 19 mpg city (adjusted for error to 17mpg) to 13 city because if I set it to 33 which it measures to using a level the speedometer and odometer return to 10% above true. I played with the diameter setting adjusting by quarter inch until it was nearly dead on. Great… the speedo/odo is right and the jeep is more than 10% less fun and 30% more expensive to drive.
Finally: Is my only option to choose between the 100k mark 10k miles early to enjoy driving the jeep or have the odometer/Speedometer correct and drive downshifting an automatic? Something doesn’t add up. Thoughts/Suggestions/Help (other than get a life… that part I already know). I want the speedo/odo to be correct, the odo especially, and the trans shift point to act like the settings it had at 33” tires. In fact everything is fixed but the auto transmission shift points.
Yes I have read hundreds of pages, multiple sites and endless threads but flame anyway. I know this is my first post here. I just need the collective knowledge of your years of Jeep experience.
05-04-2014, 07:18 AM
#2
JK Super Freak
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Hi and welcome. For a true rolling diameter you should be measuring from the center to the ground and then doubling that number. Don't worry about what the tire is supposed to measure out to, rarely is this accurate.
As far as the shift points, is this your first jeep? These are not fast snappy vehicles and you have just put stock size tires on it. Maybe going from the 29 inch tire to the 32 made enough seat of the pants difference. You can try a different tuner that will modify shift points.
As far as the shift points, is this your first jeep? These are not fast snappy vehicles and you have just put stock size tires on it. Maybe going from the 29 inch tire to the 32 made enough seat of the pants difference. You can try a different tuner that will modify shift points.
05-04-2014, 07:54 AM
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Hi and welcome. For a true rolling diameter you should be measuring from the center to the ground and then doubling that number. Don't worry about what the tire is supposed to measure out to, rarely is this accurate.
As far as the shift points, is this your first jeep? These are not fast snappy vehicles and you have just put stock size tires on it. Maybe going from the 29 inch tire to the 32 made enough seat of the pants difference. You can try a different tuner that will modify shift points.
As far as the shift points, is this your first jeep? These are not fast snappy vehicles and you have just put stock size tires on it. Maybe going from the 29 inch tire to the 32 made enough seat of the pants difference. You can try a different tuner that will modify shift points.
05-04-2014, 02:15 PM
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You didn't loose any fuel mileage by reprogramming you simply corrected it to what your actual mileage is. The onboard computer is only a measuring stick and is by no means accurate enough to make assumptions. True fuel mileage can only be obtained by manual calculation using an odometer that is known to be accurate.
There are many discussions about throttle response, shift points, and so on. The bottom line is reprogram to the accurate tire size, then drive it like you stole it for a few days so the computer can settle in like that. The computer is adaptive so if you drive it easy it will learn your ways and get sluggish.
There are many discussions about throttle response, shift points, and so on. The bottom line is reprogram to the accurate tire size, then drive it like you stole it for a few days so the computer can settle in like that. The computer is adaptive so if you drive it easy it will learn your ways and get sluggish.
