Question regarding tire sizes and on board computer

[J-Thompson]

Guys I fully understand the relation of tire size and the effect it has on speed
really I do
my question is to trans shift points
or more so to the comment on the life of the trans
the trans has no way of knowing what size tires you have
it knows it is working a little harder than it does with smaller tires but that is all it knows sure you can reprogram it and the speedo will be right
big effin deal most of us have GPS so who cares how far off the speedo is
my point is simple
there is no point in worrying about the trans just because you changed your tires
oh and if you disconnect the battery for a little while and reconnect it it will go back to normal after a while

 

[Atrus]

Guys I fully understand the relation of tire size and the effect it has on speed
really I do
my question is to trans shift points
or more so to the comment on the life of the trans
the trans has no way of knowing what size tires you have
it knows it is working a little harder than it does with smaller tires but that is all it knows sure you can reprogram it and the speedo will be right
big effin deal most of us have GPS so who cares how far off the speedo is
my point is simple
there is no point in worrying about the trans just because you changed your tires
oh and if you disconnect the battery for a little while and reconnect it it will go back to normal after a while

Agreed.

Not to mention, I had 235/70 (29") stock. I put on 245/70 (29.5"). OEM tires are stated to be 721 rev/mile, and the 245's are stated at 707 revs/mile. So, the 245's have a 2% less than stock.

I finally bought a GPS and I have my crackberry with GPS now. Both the Magellan and Crackberry show that WITH the 245/70 I am still a good 3-4 mph slow at 45. This means when my speedo shows 45, I am actually more like 41-42mph.

So, if I were to reset the factor in the PCM to account for the correct tire size, I'd be further out of calibration.

 

[Ry' N Jen]

Guys I fully understand the relation of tire size and the effect it has on speed
really I do
my question is to trans shift points
or more so to the comment on the life of the trans
the trans has no way of knowing what size tires you have
it knows it is working a little harder than it does with smaller tires but that is all it knows sure you can reprogram it and the speedo will be right
big effin deal most of us have GPS so who cares how far off the speedo is
my point is simple
there is no point in worrying about the trans just because you changed your tires
oh and if you disconnect the battery for a little while and reconnect it it will go back to normal after a while

I did disconnect the battery for about an 4 hours and then let the vehicles brains relearn everything(I'm calling them "Thr brains" as Jen reads this forum at her work and is not fully versed on all the technical terminology. ie. BCM/PCM... I'll teach her all that in time!)
Anyway, The ****** has re-learned and the Jeep drives fine.

 

[J-Thompson]

=D>

 

[tjkj2002]

how would that have any bearing on the actual speed of the KJ
if I have 225/70's then the trans and speed sensor will both be turning X rpm at Y speed trans does not know the real MPH
if I run 265/75's then the % of error will remain the same at both the trans and the wheel sensors again trans does not know
you are not changing the link between the trans and the speed sensor
that would have to be a gear change making the end of the axle shaft turn faster in relation to what it did with 3.73's
but the trans and the end of the axle shaft are "hard wired"

Your actual speed has no bearing on the whole thing,it's what the PCM is reading from all 3 sensors(and others) and comparing them to a preset parameter set in its ROM.There is a % you can be off but to much and it really starts messing with the trans.Now 245/70's and heck 245/75's might be just at the outer limits of those preset parameters and may/may not have a short term effect but jumping bigger then that it will be way out and shift funny.I know this since when I went from 225/75's to 265/75's my trans shifted like s#$t(even with the 4.10's installed) after some very fun math got the correct RPM's inputed into the PCM(VIA close freind at the dealership) and it was literlly a night and day difference in how the trans acted.

 

[kjweston]

Your actual speed has no bearing on the whole thing,it's what the PCM is reading from all 3 sensors(and others) and comparing them to a preset parameter set in its ROM.There is a % you can be off but to much and it really starts messing with the trans.Now 245/70's and heck 245/75's might be just at the outer limits of those preset parameters and may/may not have a short term effect but jumping bigger then that it will be way out and shift funny.I know this since when I went from 225/75's to 265/75's my trans shifted like s#$t(even with the 4.10's installed) after some very fun math got the correct RPM's inputed into the PCM(VIA close freind at the dealership) and it was literlly a night and day difference in how the trans acted.

I have 265/75s should I get mine adjusted? and what exactly is it called so I can tell them at the dealership? and anyone know ballpark how much this costs?? Thanks

 

[RenegadeJay]

I don't know if this even comes close to helping this discussion, but I just changed my tires from the stock 225/75/16's to 245/70/16's. After reading this thread I called a Jeep dealer near me and asked about the computer recal question. The service tech on the phone told me since my tire size is considered a "plus zero" size, no recal is needed. If I went to a plus one or higher, they would need to recal. He said the overall difference between the "plus zero" tires is about the same as over or underinflated tires so the computer compensates for it. Once the tires get beyond that, you need to bring it in. Just my input, hopefully I am in the same ballpark with this and don't sound like a complete a$$.

 

[Atrus]

I still struggle with the reasoning on why it needs to be done.

Understood that there are 3 sensors which need to be in agreement - speed sensor and 2 in the trans.

Regardless of how big or little the tires are, the three sensors will function in fixed (proportionate) ratio. The point of those three having to be in agreement is to flag and/or compensate for an error in the drivetrain. Am I wrong?

 

[kjweston]

well if the tires are bigger. I think it works out that your going faster at the same rpms you would be going at a slower speed with stock size.. there for only one factor is changed. only the speed sensor.

BUT.... now that i wrote it out im suddenly in the same boat as you i do not understand it. but i do realize it just doesn't feel right shifting and driving.. I drove my buddies stock kj granted i have 45RFE he has 42RLE it still drove way better and shifted should i say cleaner?? idk his just felt like a dependable car. mine feels like somethings wrong with it.. that's why I'm considering getting it done. plus the guru (tjkj) said so.

 

[Atrus]

You are going faster relative to the ground, yes.

The transmission, drive axles, driveshaft, differentials, everything else is turning at the same ratio. Therefore, the PCM knows no different.

The PCM knows that the engine is going X,*** RPM and the wheel speed sensor reads Y,YYYRPM and the transmission sensors read Z,ZZZ and if there are all in tolerance than it thinks everything is peachy keen. It also takes those X's, Y's, and Z's and derives a vehicle speed based on the factor stored in the PCM.

So, yeah, the KJ thinks you're doing 45 and you're doing 50, but it shouldn't matter to the PCM because everything is moving along at the same RPM it should be at 45. The ground is just moving faster below it, which is outside the system.

If there was an "out-of-the-system" measurement like a GPS or a way to MEASURE vehicle speed (not derive it) then I can see where the PCM would be like "WHAT THE HELL?!?!?" and flip out.

It's like this. Blindfold yourself and walk at 5 mph. Now blindfold yourself and walk at 5mph on one of those airport treadmill things that's going 10 mph. You'll feel the exact same as walking 5mph, but you're going 15 mph. PCM is just as oblivious as you with the blindfold.

Again, I can be totally wrong, but so far I really don't see where I am.

 

[kjweston]

yeah that's what I was realizing when I wrote my previous post. ha

well somehow it affects it. I know that maybe not mathematically.. which seems to be the only way it could affect.. idk. ill wait till troy gets back on tomorrow. He might have a further explanation

 

[tjkj2002]

Let's say you could cruise at 75mph in OD and your turning 2700rpm's(just a guess) with stock sized 235/70's.Now with say 265/75's at 75mph your turning 2500-2600rpm's(allowing for greater power needed to turn those heavier tires).The PCM see's that and does not know what to do as far as telling the TCM(for shift points) so your trans hunts for gears more often.Now the PCM and TCM "learn" your driving habits and slightly adjust but it can only adjust so much when comparing against it's ROM.Again your actual speed(not what the speedo is showing) has no bearing.

Now above would be a perfect world since we don't live in one................

My 3.7 with 225/75's at 75mph would be turning about 2600rpm's actual on flat ground,no lift and no real added weight.When I lifted my KJ and went to 265/75's(again no other added weight besides the wheels/tires) I was turning about 2800rpm's,sounds backwards but that was what it was since more power was required to maintain that speed.When I added almost all my armor and 4.10's I was turning about 2900rpm's at 75mph and my trans was shifting like crazy.After the reset with the same stuff I was turning 2700rpm's at 75mph and no funny shifting and gained about 5-6mpg's on the highway and about 4-5mpg's in town.That is my experiance.

 

[Atrus]

TJKJ - I can understand that. It's more of a weight/diameter thing (weight is self explanatory, diameter means more fore needed to get it moving - centntifugal force) than a circumference thing.

 

[tjkj2002]

TJKJ - I can understand that. It's more of a weight/diameter thing (weight is self explanatory, diameter means more fore needed to get it moving - centntifugal force) than a circumference thing.

Hence the RPM differences which relate to different input trans speed and output trans speed and thus a different rear wheel speed which the PCM can and will detect and with the same MPH reading it get's confused and does not know what to do for the correct shift points.

Now you all are making my head hurt,I'd rather not think this hard,I'm to dang old for it.

 

[J-Thompson]

let me really gum up the works
in the reasoning that the trans shift points need to be adjusted due to
the need for more power to move a given load to help the trans last longer
then in my last work truck ,which got 200K on the trans, I should have for sure
had it reprogramed
I upped the tire size by 1-2" over stock and added 3000# all the time and some times 1500# on top of that
thing is all of our trucks are the same and none get reprogramed
but they all last for close to 200K

 

[kjweston]

so bottom line it will slightly lesson the life of trans if not reprogrammed??

 

[J-Thompson]

so bottom line it will slightly lesson the life of trans if not reprogrammed??

I say no
the key factor is tire size VS gear ratio
3.73's with 32 X 11.5's on my TJ and it is not real happy
but luckily it only has a 3 speed auto so any thing over 50 and the TC locks up
the KJ has a 4 speed and I can see it gear hunting ,even on flat ground
with 32's ,265/75's, and 3.73's
and the 265/75's tend to be heavier than the 32 X 11.5's
so the answer would be 4.10's but this will introduce a problem if you have
ABS as the outer ends of the axle shaft will be turning at a different rate
then with 3.73's and the PCM will pick up on it as being an issue
when it gets the input from the trans
Trans will be at 1 speed
the ABS sensors will be at another according to the PCM
so it will have issues and a reprogram will be in order

 

[AlexKJ]

the KJ has a 4 speed and I can see it gear hunting ,even on flat ground with 32's ,265/75's, and 3.73's

The JK can be ordered with 255/75R17 (32''x10'') wheels, 42RLE ****** and a 3.73:1 axle ratio... gear hunting right from factory? :-k

 

[J-Thompson]

The JK can be ordered with 255/75R17 (32''x10'') wheels, 42RLE ****** and a 3.73:1 axle ratio... gear hunting right from factory? :-k

As long as we are talking about the same JK that will stall
going up hill in 4lo due to a lack of power
then yes
you are talking about an entirely different animal
I have wheeled with stock JK 4 doors ,non-rubi
and they have NO power to climb hills
this is from 2 things ,lack of low end torque and
numerically low gears
I really don't care what the specs on the 3.8 are I know what they
do going up steep ,drag your bumper steep, hills
the stock Rubi with 4.10 does much better but as soon as you
go to 33's you are bogging down due to a lack of gear
say what you want
they lack gearing
because I feel that my smaller, lighter, does not stall on hills
TJ on 32's lacks gearing and it has 3.73's
but no 4th gear so 4.10's are out

 

[tjkj2002]

The JK can be ordered with 255/75R17 (32''x10'') wheels, 42RLE ****** and a 3.73:1 axle ratio... gear hunting right from factory? :-k

Different engine and vehicle weight.