Question regarding tire sizes and on board computer

[Ry' N Jen]

Jen would like to know if the vehicles computer need to be reprogrammed now that we upped the tyre size from 225-70-16 to 245-75-16.
The reason she asks is: When she drives the KJ to and from work (She is the one who puts most of the km's on the clock.)
She says that the ****** shifts weird!
The only thing I noticed when driving is that it is almost like the torque converter seams to look for a gear from shifting to third to over drive. That is the only thing I noticed once in the last 160 Km I have driven.
Today I drove about 50 miles and I didn't notice any strange happenings with the auto box!
I didn't find anything really pertinent in my FSM (I didn't think I would mind you!)
So, I thought I would post this here on behalf of Jen...

Cheers
Ry'

 

[Atrus]

I don't think it'd matter as long as all 4 tires are the same size.

Sure, the speedo will be off, but even though I upgraded to 245/70 my speedo was still fast. So, I'd check the speedo with a GPS. You may actually be closer than stock.

 

[tjkj2002]

Not resetting the pinion factor will cause weird shifting and more shifting in the trans since it is confused from it's preset info on where the speed should be in relation to the 2 speed sensors in the trans.It doesn't really matter if your speedo is correct or not from the factory with stock tires,it still is programed with what it came with and it confuses the TCM.The main and only factor for getting it done is for the auto ****** survivability.


Oh and on a side note your KJ will act all funny without the t-case position sensor plugged in,even if it reading the wrong t-case position it makes a world of difference in how your KJ acts,just found that one out today(banghead).

 

[Ry' N Jen]

I don't think it'd matter as long as all 4 tires are the same size.

Sure, the speedo will be off, but even though I upgraded to 245/70 my speedo was still fast. So, I'd check the speedo with a GPS. You may actually be closer than stock.

I'll be buying a new GPS next month. I'll have to wait until then.

 

[JeepJeepster]

What tjkj said.

My speedo is around 2mph slow with my 245/70 tires.

 

[Atrus]

Ah, didn't think about that - RPM vs MPH. Good point.

So, if I've jumped 1" diameter more over stock, does this mean more wear and tear on the trans? I'd assume there's some tolerable threshold.

 

[Ry' N Jen]

Not resetting the pinion factor will cause weird shifting and more shifting in the trans since it is confused from it's preset info on where the speed should be in relation to the 2 speed sensors in the trans.It doesn't really matter if your speedo is correct or not from the factory with stock tires,it still is programed with what it came with and it confuses the TCM.The main and only factor for getting it done is for the auto ****** survivability.


Oh and on a side note your KJ will act all funny without the t-case position sensor plugged in,even if it reading the wrong t-case position it makes a world of difference in how your KJ acts,just found that one out today(banghead).

So a trip to the dealership might solve this then?
Like I mentioned, I don't seem to notice any strange shifting issues as of now.
But if it need be hooked up to Chryslers Diagnostic computer, than so be it!
I am not one to be a Jew(I am a Jew BTW!) when it comes to keeping all my vehicles in 100% top running order!

Thanks for your input TJ

 

[J-Thompson]

ours seems fine
3+ years on 245/75's and now on 255/70's
it may be that you or she just needs to adjust to the way it handles

 

[J-Thompson]

Ah, didn't think about that - RPM vs MPH. Good point.

So, if I've jumped 1" diameter more over stock, does this mean more wear and tear on the trans? I'd assume there's some tolerable threshold.

your trans has no connection to the out side world so it does not know how fast you are really going
taller tires only change your speed due to the fact that they take less revolution to go 1 mile when compared to a smaller tire but your trans does not know what size tires you have
it does know that it has to work harder at a given speed
but you are really going faster than that speed

Just did the math in the TJ when I realized that a trip I just took was 10 miles less in the TJ than the KJ which was a few miles less than the GPS
the TJ's speedo is 12%+ off this means that at 70 mph I am pushing close to 80 ,no wander it was drinking gas
I can do 65 MPH and be going fast enough to keep up with traffic
but still get reasonable MPG's

 

[tjkj2002]

your trans has no connection to the out side world so it does not know how fast you are really going
taller tires only change your speed due to the fact that they take less revolution to go 1 mile when compared to a smaller tire but your trans does not know what size tires you have
it does know that it has to work harder at a given speed
but you are really going faster than that speed

Just did the math in the TJ when I realized that a trip I just took was 10 miles less in the TJ than the KJ which was a few miles less than the GPS
the TJ's speedo is 12%+ off this means that at 70 mph I am pushing close to 80 ,no wander it was drinking gas
I can do 65 MPH and be going fast enough to keep up with traffic
but still get reasonable MPG's

Actually it does,just saying.

 

[Ry' N Jen]

Actually it does,just saying.

I was under that impression as well!
That's why I asked in the first place!

 

[HoosierJeeper]

So a trip to the dealership might solve this then?
Like I mentioned, I don't seem to notice any strange shifting issues as of now.
But if it need be hooked up to Chryslers Diagnostic computer, than so be it!
I am not one to be a Jew(I am a Jew BTW!) when it comes to keeping all my vehicles in 100% top running order!

Thanks for your input TJ

Yep....it's a dealer job. Shouldn't cost much.

 

[J-Thompson]

Actually it does,just saying.

you are going to have to explain to me how the trans can know how fast you are going based on tire size?
I do know that the trans does not have so super top secrete GPS built in
so with out that it can not know
and I know that it takes more revolution for a smaller tire to go the same distance as a larger tire thus you go faster with taller tires at a given engine speed
but the trans has no way to know the difference

 

[tjkj2002]

you are going to have to explain to me how the trans can know how fast you are going based on tire size?
I do know that the trans does not have so super top secrete GPS built in
so with out that it can not know
and I know that it takes more revolution for a smaller tire to go the same distance as a larger tire thus you go faster with taller tires at a given engine speed
but the trans has no way to know the difference

It uses the 2 speed sensors in the trans and compares it to the rear diff speed sensor(ABS sensors in '06-'07 KJ's).

 

[tjkj2002]

I did read somewhere on here that the dealer can actualy fill in the tire size somewhere in the ECU / PCM menu...

Anyway, thy are able to change it for sure.

Some years you can,some you can't and why I do not know and not sure about what years you can or can not('02's you can I do know this).Also some years have a tire size selection(only OEM tire sizes) while the others have a Revolutions Per Mile input.

 

[Atrus]

Thinking through it this morning, I tend to agree with J-Thompson. Can someone further explain this?

Engine connects to trans, trans connects to diffs, diffs connect to wheels. They are all (essentially) fixed. I can understand being able to set the PCM to have the odometer/speedo read correctly, but I don't see how the trans knows the difference. By reading the speed sensor or wheel sensors, the vehicle knows that the diff/wheels are spinning at X,*** RPM. It doesn't know how fast it's going - that's what the PCM uses the pinion factor or whatever it's called.

As far as I can think through it, the entire drivetrain is basically running unchanged by the different sized tires.

If the PCM was mesuring RPM vs ACTUAL speed then I can see where it'd get all goofy. For example, as J-Thompson said, if there's a GPS (we know there isn't) or if the vehicle was measuring actual speed with a seperate sensor that ran on the roadway, then it'd see the difference in speed vs rpm. Since it's essentially a "closed" system, the vehicle speed is derived, not measured.


What do you all think? Am I missing something?

 

[J-Thompson]

It uses the 2 speed sensors in the trans and compares it to the rear diff speed sensor(ABS sensors in '06-'07 KJ's).

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"

 

[J-Thompson]

Thinking through it this morning, I tend to agree with J-Thompson. Can someone further explain this?

Engine connects to trans, trans connects to diffs, diffs connect to wheels. They are all (essentially) fixed. I can understand being able to set the PCM to have the odometer/speedo read correctly, but I don't see how the trans knows the difference. By reading the speed sensor or wheel sensors, the vehicle knows that the diff/wheels are spinning at X,*** RPM. It doesn't know how fast it's going - that's what the PCM uses the pinion factor or whatever it's called.

As far as I can think through it, the entire drivetrain is basically running unchanged by the different sized tires.

If the PCM was mesuring RPM vs ACTUAL speed then I can see where it'd get all goofy. For example, as J-Thompson said, if there's a GPS (we know there isn't) or if the vehicle was measuring actual speed with a seperate sensor that ran on the roadway, then it'd see the difference in speed vs rpm. Since it's essentially a "closed" system, the vehicle speed is derived, not measured.


What do you all think? Am I missing something?

your first line covered it
you thought it through

 

[AlexKJ]

I did read somewhere on here that the dealer can actualy fill in the tire size somewhere in the computers menu...

But it's only to correct the speedo... idk if it has to do with the ******.

From the '03 KJ Service Manual - Section 85-E, Page 266

Tire Size - The BCM calculates vehicle speed
based upon a programmed Tire Revolutions per mile
(TIRE REV/MILE) value. The correct tire size must
be programmed to the BCM using the DRBIII.
Using the DRBIII, select Body Computer, then Program
Tire Size. The BCM must be programmed with
one of the available tire sizes in the DRBIII menu
or by programming the correct REV/MILE value.
Vehicle Speed System - ABS Equipped
- The CAB provides 12VDC hard wire speed sensor supply
to the Left and Right Wheel speed sensors and the
Rear Wheel Speed Sensor the speed sensor output
hard wire square wave signals back to the CAB. the
CAB outputs a hard wire square wave back to the
BCM. The BCM calculates vehicle speed based upon
a programmed Tire revolutions per mile (TIRE REV/
MILE) value. This calculation is based upon tire circumference
revolutions per mile. This calculation
must be programmed into the BCM. New service
BCM’s are shipped in default mode that prevents
speedometer indication until a valid tire size is programmed.
The BCM outputs a hard wired square
wave Vehicle Speed Output signal to the PCM. The
PCM transmits vehicle speed to the instrument cluster
via the PCI bus.
Vehicle Speed System - NON-ABS Equipped
- The BCM provides a 12VDC hard wire speed sensor
supply to the Rear Wheel speed sensor. The speed
sensor outputs a hard wire square wave signal back
to the BCM. The BCM calculates vehicle speed based
upon a programmed Tire Revolutions per mile (TIRE
REV/MILE) value. This calculation is based tire circumference
revolutions per mile. This calculation
must be programmed into the BCM. New service
BCM’s are shipped in default mode that prevents
speedometer indication until a valid tire size is programmed.
The BCM outputs a hard wired square
wave Vehicle Speed Output signal to the PCM. The
PCM transmits vehicle speed to the instrument cluster
via the PCI bus.

 

[Powerslave]

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"

It doesn't effect the actual speed; If you are going 60MPH then you are going 60 no matter what the tire size is. However, with LARGER tires, the speedometer would read lower. Smaller tires would show you are going faster; BUT, you are going 60. 60MPH is how fast the chassis is moving, not how fast the wheels are spinning. When you change tire size, you are changing the speed at which they rotate.