Tire size and reprogramming

[ejewels]

Hey all,

I've been doing some reading and it seems that going up a tire size or at least going from stock willys tires (32.1") to the rubi take off KO2 (32.6") doesn't require reprogramming by a bought tool or the dealer. Is this accurate? I've done the same and it drives smoother and almost exactly like it did with the stock tires. Questions that I didn't see answered are:

- are the shift points or anything auto trans affected when slightly increasing tire size?
- are odometer readings affected by any of this? In other words, if someone put 40's on a jeep, then went back to stock, would the odometer be off and the vehicle essentially could have more miles on it then it reads?

any help appreciated!

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[jludave]

The Willys tires are 32.1" as you state. The KO2 tires are actually 32.7"

The screen snip below shows the differences:

The .6" difference in size may not be causing any noticeable difference in shift points.

In terms of odometer reading, yes, the difference in tire size does cause the odometer to be off if the vehicle isn't programmed with the larger tire size. Look at the speedometer error info in the clip above. Without a tire size program update, and a larger tire, the vehicle will read a slower MPH than it is actually going, thus the odometer will be clocking miles slower than actual.

 

[ejewels]

The Willys tires are 32.1" as you state. The KO2 tires are actually 32.7"

The screen snip below shows the differences:

The .6" difference in size may not be causing any noticeable difference in shift points.

In terms of odometer reading, yes, the difference in tire size does cause the odometer to be off if the vehicle isn't programmed with the larger tire size. Look at the speedometer error info in the clip above. Without a tire size program update, and a larger tire, the vehicle will read a slower MPH than it is actually going, thus the odometer will be clocking miles slower than actual.

So is it actually necessary? I guess thats the real question. And I wonder if people buy higher mileage jeeps and didn't realize the previous owner racked up more miles than the ODO reads. And I wonder how much the ODO is actually off. So if a used jeep has 100k miles reading, but it had run say 2 or 3 tire sizes up for a lot of its life (but then taken off to sell)... how much would that mileage be affected?

 

[jludave]

So is it actually necessary?

That's your call. There are different options for programming the new tire size. It comes down to cost and effort. Personally, I want things as they should be so I updated my tire size. I'm using a Tazer now, but previously used JSCAN.

So if a used jeep has 100k miles reading, but it had run say 2 or 3 tire sizes up for a lot of its life (but then taken off to sell)... how much would that mileage be affected?

The odometer will read less miles that what the vehicle has actually travelled.

 

[ejewels]

That's your call. There are different options for programming the new tire size. It comes down to cost and effort. Personally, I want things as they should be so I updated my tire size. I'm using a Tazer now, but previously used JSCAN.



The odometer will read less miles that what the vehicle has actually travelled.

I wonder how much and if it can be a huge difference. I doubt it when I went up only a slightly larger size. Thats why I'll prob skip any programming. If I go 35's or higher I prob will.

 

[Naylia]

If you think you're going 79, and the cop clocks you going 81....you'd definitley notice the differnece here in CA between not being worth the cop's time and getting a ticket with a 3X multiplier on it....

 

[jludave]

I wonder how much and if it can be a huge difference. I doubt it when I went up only a slightly larger size. Thats why I'll prob skip any programming. If I go 35's or higher I prob will.

In your case you have over 1/2 inch difference. Per the screen clip above, at 60MPH, you will actually be travelling over 1MPH faster that what your speedo will read. Your mileage will be less that what you actually travelled.

 

[ejewels]

In your case you have over 1/2 inch difference. Per the screen clip above, at 60MPH, you will actually be travelling over 1MPH faster that what your speedo will read. Your mileage will be less that what you actually travelled.

I think I'm more concerned with getting accurate odometer readings. I just did a search and surprisingly there aren't a lot of threads on odometer readings and upgrading tire sizes. You'd think in this day and age the odometer reading would not rely on the speedometer readings only.

 

[jludave]

I think I'm more concerned with getting accurate odometer readings. I just did a search and surprisingly there aren't a lot of threads on odometer readings and upgrading tire sizes. You'd think in this day and age the odometer reading would not rely on the speedometer readings only.

Not sure what you aren't getting here. Speedometer and odometer are directly related. With a larger tire size, your odometer reading will be off. You will be travelling at faster speeds than what the vehicle is actually reading, so the odometer will be behind/slower. As per above, it all has to do with tire revolutions.

 

[ejewels]

Not sure what you aren't getting here. Speedometer and odometer are directly related. With a larger tire size, your odometer reading will be off. You will be travelling at faster speeds than what the vehicle is actually reading, so the odometer will be behind/slower. As per above, it all has to do with tire revolutions.

I get that, I was just saying you'd think they would have technology nowadays to get odo readings another way since so many change their tire sizes. And tinkering with an odometer is a criminal offense. And also, you never hear about people getting a used jeep with say, 60k on it and then ask/wonder just how off their true mileage if the jeep had been modified with larger tires previously. Seems to me that could be a concern, especially if buying one with high miles on it.

 

[jludave]

you'd think they would have technology nowadays to get odo readings another way since so many change their tire sizes.

How else would it be done? Odometer (and speedometer) readings are generally taken from a sensor in the transmission that reads the revolutions per minute the trans is spinning. This directly relates to tire size. as already discussed.

 

[ejewels]

How else would it be done? Odometer (and speedometer) readings are generally taken from a sensor in the transmission that reads the revolutions per minute the trans is spinning. This directly relates to tire size. as already discussed.

No idea, maybe GPS tracking or something.

 

[1Evil55]

How else would it be done? Odometer (and speedometer) readings are generally taken from a sensor in the transmission that reads the revolutions per minute the trans is spinning. This directly relates to tire size. as already discussed.

New invention called GPS

 

[jludave]

No idea, maybe GPS tracking or something.

New invention called GPS

Perhaps, but highly unlikely to be used as a primary means for a vehicle's odometer/speedometer readings. Cost to implement in most vehicles is too high. Also, GPS is not always 100% reliable. What will the vehicle read if the GPS signal is lost?

 

[Shibadog]

Suggest you CJ your speed on the speedo vs gps. On mine it now matches pretty closely AFTER I went to the Rubi KO2’s. YMMV

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