Tire size change

[dominator]

Hi there,

I have a bone stock 04 lib. rene. 4x4 with 235/70r16. Can i replace them with 235/75r16? they look a little bigger. I can get them cheap off a buddy.

Thanks

 

[HoosierJeeper]

Sure can. You can go up a 245/70.

 

[kb0nly]

If you got from a 70 to a 75 then your going to have to get your speedometer calibration reprogrammed.

 

[RenegadeJay]

Will you have to recal the spedo with 245/70's? Also, I thought I read somewhere they'd rub. I can't remember where I saw it.

 

[HoosierJeeper]

Will you have to recal the spedo with 245/70's? Also, I thought I read somewhere they'd rub. I can't remember where I saw it.

With 245/70's, it depends on the condition of your stock suspension...

 

[kb0nly]

Will you have to recal the spedo with 245/70's? Also, I thought I read somewhere they'd rub. I can't remember where I saw it.

Not if you stay with a 70 for the aspect ratio.

The Aspect Ratio of a tire is the sidewall height, so by going from a 70 to a 75 you add height to the tire, which makes the tires diameter larger. Any time you change from the stock diameter then your speedometer is going to be off.

Changing the width of the tire, from a 215 to a 245 for example, doesn't affect the tires diameter and so it doesn't affect the speedometer. However it does effect drag, so a wider tire can cause more drag and reduce your gas mileage as well.

And as for rubbing, as already mentioned that depends on your suspension, and in the case of the KJ the pinch weld in the rear of the front wheel wells also.

 

[J-Thompson]

Not if you stay with a 70 for the aspect ratio.

The Aspect Ratio of a tire is the sidewall height, so by going from a 70 to a 75 you add height to the tire, which makes the tires diameter larger. Any time you change from the stock diameter then your speedometer is going to be off.

Changing the width of the tire, from a 215 to a 245 for example, doesn't affect the tires diameter and so it doesn't affect the speedometer. However it does effect drag, so a wider tire can cause more drag and reduce your gas mileage as well.

And as for rubbing, as already mentioned that depends on your suspension, and in the case of the KJ the pinch weld in the rear of the front wheel wells also.

you are right and wrong at the same time
the aspect ratio is just that a ratio not a measurement
read this to better under stand


Section Width

Following the letter(s) that identify the type of vehicle and/or type of service for which the tire was designed, the three-digit numeric portion identifies the tire's "Section Width" (cross section) in millimeters.

P225/50R16 91S

The 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire's section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch).

225mm / 25.4 = 8.86"

Sidewall Aspect Ratio

Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio.

P225/50R16 91S

The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the lower the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 / 25.4 = 8.86") and multiplying it by 50% (.50) we confirm that this tire size results in a tire section height of 4.43". If this tire were a P225/70R16 size, our calculation would confirm that the size would result in a section height of 6.20", approximately a 1.8-inch taller sidewall.

 

[J-Thompson]

by the way
a 255/70 is with in a few mm size wise to a 245/75

 

[RenegadeJay]

If going from a 70 series to a 75 series would throw off the spedo, then how is it that some KJs come with the 225/75's (such as mine Rene) and some come with the 235/70's like Doninators? I thought deciding on the right tire was hard, now the right size is making it harder. I wanted to go a bit bigger (without adding a lift)to add a beefier look than the wimpy looking 225's.

 

[offroadohio]

I am running 245-75-16's and my speedo is dead on. I used to go slower than it said until I put the bigger tires on. I am running a 3"lift and my tires still rub.

 

[kb0nly]

The stock sizes on the KJ were either 215/75R16 or 235/70R16 according to the label in the glovebox and the owners manual in my 03'.

If you use the online tire calculators you can see that going from 70 to 75 for the aspect ratio does make a difference in speedo error. When you say its right on how did you check it? Radar gun? Dyno?

http://www.csgnetwork.com/speedocalibcalc.html

Go to this website...

For example, if you enter 235/70R16 for the stock size and 235/75R16 for the new size you will get the following info...

Stock tire diameter is 28.95 inches.
New tire diameter is 29.87 inches.
When the vehicle speedometer indicates 55 MPH.
The vehicle speed is actually 56.75 MPH.

Yeah i know only 1.75mph difference. But the argument is it will be dead on, no it won't. If you research it a bit, i did for a paper one time as a subject i picked after a friend of mine got stopped for speeding despite having his cruise set to 55, manufacturer's have a tolerance of +/- 3mph on their speedometer calibration. Most LEO's that i know say they won't pull someone over unless they are doing at least 5 over the speed limit, this takes the calibration error into account, and here 10 over the speed limit is a court appearance now.

So anyway... Take the above calculations into effect and lets say you have the cruise set at 55 and your +/- 3mph and then add nearly another 2mph you could be out as much as 5mph and not even know it.

Ok, another calculation... If my 03' KJ had 215/70R16 on it, i was considering 235/75R16 because a local tire shop was having a sale on that size. Using that website's calculator this would introduce a larger error.

Stock tire diameter is 27.85 inches.
New tire diameter is 29.87 inches.
When the vehicle speedometer indicates 55 MPH
The vehicle speed is actually 59 MPH.

That's a 4mph error for just making a tire size change, this doesn't even include how accurate the speedometer was in the first place.

So yes it does make a difference, big?? Not always, but it does make a difference.

Mine came with 215/75R16 stock, and i went to 225/75R16 with the new tires. Even that slight change introduced an additional error of 1.13mph. But i couldn't find tires i liked in 215's, and i didn't want to go super wide, so i went with the 225.

 

[kb0nly]

If going from a 70 series to a 75 series would throw off the spedo, then how is it that some KJs come with the 225/75's (such as mine Rene) and some come with the 235/70's like Doninators? I thought deciding on the right tire was hard, now the right size is making it harder. I wanted to go a bit bigger (without adding a lift)to add a beefier look than the wimpy looking 225's.

Because they program for which tire size. My manual says right in it that i should replace with the same size tires, a different size would affect speedometer calibration and to take it to the dealer for programming.

 

[J-Thompson]

easy speedo fix
-install new tires
-use GPS to figure out the error ,% wise
-drive that much slower

oh by the way
we were/are about 7% off as per GPS
so if the limit is 65 we do 60 and after 3 years not 1 ticket
and no hassling with the dealer

 

[HoosierJeeper]

I'd get my speedo re programed, as I don't like to take my GPS everywhere....JMO.....

 

[kb0nly]

I talked to a dealer about this when i was tire shopping. In the glove box it said your vehicle may be equipped with 215/75R16 or 235/70R16, so i did some snooping. The Mills Auto Group Jeep dealership here in MN told me the speedo calibration takes only a few minutes by plugging into the OBD port on the KJ. He said they would have to charge me a minimum service fee for it however.

I could get mine reprogrammed for the difference from the 215/75R16 to 225/75R16, he said they just connect the computer and select the tire size and it knows what to program into the vehicles computer to compensate, but since i have an error of only roughly 1mph i'm not too worried about it!

However if i would have made a bigger change i think i would have paid the $50 or so to have them reprogram, but only because i can be a bit of a perfectionist.

 

[J-Thompson]

I'd get my speedo re programed, as I don't like to take my GPS everywhere....JMO.....

you only need it once
then you know the error and can adjust your speed

and like I told my wife
"I dont care if some one other than you or me is driving the Jeep and gets a speeding ticket ,maybe they should have drove their car?"

 

[RenegadeJay]

So I am to believe that when a KJ comes down the assembly line, they put the tires on it and then the corresponding computer? In response to KbOnly, the 2006 KJ sales literature I was reading says the 2 sizes are the 225's or the 235's. Must have changed since your 03. Maybe that is why they print it inside the door when they determine which tire/computer program to install in the vehicle. Seeing as how my KJ is mostly a street machine, I will go with the stock 225/75's and forever look at tires that are smaller than some import sedans.

 

[J-Thompson]

Seeing as how my KJ is mostly a street machine, I will go with the stock 225/75's and forever look at tires that are smaller than some import sedans.

why do you say this?
you do know that a 225/75 R16 tire is 9" wide X 29.3" high
compared to a 30X9.5 at 9.4" wide X 29.5" high
now look at those numbers
.4" less width really will this matter? besides some say a skinny tire is better than a fat one
.2" lower?

I got these number from the BFG site off of the AT's

 

[tjkj2002]

Having your speedo calibrated for larger tires is not only to correct the speedo reading,it greatly effects how the auto trans shifts.If the speedo is programmed for smaller tires(even if speedo is off by 5% which is normal) and you have larger tires,by at leat 1" or more in total diameter then stock, it will confuse the trans and make it shift wrong which makes you mad and give it more gas to kick down which makes it shift more which creates more heat which more heat causes early trans failure.

 

[kb0nly]

So I am to believe that when a KJ comes down the assembly line, they put the tires on it and then the corresponding computer? In response to KbOnly, the 2006 KJ sales literature I was reading says the 2 sizes are the 225's or the 235's. Must have changed since your 03. Maybe that is why they print it inside the door when they determine which tire/computer program to install in the vehicle. Seeing as how my KJ is mostly a street machine, I will go with the stock 225/75's and forever look at tires that are smaller than some import sedans.

Yep, depending on the package ordered or the build sheet as one comes down the line they just program the computer with the size needed.

225/75R16 looks nice on the KJ, and doesn't look small in my opinion, but then i'm biased as that's what i am running. A wider tire ***** in the winter to an extent, you bog down in snow instead of cutting through it, but it can also help with stability and traction being wider, so its a tradeoff.

Here is mine, i think they fill the wells nice.