adjusting spedometer with larger tires?

[rtoney9]

1075603

I am putting a lift and tires on my 95 YJ in a few weeks. the tires are 31x10.5x15. my YJ is a 2.5L. I was told that the spedometer would have to be adjusted to indicate the correct speed mph. Is this really necessary, can I do it myself, I have 4.11 gears so will the spedometer change at all?[addsig]

 

[osito129]

1075607

I am not sure about the speedo changing with new gears but after checking mine with our standing radar on post many times it seems to be right on. I have stock gears and am running 31x10.5 Wranglers. Like I said I don't know if the gears make a diff or not.[addsig]

 

[jterp]

1075608

You don't have to change the speedo, but with different tires it will be incorrect.. What size tires were you running before?
Changing the speedo just requires pulling the gear that spins it and replacing it with a different size... Not sure of what the size would be though.[addsig]

 

[rtoney9]

1075612

I am running p225s. they were on the jeep when I bought it. they are about 27.4" tall and 8" wide. I think i would be okay having 4.11 gears but I am not sure? How much will the reading be off 5-10 mph or what? it will read faster than I am acutally going[addsig]

 

[cj5steve]

1075615

your speedo wont be off by a set amount but rather a %.i went from 33's to 36's not a huge change in the speedo but it was a change as long as you are aware of it you just have to pay attention.[addsig]

 

[K_K_PSI_GUY]

1075616

I'm not sure if you do or if you do not have to adjust your speedometer, but if you do, they sell a device on one of the Jeep Catalogues that adjusts your speedometer. It even has two settings for those of you that have a different set of wheels for the winter. Hope this helps. -Al[addsig]

 

[jterp]

1075619

I forgot we were talking about a computer controlled vehicle here, so there may just be a computer controller device to change the speedo-- On old cars there was a gear that controlled the speedo that corresponded to different rear end ratios that could just be pulled and replaced..
If you go from a 27" tire to a 31" tire your speedometer will read slower than your actual speed eg. if you are travelling at 65mph then your speedo will read 58. (approximate numbers) ----I'm 99% sure about this, please correct me if i have it backwards.. [addsig]

 

[Bounty__Hunter]

1075628

No computer adjustment on the '95, need to change the speedo gear in the transfer case rear output housing. Your speedo will read 3 to 4mph slower than your actual speed, it's what I've found on my '95 2.5L 4.11 geared YJ with 31" tires.

Recalibrating is not a necessity if you can live with the error. I always lend my GPS to fellow Jeepers 'round here, it has a speedo on it that is very accurate, good way to determine speedo error.[addsig]

 

[rtoney9]

1075636

thanks. you have been a lot of help. I think I will leave it and just pay close attention to the difference. thanks guys.[addsig]

 

[PASmokeater]

1075675

I just adjust my foot to the speed I should be going...after I put on the lift and went to the 33" tires, the differeance is very slight. I hooked up with a friend with a GPS and found that at 65 mph I was only off by a couple of mph.
If you feel you MUST be back to normal, you can get a new gear from the dealer. The gear goes into an easy to remove section of your t-case....somewhere around here I saw a write up on it...let me know if your interested, and I'll dig it up for you.





Rob [addsig]

 

[ecor]

1075678

To get the adjustment you need to change the gear in the transfer case or you can by the recalibration kit from superlift. The easiest thing to do is take your new tire size and divide it by you old tire size. That number is the percent that your speedo is off. say you want to know what how fast you are going when your speedo say 60. Take Rob for instance. He had a 30 inch tire and went to a 33. The difference is 1.1 now multiply that by 60 it equals 66. He's actual speed is 66. It works the same in the opposite way say you want to know how fast your speedo needs to read to be going 60. Divide the 30 by 33 = .91 now multiply by 60 = 54.6 So his speedo would need to read about 55 to do 60.

Hope this helps

[addsig]

 

[Inspector-Gadget]

1075740

By way of supplementing the great info above, an excellent web article on the topic of speedo correction after big tire installation and/or differential gear ratio change, with enough tables and lists to intoxicate your average spreadsheet freak, will be found here:

http://www.gojeep.rockcrawler.com/How to4.htm

The above mentioned web article includes a step-by-step pictorial of the changing of a transfer case speedo pinion drive gear.

A nit-picking caveat: Web articles that I have read on this topic make reference to tire diameter. Those articles -- to include the article mentioned above -- typically do not make the reader aware that it is "rolling diameter," not "measured diameter" that should be used in the calculations. As a "rule of thumb," rolling diameter is 0.95 of measured diameter. This correction is admittedly hair splitting, but it is mentioned here for the sake of completeness. For more info on tire rolling diameter, check out the following URL:

http://www.partnumber.com/ev/handbook/tires.html

The rolling diameter of a tire will change with tire inflation pressure, tread wear, vehicle loading, yada, yada, yada. An out in the driveway practical method of dealing with the rolling diameter correction is to ignore it. ;-)

Inspector-G
[addsig]