So other day I drove past one of those cops 'your speed is' things. And I thought i was doing 50 (I made sure I was doing as close to it as possible) And the thing showed up as 45. Now I normally do 5 over all the time. (I know, I know, you should speed blah blah blah..Go spill some coffee on your shirt or something as equally as day ruining) I just thought people were actually starting to do the speed limit even old people but I guess not

Now my question is, what could be causing this? I just got my Jeep from an auto auction and I have no idea about anything the PO did.
Also my tires are kinda small, theyre like 27s on 16 inch stock rims.

 

Look at the sticker on your drivers door. It will tell you what size tires and rims came with your XJ.

15" was common, but 16" rims did exist.

My guess is that your tire size is not stock.

 

i have never ever been driving a car where the speedo was accurate. it has always been showing between 5 - 10% to much.

i dont know why they make it like that. it should be posible to have it more accurate.

 

I used can use a tape and it reads 16 lip to lip. I'm guessing that the rim size isn't its actual size then?


But yeah I'm guessing its cause there so small, is there a way to tell if its been regeared for bigger tires?

Edit: it's a 16" rim..it says R16 lol

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Look at your tire size it will say r15 or r16

 

The bigger the size of tire you go the slower your speedo' shows. Rim size won't affect it, its the actual size of your tire. I have 235's and at 60 mph on the speedo' I'm actually running at 63-64ish. And the difference increases the faster youre going.

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If you have a 265.70R15 and a 265.70R16 the 16 is one inch bigger take a look at 1010tires.com really good info

 

In the end the actual outside diameter of the tire is what's affecting what the speedo is displaying. The car doesn't know how big the rims are; it knows how many revolutions the tires are spinning vs. the revolutions of the speed gear.

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No kidding! Lol plus size rims can and will affect overall diameter unless you go to a lower profile tire

 

Stock tire size for most XJ Cherokees is 225/75R15. This translates to 28.287" by actual conversion. Each tire manufacturer has slight variances that can be as large as half of an inch (sometimes more) in overall tire diameter. Tire diameter is also affected by the inflation level of the tire and the load on it. All of this can have an effect on your speedo. Right now, I've got 30k miles on my Kelly Safaris, and the speedo registers 65 MPH when I'm doing 64. My snow tires (215/75R15) make my speedo register 5 MPH over my actual speed.

 

if the tires are bigger than stock, the speedo will read slower.
if the tires are smaller than stock, the speedo will read faster.

put the right size tires on, and your speedo will be more accurate.
or get a GPS.

 

I use a speedometer app on my phone.

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So do I, that's funny man. That's how I check my speedo accuracy. I need to do 0-60 sometime and quarter mile...

 

No they won't. Theres no info supporting that on your site 1010. Check the chart out on this site scroll down to the chart where it shows a 235 on a 15 and a 235 on a 16 are both 29" http://clubs.hemmings.com/clubsites/...conversion.htm

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That's because the values are rounded man. Do the math and you'll figure it out to exact values.