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Unread 04-21-2013, 12:42 AM   #1
Charley3
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Do I need to adjust my speedometer for 30" tires?

My XJ came with 225/75R15 tires.

Is 30" tire tall enough to need to adjust speedometer?

How far off is my speedo?

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Unread 04-21-2013, 04:58 AM   #2
xjfever
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No it shouldn't be off by more than a couple of mph at 55. My speedometer was actually correct with 29" tires versus the 28" tires it came with.
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Unread 04-21-2013, 07:26 AM   #3
Motorcharge
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Check your speeds with the speedo and GPS and see how far it's off. There's always variance in how accurate your speedo is because there's variance in tire size within the same sizes which is why most jurisdictions give you a 5 mph or so leeway in speeding.
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Unread 04-21-2013, 09:21 AM   #4
habbyguy
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Basically all cars leave the factory reading slightly faster than the actual speed, so larger tires - to a point - correct this. My 31's took me about 10% over the indicated speed. So I just used a GPS and calculated how many percent the actual speed was over the indicated speed, and divided the 30 teeth on my existing gear by that number, and ordered the closest whole number speedo drive gear (had to build it, but that's another story).
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Unread 04-21-2013, 09:41 PM   #5
Charley3
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I figured it'd be close. Maybe even dead on. I'll GPS test.
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Unread 04-22-2013, 02:59 AM   #6
sAe23
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According to this web site, you will be about 6% high, so when you're doing 62MPH the speedometer will read 66MPH.
http://www.1010tires.com/Tools/Tire-...R15/30x9-50R15
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Unread 04-22-2013, 04:19 AM   #7
Charley3
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Quote:
Originally Posted by sAe23 View Post
According to this web site, you will be about 6% high, so when you're doing 62MPH the speedometer will read 66MPH.
http://www.1010tires.com/Tools/Tire-...R15/30x9-50R15
Don't you mean the other way round? When doing 66 it will say 62?

In any case, it won't be more than 2 mph off, IMO. I'll test with GPS.
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Unread 04-22-2013, 09:40 AM   #8
habbyguy
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You can also get a pretty good idea of the calibration by driving by one of those helpful "You are going XX MPH" signs, if you have any in your area. Not really as good as a GPS, but you'll get a good feeling for the error.
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Unread 04-22-2013, 11:13 AM   #9
lostatindo
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Just get a new speedometer gear http://www.jeep4x4center.com/jeep-sp...ears/index.htm
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Unread 04-22-2013, 11:43 AM   #10
habbyguy
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Quote:
Originally Posted by lostatindo View Post
It's not always quite that easy. First, there are variations in tire size between tires of the same "size", and then there are variations in the actual vs. indicated speed (normally always with the speedo reading a little faster than actual). Add to this that the increments you're going to make are in approximately 3% jumps, and there's every opportunity to end up with some speedo error if you just go by the chart. Granted, that chart IS a great resource, and starting point... and what I'd use if I couldn't check my actual vs. indicated speed.

Also, notice that you can't get a 27 tooth, short-shaft gear (which is what's needed for us five-speed, original gearing guys with 3.07 diffs who mount true 31" tires). I had to make one by ordering a long-shaft 27 and melting the gear off the long shaft and then onto the short shaft from my original gear. I did the opposite as well, and now have a long-shaft 30 tooth gear if anyone needs one cheap.
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Unread 04-22-2013, 02:02 PM   #11
Charley3
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That makes my head hurt.

Quote:
Originally Posted by habbyguy View Post
It's not always quite that easy. First, there are variations in tire size between tires of the same "size", and then there are variations in the actual vs. indicated speed (normally always with the speedo reading a little faster than actual). Add to this that the increments you're going to make are in approximately 3% jumps, and there's every opportunity to end up with some speedo error if you just go by the chart. Granted, that chart IS a great resource, and starting point... and what I'd use if I couldn't check my actual vs. indicated speed.

Also, notice that you can't get a 27 tooth, short-shaft gear (which is what's needed for us five-speed, original gearing guys with 3.07 diffs who mount true 31" tires). I had to make one by ordering a long-shaft 27 and melting the gear off the long shaft and then onto the short shaft from my original gear. I did the opposite as well, and now have a long-shaft 30 tooth gear if anyone needs one cheap.
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Unread 04-22-2013, 02:10 PM   #12
tucsonzj
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yea head hurt seriously...according to that website, going from 225 (which i'm really running 235 as my stock size) to 31's when my speedo says 62 i'll be doing 70
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Unread 04-22-2013, 02:36 PM   #13
Charley3
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Quote:
Originally Posted by tucsonzj View Post
yea head hurt seriously...according to that website, going from 225 (which i'm really running 235 as my stock size) to 31's when my speedo says 62 i'll be doing 70
The speed is off with stock tires. Your first size increase actually fixes that so it's accurate. Second size increase will make it off a little, like 2 to 3 mph. So no worries. Drive with speedo reading speed limit and no problems.

It's when you go up 3 sizes that it gets off enough to consider fixing.

So with 235 tires speedo would be accurate. With 30 tires speedo would be 2 to 3 mph off. With 31 tires speedo is off enough to give some thought.

With 31 on your Jeep, when your speedo says 62 mph, you'll really be going around 66 mph.

A more useful way to say that is with 31" tires drive 70 mph on speedo and you're actually going approx 75 or 76 mph. So you won't get any tickets with 31" tires if your speedo says you are going 2 mph under speed limit. So no worries. Just drive with speedo saying 68 mph when speed limit is 70 mph and you'll be fine. IMO

Defineately nothing to worry about with 30" tires. Just drive with speedo saying speed limit and you're fine. I'm not going to change speedo gears for 30" tires.
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Unread 04-22-2013, 02:43 PM   #14
Charley3
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On an LJ I used to have, it was dead on accurate with 32" tires. With 33 tires it was only about 3 mph off. It was a Rubi that came with came with 31" tires stock.

The dealership told me a tire size increase of 1 size results in an accurate speedometer on a Jeep. I think that is also true of other brands of vehicle..

That is because stock Jeeps read a few mph faster than really driving. It takes 1 tire size increase to correct that.

So 235 on an XJ would give an accurate speedometer, 30 tires would be 2 to 3 mph off, but no problem if you drive with speedo saying speed limit. With 31 tires would be several mph off, like maybe 4 to 6 mph off, but that causes no problems if you drive with speedo saying 2 mph below legal speed limit.
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Unread 04-22-2013, 02:50 PM   #15
habbyguy
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Be VERY careful. You can't tell a cop an anonymous person on a Jeep forum told you you weren't really speeding....

When I put 31's on my XJ, I was going 10% faster than the speedo indicated (so when the speedo said 60, I was actually going 66mph). My wife got a ticket doing about 50 in a 45 while "driving the speed limit" according to the speedo. Changing to a 27 tooth from my 30 tooth gear exactly corrected the error, which (if you can divide 30 by 27) is that 10% I was seeing.

But this really isn't rocket science. Borrow a GPS, or drive by one of those speed-registering signs, and you'll know what's REALLY going on. Or check your odometer against interstate mileage signs (I was registering 9 miles when I was driving 10, for example).

Or just hope you don't get a ticket, I suppose...
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