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Unread 04-25-2013, 04:42 AM   #31
Kevin108
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Quote:
Originally Posted by lostatindo View Post
This thread makes me want to stab myself in the eye with a speedo gear.
I have a spare 34-tooth I can send you.

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Unread 04-25-2013, 08:11 PM   #32
Charley3
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Got GPS speedometer and used it to test my XJ speedometer.

When XJ speedometer says 30 mph the GPS says 28.5 mph.

When XJ speedometer says 60 mph the GPS says 60 mph.

When XJ speedometer says 70 mph the GPS says 70 mph.

That is with 30" tires. So 2 tire sizes larger than stock gives accurate speedometer on my XJ.
That is exactly what Jeep dealership told me was the case with LJ. Stock tires on LJ are 30", and the dealer service dept said 32" tires would probably read accurate on LJ speedo because (they explained) the speedo reads faster than reality with stock tires, and in their experience increasing 2 tire sizes reads approximately accurate.
In my 30" XJ tires I was running 28 psi front, 25.5 psi rear, which are the tire pressures I had previously determined by formula and confirmed by chalk tests and driving tests.
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Unread 04-25-2013, 08:44 PM   #33
sAe23
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One time I found a formula and came up with similar PSI settings, but I can't find it anymore.
And the one time I tried a chalk test, either my tires were at the proper PSI or it didn't work. The chalk was gone after two rotations
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Unread 04-25-2013, 10:18 PM   #34
Charley3
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Originally Posted by sAe23 View Post


One time I found a formula and came up with similar PSI settings, but I can't find it anymore.
And the one time I tried a chalk test, either my tires were at the proper PSI or it didn't work. The chalk was gone after two rotations
If you're serious about max formula, I can give it you. Works really good. Though my version is the standard formula +10%. I find that works best.

I actually don't use chalk for "chalk test". I use duct tape. Much more convenient to put duct tape across tread, and the quizzical looks I get from people are hilarious. I once had a woman ask if I used duct tape to patch a leak in my tire. Even funnier was the look my 12 year old nephew gave her. Like she was the stupidest woman on earth. Good times!
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Unread 04-26-2013, 11:43 AM   #35
Charley3
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I encourage everyone with 235 and 30" tires to get a GPS and test the accuracy of your speedometer at 30, 60, and 70 mph and then post your findings in this thread.

I'd be very interested to see others results. Also, it'd be educational and useful info for everyone.

I didn't actually get a GPS. I just downloaded and installed a GPS speedometer program for my smart phone. It was free and worked great. So if you have a dedicated GPS, or a smart phone with a GPS antenna, either will work.
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Unread 04-26-2013, 12:36 PM   #36
habbyguy
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It's an interesting exercise, but it won't result in any kind of absolute results for a couple reasons. First is that the stock tires on XJ's ranged from 195 to 235 profile (that's a difference of 60mm or about 2.5" in diameter), and the original speedo gearing would have been installed accordingly. That's an almost 10% range that your XJ could have been "speedo geared" for.

Second is the variation between tires of the same "size" (which is considerable), and wear (which also can result in the better part of an inch in deviation).

For example, the ubiquitous BFG A/T 30x9.5.15 30" tire is really 29.5" (see http://www.bfgoodrichtires.com/tire-...ails#techspecs)
The same tires has 16/32" (aka 1/2") of tread, so by the time it's bald, the tire diameter is 1" smaller. So a "30 inch" BFG A/T that's nearing the end of its life is actually 1/2" smaller than a new "true size" 225/75/15 metric tire.
Some tire sizes vary even more than the 1/2" of the BFG tires, FWIW.

This is why it's necessary to calibrate your speedo using GPS vs. indicated for your particular vehicle and tire combination.
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Unread 04-26-2013, 04:23 PM   #37
Charley3
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Originally Posted by habbyguy View Post
It's an interesting exercise, but it won't result in any kind of absolute results for a couple reasons. First is that the stock tires on XJ's ranged from 195 to 235 profile (that's a difference of 60mm or about 2.5" in diameter), and the original speedo gearing would have been installed accordingly. That's an almost 10% range that your XJ could have been "speedo geared" for.

Second is the variation between tires of the same "size" (which is considerable), and wear (which also can result in the better part of an inch in deviation).

For example, the ubiquitous BFG A/T 30x9.5.15 30" tire is really 29.5" (see http://www.bfgoodrichtires.com/tire-...ails#techspecs)
The same tires has 16/32" (aka 1/2") of tread, so by the time it's bald, the tire diameter is 1" smaller. So a "30 inch" BFG A/T that's nearing the end of its life is actually 1/2" smaller than a new "true size" 225/75/15 metric tire.
Some tire sizes vary even more than the 1/2" of the BFG tires, FWIW.

This is why it's necessary to calibrate your speedo using GPS vs. indicated for your particular vehicle and tire combination.
My original tires were 225 and my Grabber AT2 are 29.8" tall according to General tire. As they wear they'll get shorter. So then my speedo will read faster than I'm really going. With new AT2 it reads dead on accurate.

I have no disagreement with using a GPS to test. It's easy and free since any GPS or smart phone can do it.

Do you know Chrysler used a different speedo gear for each tire size, or is that an assumption? It sounds resonable, but just wondering.
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Unread 04-26-2013, 04:40 PM   #38
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I assume that most different size tires did get different speedo gears (which would account for the fact that pretty much everything from 30 teeth gears on up are available on a tooth-by-tooth basis). A one-tooth jump in speedo gear would roughly correspond to a 1" tire size change, so I doubt it was likely to nail every tire size with an exact match - I'm sure some were further off than others (sounds like the 225 tires is one case, if a 1.5" plus (5%) jump in actual tire size yielded an accurate speedo. When you think about it, even when you're shooting for perfection, you've got to make changes with a 3% minimum gearing change (one tooth), so some combinations are going to be 1.5% high or low no matter what you do. Add in the tendency of auto makers to build "fast" speedos and a 5% deviation that would be corrected with a tire swap like yours really isn't hard to imagine. OTOH, my tires are only 0.7" taller than yours (they're BFG A/Ts) and I was getting a 10% speedo error, probably because my XJ came with 215 tires from the factory (can't even imagine what that would look like now!!!).
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Unread 04-26-2013, 05:45 PM   #39
Charley3
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Did most XJ come with 225 tires?

Seems likely to me that all XJs that came with stock 225 tires would be accurate with 30" tires. Mine is.

I'm not convinced that every tire size came with a different speedo gear. That'd be unnecessary because my XJ (for example) could use same gear for 225 and 235. The same stock gear (that came with my 225 stock ties) would work for 235 tires too. It also works for 30" tires.

It does seem plausible that the 195 and 215 tired stock XJ woukd have come with a different speedo gear than XJs with 225 and 235 tires. So I agree with you there.
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Unread 04-26-2013, 06:19 PM   #40
tucsonzj
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So did a test on my way home, mine came stock with 225's and i'm currently on 235's...speedo was perfect with the gps.
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Unread 04-26-2013, 06:48 PM   #41
sAe23
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Originally Posted by tucsonzj View Post
So did a test on my way home, mine came stock with 225's and i'm currently on 235's...speedo was perfect with the gps.
Same here. 225's listed on build sheet, but it has always had 235's and the speedometer matches my Garvin Nuvi and the GPS apps on my phone.
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Unread 04-26-2013, 08:03 PM   #42
habbyguy
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Interesting results (not surprising though). A 235 profile tires (all other things being equal) is about 2.1% taller than a 225 profile tire. Changing one tooth in a speedo gear will change the ratio by around 3%, so whether that makes sense or not depends on how close the speedo is on the next-larger size tire.

It appears that the guys who have accurate speedos with 235 tires replacing the original 225 tires had an original 2% error in their speedo (reading fast), which was corrected by the larger 235 tires. However, that means that if the same speedo gear was used on the XJs equipped with 235 tires out of the factory, the speedo would be reading dead on, or (and here's the rub...) a little slow. That seems to be a big no-no in the industry (to build a vehicle that might be going ever so slightly faster than the speedo indicates), so my guess is that the 235-equipped vehicles have one less tooth on their speedo drive gear than the 225-equipped vehicles. That would produce an approximately 3% change in the speedo's output (making it read 3% faster than the actual speed).

Charley3's XJ started out with 225s as well, but seems to have been corrected with a change in tire size of over 5%, which is at odds with the other guys' results. If his XJ had come out of the factory with 235's, it would make perfect sense (since the jump from a 235 to a tire with an actual diameter of 29.8" is almost exactly 3%, which would correspond to that one-tooth difference in speedo drive gears).

I guess the only question at this point is do the three vehicles that all were supposed to have been shipped with 225 tires have the same speedo drive gear?
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Unread 04-26-2013, 08:28 PM   #43
tucsonzj
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tell me how to check and Ill let you know hahaha
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Unread 04-26-2013, 11:09 PM   #44
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The speedo drive gear is held in under a small cover with one bolt - you can't miss it - it's on the top of your tranny and has a wire coming out of it. Just take note of how it's installed (better yet, make a small mark that you can use to realign the cover), unscrew the bolt, and pull the cover (and gear) up and out. Count the teeth, put it back in, align the marks you made (it's not super-critical, BTW) and replace the bolt.

If your ZJ is an auto with a 3.55 gear ratio, I'm betting you'll find a 35 tooth gear (appropriate for a 28" tire and 3.55 gearing). Here's a link with both gear charts and a photo essay on changing (or checking) your speedo gear: http://jeep-xj.info/HowtoSpeedoGears.htm
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Unread 04-27-2013, 07:31 AM   #45
tucsonzj
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hmmm i just reviewed my build sheet apparently it actually came on 245's and has 3.73 gearing
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