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Unread 04-27-2013, 08:12 AM   #46
habbyguy
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Interesting that you're running SMALLER tires and your speedo seems to be spot-on. The 245 profile tire should be about 29.5" tall (the same as the BFG A/T 30" tire), while the 235 tires you're running now (if they're "true size") should be about 28.9".

I guess the lesson here is that Jeep wasn't consistent about speedo alignment, or possibly that they used a really, really small 245 tire, and you're running a really, really big 235 tire now. It might be interesting to measure the actual height of your 235 tires, just for a data point.

But at any rate, this is why I recommend checking with a GPS instead of relying on a formula or "typical results" others have had. If you'd installed tires bigger than stock, your speedo would be reading over 6% slow (with 31" tires) or 9% slow (with 32" tires), assuming those tires were "true size". That's more than enough to get you popped for speeding, of course.

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Unread 04-27-2013, 09:09 AM   #47
tucsonzj
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That was my thought, later I'm going to measure since I did put on some a/t which we're relatively aggressive tread. Will be interesting to see what my speedo reads when I get my 31s on.
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Unread 04-30-2013, 01:17 AM   #48
Charley3
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Quote:
Originally Posted by tucsonzj View Post
hmmm i just reviewed my build sheet apparently it actually came on 245's and has 3.73 gearing
You lucky dog. I wish my XJ had 3.73 gears. Those are ideal for 30" tires, IMO, and I love the 30s!
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Unread 04-30-2013, 01:24 AM   #49
Charley3
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Speaking of tire height variations...

Air pressure affects tire height. I have a bad back. So I run my 30s on lower psi.

The tire pressure formula + 10% is what I run. The +10% was recommended by an online tire store and it works good. Chalk test and test drives confirm it is correct pressure.

27.5 psi front, 25 psi rear In my 30x9.5R15 tires

If a person with 235/75R15 tires runs 30 or 35 psi in the rear, then their tire radius between axle and ground might be similar to mine with my lesser inflated 30s. I'll have more traction of course, and they'll have more gas mileage, but our speedometer readings will be similar. At least that's my theory. It explains how people with 235 might be getting similar speedo results as I am getting with 30, even though we each had 225 stock.

Plus height differences between brands of tires...

What do you think of my theory?
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Unread 04-30-2013, 06:11 AM   #50
tucsonzj
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Quote:
Originally Posted by Charley3

You lucky dog. I wish my XJ had 3.73 gears. Those are ideal for 30" tires, IMO, and I love the 30s!
All in all I'm pretty happy with how my zj came set up from factory. The tow package sets it up pretty nice in the 5.2. I wish I had a d44 in the rear though.

It'll be fun to see what the 31's (hopefully going on next week) will do to my speedo.
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Unread 04-30-2013, 09:21 AM   #51
habbyguy
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Quote:
Originally Posted by Charley3 View Post
If a person with 235/75R15 tires runs 30 or 35 psi in the rear, then their tire radius between axle and ground might be similar to mine with my lesser inflated 30s. I'll have more traction of course, and they'll have more gas mileage, but our speedometer readings will be similar. At least that's my theory. It explains how people with 235 might be getting similar speedo results as I am getting with 30, even though we each had 225 stock.

Plus height differences between brands of tires...

What do you think of my theory?
While tire pressure does affect the height of the tire (by adding or subtracting the sidewall flex-produced change in height of the tire), it doesn't affect the circumference. Think about it - the length of the tread around the circumference of your tire doesn't change when you add or subtract air, so to get a different speedo reading, the tire would have to be slipping by the amount of the difference. In the example you gave (up to 7% difference), that would mean that your tires are slipping on the road surface 7 miles for every 100 you drive. I doubt you'd make it 100 miles if that was the case...

But yes, the differences in actual height (or more accurately, circumference) in different tire brands (or probably even models) is really hard to guess. It's all over the place - I've read about some of the larger (35") tires being as much as 2+ inches off. The same thing happens in the bicycle industry - tire manufacturers will market a 21mm wide tire as a 23mm tire so they can claim their "23mm tire is lighter than the competition". Of course, all it really means is that their 21mm tire is lighter than the competitor's 23mm tire, and the consumer is left in the lurch, making buying decisions on a bunch of faulty information.

I'm not entirely sure what to make of tucsonzj's results, since it would appear his Jeep left the factory with a speedo that read considerably low (dangerous in AZ, since there are SO many speed cameras - ask me how I know....). Maybe when the factory ran out of a particular speedo gear, they just grabbed the closest one they had at hand and stuck it in. But at any rate, that's why I suggest doing an actual vs. indicated speed check before investing in new speedo gears - they're just too expensive to make any assumptions, IMHO. And there are SO many variables that it's impossible to just rely on the gear charts (though they ARE a great starting point).
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Unread 04-30-2013, 10:55 AM   #52
tucsonzj
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With modern technology its not hard to know instead of guess. I couldn't say why mine is so strange or possibly had a replacement done at some point...who knows.

As for speed cameras easy, just duck but I am no longer in arizona so now I just have to dodge normal radar.

Can't say I advise it, but I've dodged my fair share of camera tickets. Unconstitutional and take all reality that a cop can see in a situation
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Unread 04-30-2013, 12:20 PM   #53
habbyguy
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Ouch - don't get me started on photo radar tickets. I've tried to dodge them as well (if you aren't served a physical summons within a certain amount of time, they essentially disappear). Dang servers are clever though - last time, one came to my front door on Halloween night (sneaky, sneaky, sneaky). I'm really OK with getting nailed if I'm doing something awful, but there's a photo radar in Star Valley (a crummy little dirtbag town in the woods east of Payson) that installed a photo radar site about 20 feet from an obscure 40mph (from 60mph) sign on a steep down hill. You have to be on the brakes almost before you can see the sign to prevent getting popped. Over half the "town's" finances come from that speed trap. As a result, I won't spend any money for any reason anywhere on AZ 260 (Payson, Heber, Overgaard). I'll gas up before I get there, and will wait to eat until I'm somewhere else.
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Unread 04-30-2013, 02:15 PM   #54
tucsonzj
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When I was in Tucson we had a few speed spots and several light cameras. Mayor and everyone else would come on and say "accidents have been reduced at these intersections." Yea, because everyone goes around them now, notice how accidents at other intersections have increased? Not to mention overall accidents decreased, but accidents falling into the "rear ending" category increased at these intersections. It's all about the money. But all of this is for a different thread.
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Unread 05-01-2013, 04:37 PM   #55
Charley3
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Quote:
Originally Posted by habbyguy View Post
While tire pressure does affect the height of the tire (by adding or subtracting the sidewall flex-produced change in height of the tire), it doesn't affect the circumference.
It affects the radius of the tire between the axle and ground. That is what matters to gearing, and therefore to speedo too. IMO
Few people (if anyone) believed me earlier in this thread when I thought my speedo was probably accurate with 30" tires. My GPS proved me correct. My speedo is dead on accurate with 30" tires. My instincts and understanding of physics are usually good.
I could test if tire inflation matters to speedo by testing speedo again with substantially higher tire air pressure, but I don't have time this week. Perhaps someone else could test their speedo with a GPS at low tire inflation psi, and then again at high psi.
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Unread 05-01-2013, 04:56 PM   #56
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I was part of a petition drive that indirectly led to abolishment of red light cameras in my town.

The city politicians sued the petition organizers personally to stop petition gathering. That was a blatant harassment suit and an attempt to subvert democracy, which failed to win in court for city, but did inflict financial harm on petition organizers. We had to pass the hat because the judge didn't make them pay our legal fees.

Then city sued again saying the public had no right to have a petition regarding traffic enforcement and our petition (which had enough signatures) was declared legally irrelevant by a judge (who gets part of his income from the ticket money because the tickets help pay city employees' wages and PAY RAISES that year). The city and judge were bilking the public for city employees' pay raises during a recession! Is anyone surprised?

However, the next election soon followed and then those same city politicians were whining that they were being driven out of office over the red light cameras. i.e. - held accountable for their disgusting behavior and subversion of democracy. One was voted out and replaced by a petition organizer from our group. The rest of those politicians have been claiming ever since that they were really against the cameras all along.
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Unread 05-01-2013, 06:06 PM   #57
habbyguy
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Originally Posted by Charley3 View Post
It affects the radius of the tire between the axle and ground. That is what matters to gearing, and therefore to speedo too. IMO
Sorry, but no. The actual circumference of the tire doesn't change materially with underinflation. That is, the length of the tread is still the length of the tread, and one rotation of that tire will produce essentially the same distance, whether underinflated or overinflated. There IS some slight deviation in this concept due to the "stretching" of the material under inflation, but it's a tiny amount compared to the change in radius caused by an underinflated tire.

Think about it - if you remove enough air from your tire to allow the lower portion of the tire to "sag" by 3" (really not a huge amount on a 30" tire), you're saying that the vehicle would only be moving 90% as far as previously, per revolution of the tire. Since all the tread length is still there, even though it's flattened on the bottom, you'd have to have a 10% slippage to claim that you're going to go 10% slower. That is, you'd essentially be doing a continuous burnout when driving. Your tires would last only a couple hundred miles before being entirely trashed under these conditions.

You can test this theory if you like, it's as simple as using a cloth tape (or even a string) to measure the circumference of your tires. Then let out a whole lotta air, and you're going to find that the circumference is pretty much the same (minus that very minor "stretching component"). So then, your challenge would be to explain how the distance the tire rolls changes dramatically when the circumference doesn't. Every inch of that tread still touches the ground every revolution, after all.
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Unread 05-04-2013, 08:39 AM   #58
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Here's my results. Didn't use a GPS, used the known good, calibrated chassis dyno for the speed measurement. Tires are Falken Wildpeak ATs, 30 x 950 x 15 on stock steel wheels. First number is truck speedo and second is dyno roll speed... 10-10, 20-21, 30-32, 40-45, 50-55, 60-65,66...dyno readout kept bouncing between 65 and 66 on this one, and last, 70-76. I got a 31t gear to put in and try. I'll post the results. It'll be a week or so. The tires measure 14.625 static load radius for an overall loaded height of 29.25 inches and revs per mile of 639.357.
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Unread 05-05-2013, 05:53 PM   #59
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Well, put the 31t gear in. Borrowed my sons g/f's phone gps. I am now 2 1/2 mph fast on the speedo v.s the gps. Guess the info Woods supplies with his kit is incorrect. Think a 33t gear is in order. Bummer, 30 bucks spent for naught. Maybe the guy will trade A 33t for 2 gears.
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Unread 05-09-2013, 11:29 PM   #60
tucsonzj
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Originally Posted by habbyguy View Post
I thought that formula was interesting, but it too relies on some assumptions I don't like to (like that the speedo was spot-on to begin with). My "formula" uses actual speed, obtained with a GPS or a drive-by "You are going XX MPH" sign.

For the sake of this example, let's say the vehicle is a 5-speed, 3.07 geared XJ with 31" tires (mine, that is...).

Here's how you do it...
1) Get your Jeep up to 60mph (any speed will work, but that's kind of in the middle of the "speeds that matter", and at least my speedo is pretty much spot-on at every speed if it's right at 60mph)
2) Check your ACTUAL speed with a GPS or drive-by sign. Let's say it says 66mph.
3) Divide your ACTUAL speed (66) by INDICATED speed (60). This time, it comes out 1.10
4) Count the teeth on your current speedo gear. In this case, it was 30
5) Divide your teeth count by the result of step 3. In this case, that ends up being 30 divided by 1.10, or 27.27
That's the number of teeth you need. Obviously, you can't get a gear with 27.27 teeth, or even 27.3 teeth, so you settle for 27 teeth (which will be within 1% of accurate, or 1mph at 100mph).

School zones and speed traps suddenly hold no unnatural fear, and your orienteering will be much improved (when following directions like "drive 5.2 miles and turn left").
ok so jeeps coming back tomorrow with 31's on it. quoting this so I can find it easily so i can do it and see what teeth count I need to switch to.
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