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Old 05-05-2018, 08:55 PM
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over size tires and odometer reading

When driving 65mph and showing on the speedo and GPS shows approx. 71mph. At a loosely estimated 10% difference from stock tires to 30" tires, so if I drive 10,000 miles showing on odometer but actually have driven 11,000 miles. Does this sound correct? I see Quadratec has a neat conversion chart and a speedo gear to make the correction. Recently when checking my last fillup was 11.2mpg, so 10% difference would be closer to 12.3mpg.
My math ain't all that good.

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Old 05-05-2018, 09:00 PM   #2
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Your math seems good to me.

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Old 05-05-2018, 09:33 PM   #3
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Yep^
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Old 05-09-2018, 08:39 AM   #4
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It's been a minute since I looked into this so correct me if I'm wrong. The easiest method is to divide your current tire size by the stock tire size (if using original speedo gear) then multiply the result with your mileage. For example, 30/27= 1.111. So if you drove 50 miles then you really went 55.555 miles. To be more accurate measure your tire (on the jeep on the ground). Probably little less than 30".

You should get the same value by dividing the GPS speed by your speedometer speed.
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Old 05-09-2018, 10:40 AM   #5
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I have 31" BFG KOs, a stock speedo gear and 3:07 axle gears.

From time to time the cops will put out a radar activated speed limit sign in various locations showing the posted speed limit and 'your speed'. They'll usually put these signs out to slow folks down in areas where drivers tend to exceed the limit.
When I pass any of these signs, my speedo very closely matches the speed that the sign says I'm going.

It's long been my opinion that vehicle manufactures purposely make speedometers read faster than the vehicle is actually going. With my 31" tires and a stock speedo gear I'm more convinced than ever that that's the case.

I keep the "locations" on my phone turned off because I don't think anyone needs to track my comings and goings. Thus, a phone based GPS speedo won't work for me. I certainly wouldn't buy a vehicle based GPS so that anyone could track the comings and goings of my Jeep.

As far as I'm concerned, Big Brother can go give himself a swirlee.

Good Luck, L.M.
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Old 05-10-2018, 07:38 AM   #6
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Luckymac, why don't you like your brother? Haha. Yeah, I think not use GPS to verify on occasion. I've heard about the speedos being faster by the manufacturer before so I'm sure there is some truth in that.
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Old 05-10-2018, 10:50 AM   #7
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The "Big Brother" reference was to a book by George Orwell titled "1984" where the government stuck their nose into every detail of citizens life.
It seems like we're approaching that ability now.
"Big Brother" was the all-seeing, all-knowing entity that directed peoples lives.

The book's a classic, but it might be a generational thing. I don't think folks read as much as they did when I was young.

Good Luck, L.M.
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Old 05-10-2018, 11:48 AM   #8
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Quote:
Originally Posted by Luckymac View Post
...it might be a generational thing...
When "1984" was written back in 1949 readers, me included, believed all the possibilities of the future.
In 1990 we were supposed to be driving flying cars.
My 1990 YJ has a carburetor just like my dad's 1949 Desoto. What a gyp!

Luckymac, we just need to face the fact that one of us is old (and it ain't me).
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Old 05-10-2018, 03:38 PM   #9
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Fwiw gps is one way... a gps receiver cannot report your whereabouts ti big brither without additional technology... (such as a cell phone)
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Old 05-10-2018, 03:39 PM   #10
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Quote:
Originally Posted by JJYJ View Post
It's been a minute since I looked into this so correct me if I'm wrong. The easiest method is to divide your current tire size by the stock tire size (if using original speedo gear) then multiply the result with your mileage. For example, 30/27= 1.111. So if you drove 50 miles then you really went 55.555 miles. To be more accurate measure your tire (on the jeep on the ground). Probably little less than 30".

You should get the same value by dividing the GPS speed by your speedometer speed.
Tire diameter won't work.... circumference will though.
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Currently in the stable: ... 97 tj 4.0 5 speed stock....04 tj 4.0 auto 4" and 33's... 98 tj roller for future build... 94 yj sahara 4.5" RE and 33's... 92 yj sbc 3/4 ton axles and 38's... Cj5,7,yj,xj,ex mud dragster project

TJ dash speakers that actually rock! Kicker sub and amp in center console
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Old 05-10-2018, 04:56 PM   #11
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Isn't circumference relative to diameter. It's been almost 50 years since I took high school algebra so I might have forgotten if pies are square or round.

Good Luck, L.M.
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HEI distributor with computer and all related relays and wiring removed.
12K Badlands winch with dual batteries.
Warn front bumper. Matching imitation Warn bumper on the rear.
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Old 05-10-2018, 08:45 PM   #12
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The formula works. Mine comes out to 1.09. I multiply it by my speed and it matches gps.

Luckymac, I understood the reference about Big Brother. You pretty much have to live off the grid to not be monitored.
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Old 05-10-2018, 08:57 PM   #13
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The formula for circumference is
C=Pi times diameter.
Therefore, when the diameter increases by 10%, the circumference does also,
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Old 05-10-2018, 09:37 PM   #14
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Idk why but I can't edit my previous post. Circumference is Pi times diameter.

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