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#19 ƃop ǝnןq ǝןʇ&#
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in English and so I understand

I was thinking of getting a taller tire for the rear of my 94 Dakota....will changing my tire size screw up my speedometer reading?

I think yes, but have heard no, it wont. I want a wider tire and maybe a bit larger as well

There is no speedo cable on my 94, it's electronic driven

specs: 94 Dakota, V6, 5spd, 3.21 rear gear, 215-75-15's on truck now

Bryan
 

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A larger tire has a bigger circumference so it will travel farther for every revolution that a smaller tire would. Say your small tire has a circumference of 100" so for every 500 rpm it will travel (500rpm x 100") or 50000 inches per minute. The larger tire is 120" in circumference so at 500 rpm it will travel (500rpm x 120") or 60000 inches per minute so your vehicle will travel (60000ipm -50000ipm)=10000ipm farther in the same amount of time. Your speedo still reads the same but it will travel faster due to the larger circumference.
BTW Dakota Digital makes a box that can be spliced in line with the electric speedo to give an accurate readout for both speedo and odo.
http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/prd126.htm
 

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Home School Valedictorian
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Yes, a taller or shorter tire will affect the speedometer reading.
 

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Home School Valedictorian
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or an easier way to do this would be to find a wider tire that is the same height as what I have now......

Yes??

Bryan
it depends on what you are wanting to accomplish, maintaining current speedometer calibration...then yes.
 

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Tire Hater
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Just a poor shlep......
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I did a whole spreadsheet on this when I was trying to figure out what tires I could run on my 95. I went from the stock 215/75r15 to a 225/60r15, a tire 2" shorter, which caused me to change to a white 21T speedo gear with my 3.55's.
 

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No Boost? Why Drive?
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It wont, I switched from the factory 215/70s to 275/60s which are ~2 inches taller. And my speedo hasn't changed, it matches the speed of my SatNav and the GPS programs on my phone.
That is physically impossible.
 

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Call me a liar if you want, but I've actually done it. You guys are just relying on physics and other kind of maths.
 

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Lol it will change with tire diameter. I believe the dealer or anyone with a nice scan tool (ie snap on modus) can usually change the programming so it'll read correctly. But without correction a different diameter tire will give a different speedometer readout. Tested personally with common sense, physics, math, and in real life with my shorter drag radials lol.
 

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^^ I thought that only applied to OBD2 vehicles. On the earlier ones don't you just swap the speed sensor for one with more or less teeth depending on whether you're going bigger or smaller?
 

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"][" {[]} {[]} ][_ 
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Its changing gear ratio that will not affect speedo. Tires will definitely change it.

At least on gen 3
 

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No I'm not talking about the tone ring sensors on OBD2.. The earlier trucks had a "pulse generator" sender that was gear driven by the output shaft. When you change tire sizes, you corrected the "pulses per mile" by changing to a larger or smaller gear on the sender.
 

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You very well could be right about the obd2 thing. I thought he said 96 for some reason.
 

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Just a poor shlep......
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or an easier way to do this would be to find a wider tire that is the same height as what I have now......

Yes??

Bryan
Yes, you can "Plus Zero" yourself, but finding tires in that size that works the math right might not exist.

You stated 215/75r15 as OE or starting size, so using plus zero "logic", your sizes would be 225/70, 235/65, 245/60, 255/55 and on... But soon you run out of rim width, which the factory didn't give any. 225/235 are the safe limit for a stock 6" wide rim.
 

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Home School Valedictorian
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Call me a liar if you want, but I've actually done it. You guys are just relying on physics and other kind of maths.
Damn...I hate having to rely on physics or math, you cant count on that stuff.

I personally have run both of my sets of different sized tires past the same police speed warning rig, I get two different readings on their machine for the same speedo reading.
 

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Damn...I hate having to rely on physics or math, you cant count on that stuff.

I personally have run both of my sets of different sized tires past the same police speed warning rig, I get two different readings on their machine for the same speedo reading.
Next time, try for high score.:stirthepo
 
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