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Old 07-24-2010, 05:47 PM   #1
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speedo vs tac

I dont know if this was ever covered...searched the forums and didnt see anything.

Here goes:
My speedo is off by 5mph due to smaller tires I put on my jeep for hwy travel. I was checking my tac to see where my RPMs should be with 4.10 gears in my diffs. My question before I go any further in this matter is this:
Does the speedo in my jeep dash interfere with my tac giving me my true RPM reading?

Like I say my speedo registers 5 MPH over what my actual speed is. (example - if my speedo says im doing 60, im really doing 55)

any help on this matter will keep my from cracking open the diffs tomorrow and counting teeth...

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Old 07-24-2010, 06:56 PM   #2
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There are some tire/gear charts on here that tell you the rpm at diff. speeds. What size tires did you have and what size tires do you have now? Also speedometers are not exact the can be off a few mph from the factory.

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Old 07-24-2010, 07:35 PM   #3
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Quote:
Originally Posted by TexasSam View Post
There are some tire/gear charts on here that tell you the rpm at diff. speeds. What size tires did you have and what size tires do you have now? Also speedometers are not exact the can be off a few mph from the factory.
I use a rpm calulator. All i need to know is if my stock speedo dictates what my tac reads. If my speeds says im going 60 but im really doing 55, will my tac be reading off the speedo(60) or my actual speed(55).
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Old 07-24-2010, 07:35 PM   #4
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Check your MPH with a GPS
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Old 07-24-2010, 07:47 PM   #5
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No your tac will not be off because your speedo is. Rpm will be diff. at 60 because of shorter tire and you will turn more rpm than before at a true 60mph.

Sorry if I am misunderstanding your question.
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Old 07-24-2010, 07:51 PM   #6
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Quote:
Originally Posted by 03 RUBI View Post
Check your MPH with a GPS
thats how i know my speedo is off :P
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Old 07-24-2010, 07:52 PM   #7
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Quote:
Originally Posted by TexasSam View Post
No your tac will not be off because your speedo is. Rpm will be diff. at 60 because of shorter tire and you will turn more rpm than before at a true 60mph.

Sorry if I am misunderstanding your question.
thanks! my gears are 4.10 according to my rpm calulator
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Old 07-24-2010, 08:30 PM   #8
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All TJ/LJ rubicons come stock with 4.10 gears.
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Old 07-25-2010, 05:34 AM   #9
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RPM is you engine speed.
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Old 07-25-2010, 07:39 AM   #10
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Why would you put on smaller tires for highway travel? Ratio's make my head spin but don't smaller tires mean higher revs, and therefore worse milage?
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Old 07-25-2010, 07:56 AM   #11
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Quote:
Originally Posted by cabin_guy View Post
Why would you put on smaller tires for highway travel? Ratio's make my head spin but don't smaller tires mean higher revs, and therefore worse milage?
I am getting 260 miles out of 1 tank of gas. I had 33 inch M/T on it and was only getting about 200. I put 31 A/T on it and the ride and milage is alot better.

and yes rubicons came with 4.10 gears but I bought mine used. The diffs dont look like they have been opened but I was just making sure noone re-gear it before I got my hands on it

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