Speedo change going from 33s to 37s?

Mikeyut3

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So I went to 37s (and I have a Mopar 2" lift) and I know what I am about to say is not perfect, but...

There is a speed reader on a street near me (one of the large rolling temporary trailer ones) and I have driven by it several times now. My rough read is that my speedo is "off" by 2 MPHs...i.e. when I am going 20 it reads 22 and when I am going 25 it appears to read 27. I recognize (1) I am looking at my speedo and then looking up and (2) maybe the reader is wrong....but...

Is this normal, to "only" be off by 2ish MPHs? Because I am thinking to myself for this "small" difference why recalibrate either at the dealer or with a programmer - it seems very minor.





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gijohn56

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at 20 miles and hour you are off by 2mph but as you go faster that gap increases so that at 70 you might be doing 77-79 mph.... buy a cheap garmin gps from a pawn shop and you can see the difference. There is an app called Jscan that you buy for $5 and a obd2 reader off amazon. then you purchase the $20 right to list your VIN and it allows you to change the calibration and a whole lot of other things as well. so for $50 you can buy a great programmer that works and does other things as well.
 
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Mikeyut3

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you purchase the $20 right to list your VIN
you buy this from who?


also...why do people buy tazer or aev for 3-5x this price vs jscan? thx!

also...why not just get a speedo app too? seems there are plenty on app store.
 

gijohn56

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the app goes on your phone so either iphone store or the android store. once you get into the app it gives you the method to pay for the vin
 
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Mikeyut3

Mikeyut3

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the app goes on your phone so either iphone store or the android store. once you get into the app it gives you the method to pay for the vin
Thx!
 

Arterius2

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So I went to 37s (and I have a Mopar 2" lift) and I know what I am about to say is not perfect, but...

There is a speed reader on a street near me (one of the large rolling temporary trailer ones) and I have driven by it several times now. My rough read is that my speedo is "off" by 2 MPHs...i.e. when I am going 20 it reads 22 and when I am going 25 it appears to read 27. I recognize (1) I am looking at my speedo and then looking up and (2) maybe the reader is wrong....but...

Is this normal, to "only" be off by 2ish MPHs? Because I am thinking to myself for this "small" difference why recalibrate either at the dealer or with a programmer - it seems very minor.
At 20mph you are off by 2, so at 70mph you will be off around 7-8.
 

UTES

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If you have an Android phone you can download DigiHUD off the web and put it on your phone. It'll always show how fast you're going, It's pinged off satellites.

Hunter
 

hoag4147

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If you have an Android phone you can download DigiHUD off the web and put it on your phone. It'll always show how fast you're going, It's pinged off satellites.

Hunter
Waze app also shows your speed
 

Htfan

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also...why do people buy tazer or aev for 3-5x this price vs jscan? thx!
Great question! This must be a personal preference. Unless people are unaware of jscan.

However, jscan does not have a ESS bypass so that could be a reason to spend 3x - 5x the $$
 

limeade

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When you change tire sizes, your speedometer will be off by the corresponding percentage. For example, going from 33's to 37's is four inches, but is a 12% difference in tire size.

Now, I don't know of any tire which measures it's true sidewall or advertised size. A lot of 37's true diameters are listed as 36.5" to 36.8". A stock BFG KO2 in 285/70R17 has a diameter listed as 32.8".

Since I don't know the tire you're running, I'll make an assumption. A Nitto Ridge Grappler's diameter is listed as 36.77". The difference between the stock KO2 and Nitto is 3.97", or 12%.

So regardless of speed, you speedometer will be off (slower) by 12%.

If you don't want to buy a programmer, just estimate your speed as 10% higher (used 10% since that's easier to calculate on the fly vs 12%) than what the speedo shows and you should be close enough to your true speed.
 

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