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New tires - is a speedo adjustment necessary?

Discussion in 'Chevy Truck Talk & GM News' started by GeorgiaZ71, Nov 20, 2013.

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  1. GeorgiaZ71

    GeorgiaZ71 New Member

    Hi All,

    I tried using the search function, and I didn't find anything. So sorry if this has been asked a thousand times!

    So within a 2 weeks of getting my 2014 (150 miles on her), I had the level kit and bigger Wrangler Duratracs put on.

    My question is, going from a 265/60/18 to a 275/70/18 - is my speedometer / odometer off you think? I guess I could download some GPS app on my phone and test it...but thought I would ask here first.

    If it is - what's the recommended point for getting the old tune and reflashing the ECM? If the speedo is off by just a little - would you still recommend? (Obviously if it's like 7+mph off, I think I would... but what if it's only 2-3mph off?)

    Thanks!
  2. 99'HEARTBEAT

    99'HEARTBEAT Moderator Staff Member Platinum Contributor 1000 Posts 100 Posts


    There are a couple of reasons you would want to have the Trucks Computer Re-Programmed to Correct your Speedometer Readings,
    which will Be Off with this change in Tire Size(As Shown Below).....as the Vehicle’s Computer Calculates your Speed based on the Diameter of the Stock Tires, and with this Change in Tire Size, the Vehicles Computer doesn’t know how Fast your Vehicle is going......also with this change in Tire Size, your Transmission may Not Shift Correctly......

    Most Hand-held Programmers have the feature of Correcting the Speedometer Readings, when there's a Change in Tire Size and/or Change in Gear Ratio.



    Here are the Specs for Both Tire Sizes you've mentioned

    [TABLE="class: withlines, align: center"]
    [TR]
    [TH]Typical Weight[/TH]
    [TD="align: center"]~ 26 lbs / 12 kg[/TD]
    [TD="align: center"]~ 18 lbs / 8 kg[/TD]
    [/TR]
    [TR]
    [TD="align: center"]Tires:[/TD]
    [TH]Tire 1[/TH]
    [TH]Tire 2[/TH]
    [/TR]
    [TR]
    [TH]Tire size[/TH]
    [TD="align: center"]265/60-18[/TD]
    [TD="align: center"]275/70-18[/TD]
    [/TR]
    [TR]
    [TH]Section width[/TH]
    [TD="align: center"]265 mm (10.4'')[/TD]
    [TD="align: center"]275 mm (10.8'')[/TD]
    [/TR]
    [TR]
    [TH]Sidewall[/TH]
    [TD="align: center"]159 mm (6.3'')[/TD]
    [TD="align: center"]193 mm (7.6'')[/TD]
    [/TR]
    [TR]
    [TH]Tire Diameter
    [/TH]
    [TD="align: center"]775 mm (30.5'')[/TD]
    [TD="align: center"]843 mm (33.2'')
    68 mm (8.8%) taller.
    [/TD]
    [/TR]
    [TR]
    [TH]Rim sizes[/TH]
    [TD="align: center"]18x7.5 to 18x10[/TD]
    [TD="align: center"]18x7.5 to 18x10.5[/TD]
    [/TR]
    [TR]
    [TH]Circumference[/TH]
    [TD="align: center"]2434 mm (95.8'')[/TD]
    [TD="align: center"]2647 mm (104.2'')[/TD]
    [/TR]
    [TR]
    [TH]Revs per mile[/TH]
    [TD="align: center"]661.3[/TD]
    [TD="align: center"]608[/TD]
    [/TR]
    [TR]
    [TH]Speedometer[/TH]
    [TD]Assuming that this is OEM tire and speedometer readings are correct >>>[/TD]
    [TD]Speedometer reads 60mph actual speed will be 65.3mph 8.8% faster.
    [/TD]
    [/TR]
    [/TABLE]

    Edit:....IMO....you should have the Speedometer Corrected, for this Change in Tire Size.

    Last edited: Nov 21, 2013
  3. JimmyA

    JimmyA Member 100 Posts

    GPS tells you how fast, you are going! If not, GPS model? Yes.. Get a "SCT" programmer, http://www.sctflash.com/index.php I am not a rep, just a Owner of their product.
  4. Pikey

    Pikey Moderator Staff Member 1000 Posts 100 Posts

    Sounds like an excuse for a custom tune with a diablo tuner from @06MonteSS to me!! There are a bunch of satisfied customers of his here.
  5. Jrod89

    Jrod89 New Member

    I second the tune from Lew. My truck is a beast since he's been working on it. And we're not yet done! For the price you can't beat it!
  6. GeorgiaZ71

    GeorgiaZ71 New Member

    Well, I just use my Chevy MyLink system for Navigation now... and sometimes my phone. But now that you mention it, my old TomTom GPS did give me a speed readout. But the current system doesn't display my actual speed. I will check into both an SCT or Diablo Tuner. I was thinking about exhaust and CAI...I might as well order it all with the tuner. Shopping time! Thanks everyone!
  7. ChevyFan

    ChevyFan Administrator Staff Member 1000 Posts

    There are some iPhone / Android apps that will give you your speed and other driving information as well, they can be very useful in helping to determine how much off you are.

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