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Larger wheels & tires question

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by Necred, Apr 1, 2013.

  1. Necred

    Necred Member

    On my 00Z71 is 16 inch wheels, I have an opportunity to buy off 2012 the 18in tires & wheels. I was told today that one of the gear shows said larger wheels & tires would foul up the computer & eventually ruin the OD in the tranny. Now, I don't know if he heard correctly but im sure he did hear something along those lines, Can anyone elaborate or confirm?
    thanx
  2. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    Bigger tires and wheels means more weight. More weight means more torque is required to get/keep that weight rolling. Unless you (re)gear appropriately for the new tire size to get/keep that weight rolling (so that you provide the needed torque), it's going to be more work for the tranny.

    Think of running bigger wheels/tires without appropriate gearing in terms of running your truck on a mild incline all of the time, as that's effectively how it translates into what the transmission will feel in terms of stress and added heat.

    So, your first step should be to check your current overall diameter tire size and gear ratio on a gearing chart and see where it falls ... then find your new tire size on the same chart (with your current ratio) and compare. The following chart assumes a 1:1 ratio (i.e. no overdrive engaged) at 65mph for the rough RPMs shown in a given block. Yellow is biased for fuel econ at the expense of power, black is optimal balance between fuel econ and power, and blue is biased for power at the expense of fuel econ. Following/staying within the same colour band as your OEM tire size and gear ratio with your new tire size ... will show you roughly what gear ratio you should have for a given tire size -- and you can, of course tweak it from there to bias for fuel econ or power as you see fit. (I'll leave it to you to convert metric nomenclature to inches before plotting overall tire diameter on the chart, as that should be simple enough for anyone who knows how to use Google.)
    gear-chart.gif
  3. Necred

    Necred Member

    OK thanx,, I have 3.4 ratio & 245/75/16 96 in in dia & 30.5 dia compared to 265/65/18 at 99in cur & 31.5 dia. I don't see enough diff to be concerned?
  4. 95C1500

    95C1500 Member 2 Years 100 Posts

    A lot of times there isn't enough to be concerned. I went from a 235/75r15 to a 30x9.5r15 and I'm off by 2 mph at 65 mph.
  5. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    1" overall diameter change is no big deal from a gear standpoint. You -should- correct your speedo error, though ... which means a tuner ... unless you already drive precisely the speed limit or unless you're willing to risk a ticket for a higher speed than you already ... speed.
  6. steved

    steved Former Member

    I know this affects my 2500HD, did they change the bolt pattern on the 1500s? I know they changed the 2500HDs from a standard 8x6.5" to a 8x180mm in 2011.

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