leveling with 33's - yep another gearing question thread

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by cain73, Jul 13, 2010.

  1. cain73

    cain73 Rockstar

    I have gone back and forth between 35's and 33's. I love the 35's but hate the need of the 6" lift, regear, etc. etc. I had 33's on my Tahoe and they were fine, BUT my Tahoe had 3.73 gears. I know it's a different vehicle than my 2010 Silverado so I don't know how much that throws into the equation. I have read that with 33's I should go to 4.10 gears.
    I looked at a couple of the gearing charts - couple of questions:
    1 - when it says tire size does that mean pre-install or the actual tire diameter when on the truck under normal load ? I ask because on my Tahoe I had a Superchips programmer and they told me to measure the actual tire size under normal load to adjust for the tire change. Are the gearing charts the same way or do they mean the uninstalled full 33" size?
    I ask because if you look at the uninstalled tire size of my stock 265/65/18's it basically says my stock 3.42 gears are wrong. They have the highlighted for MPG/overall/towing but with stock tires and 3.42 gears I'm on the other side of MPG which gives no umph.
    2 - Assuming the stock tire and gear is fine for overall - would a change from 31.5" to 33" make that much difference ? would just my tuner make up for it or would I still need to regear ?
    Sorry for the long post but I've been reading a lot and have thoroughly confused myself and need to be pointed in the right direction before I spend the money. I really appreciate it.
     
  2. 04-silverado

    04-silverado Rockstar 100 Posts

    I would think that "they" mean height of an installed tire, properly inflated, under normal load... as that is going to yield the "average circumference" which determines "average number of revolutions" (i.e., rotations over the course of a mile, as is given in most tire charts).

    As for the 3.42s placing results on the side of "economy" ... that's been Chevy's claim to fame (so to speak) in all of their advertising about getting the best "overall" MPG. Of course, those who want/buy trucks capable of heavy duty hauling... can get them with a different gear ratio, an engine with more muscle, and beefier suspension.

    ...just my thoughts
     
    Last edited: Jul 14, 2010
  3. Greg84

    Greg84 Rockstar 100 Posts

    What transmission is in your truck? If you've got the 6 speed with 3.42 gears its going to be pretty much the same as your tahoe was with a 4 speed and 3.73 gears.
     
  4. cain73

    cain73 Rockstar

    I know there are heavier duty options available, but I didn't want to go to the 6.2 or the 2500 HD as I don't haul that often, or incredible amounts when I do. I did go with the 5.3, 6 speed trans with manual, HD cooling, upgraded Rancho suspension, etc. The thing is that it's a daily driver, not a daily hauler/work truck. I'm trying to get the best of both worlds here. The only way to have gotten the 3.73 instead of the 3.42 or the 3.08 was to get the max trailering package with the 6.2. I have all of that stuff except the mpg killing 6.2 engine. Yes I would have liked the added power, but for as little as I would actually NEED it , it wasn't practical.

    Greg84 - I do have the 3.42 and the 6 speed - So I should be fine with 33's , 3.42, and my programmer then - right ?

    stock as it sits tires are about 30.25" on the gearing chart http://www.4wheelparts.com/aux_incl/gear_ratio.html
    it puts me at about 2490 RPM right in the middle of the best MPG range (according to the chart)

    If I go 33" (assuming about the same variance due to normal load and mounting) I should be around 31.75" putting me at about 2330 RPM just outside the best MPG range according to the chart. I'm assuming that's still fine with the 6 speed and the programmer to compensate for the larger tires. especially considering the chart has this note " If you have an automatic transmission your RPM will higher due to slippage in the transmission and the torque converter. " so in reality I should be a little higher in RPM putting me somewhere around 2400 RPM ???


    Knowing that and assuing I'm correct on the chart info - and that I only tow occasionally - and then it's considerably less than the manufacturer limit of about 10,000 pounds - sounds like it should be ok right ?
    input ? what ya think ?
    Is it worth the $1,600 to regear and get closer to the 4.10 gear rate of about an additional 400-500 RPM's
     
    Last edited: Jul 14, 2010
  5. silveradotrailblazer

    silveradotrailblazer Epic Member 5+ Years ROTM Winner 5000 Posts

    I dont want to sound like a jerk but I think your over analizing everything. Take it from some one that has regeared to many trucks to remember, its worth it. Go with the 4.10s because the 6 speed is a overdrive transmission so you wont be turning crazy highway rpms, but will give you more bottom end grunt.
     
  6. 04-silverado

    04-silverado Rockstar 100 Posts

    Actually, on further thought, I think you should go by the manufacturer's specifications for YOUR particular tire. Likely, this is NOT your tire... but, for example, SPECS on a particular P265/65R18 which I found at:

    http://www.lrhooktire.com/showcaseprintproductdetail.htm?productId=3158244

    ...are as follows:

    TREAD WIDTH: 8.7
    RIM WIDTH (MIN): 8.0
    SECTION WIDTH: 10.5
    OVERALL DIAMETER: 31.7
    WARRANTY: Limited
    TIRE WARRANTY: 60000
    TIRE WARRANTY UOM: Mile
    SIDEWALL: BSW
    TREAD DEPTH (32NDS): 11.0
    SPEED RATING: S
    STATIC LOADED RADIUS: 14.3
    REVS PER MILE: 657
    TIRE SIZE: P265/65R18 112S ​

    At any rate: the overall diameter of any tire will change, as the tread wears down... if the tread-depth wears down by 1/4 of an inch, the overall diameter will change by 1/2 an inch. IOW: the tire size/height will never remain the same (over the life of any tire)... so, I would go with the original manufacturer's specs, and then use the "ballpark rule of thumb"

    tire height in inches ... times 0.12 = approx optimum gear ration ​

    With that result... and referencing http://www.4lo.com/calc/geartable.htm

    ...use the next larger number gear ratio for more pulling power and less MPG--but more than adequate pulling power for heavy duty hauling/towing

    ...or use the next the next lower number gear ratio for better MPG--and likely more than adequate pulling power for average driving/hauling/towing ​

    bottom-line? I suggest taking silveradotrailblazer's advice
     
    Last edited: Jul 14, 2010
  7. cain73

    cain73 Rockstar

    Am I overanalyzing - yes. I'm prone to that. I guess I'm trying to find evidence that suggests I shouldn't spend the extra $ and that I'll be ok with the 33's.

    I looked up the exact tire I have and the specs:
    Bridgestone Dueller A/T RH-S
    41 pound
    sect width 10.7"
    tread width 8.4"
    tire diameter 31.5"

    so using that info and multiplying the 31.5x.12= 3.78 so from factory I'm screwed with an inappropriate gear ratio ?

    man if that's so I don't have a choice but to regear especially if I go up in diameter.
     
  8. ToTallScott

    ToTallScott New Member

    hey here is the one thing to think about if u regear the factory warranty is gone because they will say the gears caused the issue and u have a new truck witht the 5year 100,000 warranty. also have a 2010 with 35s and a 6 in lift from chevy and it has the 342 in it. I talked to them about regearing and they said no I got into it with them about the trany and it not going into 6th only sometimes i do not care that much about gas but was worried about the tranny and they said i was fine. so i did not regear it does lack some power but this thing is not a race car it is a truck. good luck on whatever u do that price is good if u do regear.
     

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