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Looking into getting a new gear ratio into my truck and need a little help.

Discussion in 'General Chevy & GM Tech Questions' started by redneckchevy, Jan 25, 2012.

  1. redneckchevy

    redneckchevy Rockstar

    So I've been looking around the internet about going into a lower gear ratio to give me back some low end power, since i gotten larger tires. I know that current (stock) ratio is 3.42 and I have a G80 limited slip differential. I got GM 8.5" 10 bolt differential, and I have 35's on my truck currently and my stock tire size is 265/70x17. I was looking into 4.56 gear ratio or maybe a 4.10 (or 4.30) but really unsure of what to get really. Cause eventually after my 35's wear out im probably stepping down to 33's. Im looking to see what would give me a little bit better mpg increase and more power since those 3.42 pushing those big ol' tires and using more fuel to get those tires going. I off-road a little bit and drive highway maybe a couple times a week, but i do a lot of driving in town.
    I want to buy the pinion and gear my self, i know i got to buy 2 sets cause my truck is 4WD. But really dont know what to get or what would be a good brand without spending too much money, and have a mechanic install it for me. Any suggestion guys?
     
  2. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    Since you plan on stepping down to 33's in the future i'd recommend 4.30's.
    4.56's would be great for your present tires but probably to much for the 33's.
    If your having a shop install the gears you might want to talk to them before buying them, most shops charge a surcharge when you bring in outside parts, they buy parts at wholesale and invoice you at retail, usually 10% a difference.
    Most shops will bargain a little over parts price if you let them know you understand their making money off the parts.
    Otherwise you'll pay retail for the gears and install kit then have to pay the shops 10% surcharge.
     
  3. redneckchevy

    redneckchevy Rockstar

    Ok then thanks, i just got to make sure they make 4.30 for my rear and front. and i'll talk to some shops then to see about the pricing.
     
  4. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    I just went through this dilemma, myself. You may find the formula and chart on this page useful in determining approximately where you want to fall, gear-wise: http://www.4x4offroads.com/gear-ratio-chart.html


    Using my own truck as an example, it came stock with 235/75R16's (29.9" diameter ... aka 30" tires) using 3.73:1 as the gear ratio. At 65mph that means the truck WAS turning 2715 RPM @ 65 MPH.

    I'm trying to get back to close to stock performance with a leaning more toward power than highway fuel econ. I average about 35 MPH over a tank of fuel, with maybe 1/3 of the tank at highway speeds and the rest being city or off-road speeds that drag my average way down, so I'm optimizing less for highway but still considering it. I'm now running 315/70R17's (34.3" diameter ... aka 34" tires) ... so using the formula on the page I linked I can choose 4.30's to get 2762 RPM @ 65 MPH (a mere 47 RPM @ 65 MPH higher than stock) or 4.56's to get 2929 RPM @ 65 MPH (214 RPM @65 MPH higher than stock).

    For me and how I use my truck, 4.56's seem ideal. I'll gain power and probably even gain back some MPG at slower speeds as I'll get back the mechanical advantage I've lost via modifications ... but I'll lose MPG's at highway speeds. Since 2/3 of my driving is not at highway speeds, this should be fine. 4.88's would kick butt around town and off road ... but would likely be absurd on the highway. :)
     
    Last edited: Jan 26, 2012
  5. vncj96

    vncj96 Epic Member 5+ Years 1000 Posts

    surreal is that right your truck turning 2715 at 65?? am I missing something?? both my trucks have 373 and are at 1900-2000 at 70mph, [MENTION=11630]RedNeck[/MENTION], my pickup has 285 on it (32.8) with 373 and I still pull 17-18 on the hwy
     
    Last edited: Jan 26, 2012
  6. sstoner911

    sstoner911 Epic Member 5+ Years 1000 Posts

    I dont know if he figured in his OD. I went from 3.23 to a 4.11's with a 34in tall tire. At 85mph Iam running at 2500 rpm. This is a perfect spot for the powerband of my baby V8 (4.8).

    Every online calculator was putting me closer to 3000 RPM or higher but the OD is pretty efficient in the newer trucks.
     
  7. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner Gold Member 1000 Posts

    [MENTION=17440]vncj96[/MENTION] the charts are meant to be read with the transmission in a 1:1 ratio. so in surealone's case 3rd gear not overdrive. the 1900-2000 for you is probably in overdrive as that is similar to what I would run at that speed in overdrive and closer to what surealone is quoting when I am in 3rd.
     
  8. mfleetwood

    mfleetwood Rockstar 4 Years ROTM Winner 5000 Posts

    <br>
    <br>
    That is correct about the 1:1 ratio and those numbers on that chart are not the actual RPMs you'll get with your vehicle (or at least my vehicle)....you need to adjust based on your vehicle's specifics. That chart is great for comparison purposes with all things being equal. For me, with 4:56s, my tire size, and my transmission, I am running in the neighborhood of 2,100 RPMs, give or take a 100.
     
    Last edited: Jan 26, 2012
  9. dsfloyd

    dsfloyd Epic Member 5+ Years ROTM Winner Gold Member 1000 Posts

    the chart is a good reference but you can also do it mathematically (still easy) .

    New ratio = (new tire size x old ratio) / old tire size
    ie. (my truck) New ratio = (35x3.72)/30
    NR= 130.2/30
    NR= 4.34

    If you dont feel like doing the math if you go down to the formula on the webpage (site link is down below) click where it says tire size calculator and on that page it has all the formulas where you can just type in the numbers and it will do it for you.

    From there go up or down based on what type of driving you do. I went with 4.10 instead of 4.56 because I do mainly highway driving and only occasionally tow. I have a commuter car now and will eventually go up to 4.56 since the truck wont be doing 300+ miles a week highway.

    if you want to figure out an estimate of RPMs here is another formula (made for manual tranny, but seems accurate in a 1:1 ratio, so not in overdrive when I did it for my truck)
    RPM= (mph x gear ratio x 336) / Tire size

    As Mike said it is all an estimate but should be close as there are a lot of factors involved in your vehicles actual numbers.

    All of this can be found at http://www.4x4offroads.com/gear-ratio-chart.html (which yes is the same site that surrealone posted. just thought I would do it again:lol:)
     
    Last edited: Jan 26, 2012
  10. redneckchevy

    redneckchevy Rockstar

    Thanks for the replies guys, also just wondering what kinda mpg i would get with the setup i want. I went to richmonds website ( http://www.richmondgear.com/101032.html) and got these rpm calculations, this is with the OD my truck has, currently with 35's with 3.42 @65 = 1515 and thats roughly what my readings are at on the truck.

    Here is what itll look with OD:
    33's
    4.10 @65 = 1927
    4.56 @65 = 2143

    35's
    4.10 @65 = 1816
    4.56 @65 = 2020

    Im averaging about 13-14 highway, and like 9-11 city (maybe like 8 for giving it sum gas for a better takeoff lol). What kinda of mileage would i be looking at with those gears at those rpm's? Also the highway i drive is 60 mph and i only drive it maybe 150 miles a week if that.
     

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