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So what does a hundred rpm feel like.....really???

Discussion in 'The Garage' started by BigJohnson, Dec 2, 2002.

  1. BigJohnson

    BigJohnson 1/2 ton status

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    It's the age old question can I get by with larger tires and no gear change or should I wait for the axle swap?

    I know its probably been asked a million times before but here goes, how many inches of tire increase does it take to really feel it in acceleration and power?

    I am faced with a dilemma, I desperately need tires and of course I want the largest I can fit, turn, and afford. And you guessed it, I currently have 31's on stock axles, no big surprise there. My current setup is a stock Vortec 350, TH400, 10 bolts with 3.08 gears, and 31's. As of now I am pleased with my power and acceleration. I have read many similar posts with mixed reviews, some feel a major decrease in power and others don't. I have even heard of ridiculous sounding gear and tire combos like 2.73's and 38's that seems to work for some people and others with 4.10's and 33's that complain about excessive highway rpm. So what should I base my decision on?

    Is there a way to quantify the effect of seemingly slight decreases in overall engine rpm due to an increase in tire diameter? I would suspect that this would vary greatly and be effected by several variables. Some obvious variables might be engine, cam profile, intake and fuel delivery, transmission gear ratios, torque converter stall speed, tire weight, and overall vehicle weight. With that said, let me specify the details of my rig. The Vortec 350 is bone stock with an Edelbrock performer rpm intake and q-jet carb, the TH400 is stock with a stock diesel torque converter ( I am unsure of the stall speed, it could be low), the t-case is a stock NP208, and of course the last and the least the 10 bolt axles geared 3.08. So without wasting my money on a set of tires I might not be happy with, I am putting my faith in the ColoradoK5 family with their extensive knowledge and varied experiences to guide me in my tire selection.

    Here is the information I have:
    *All actual tire heights are Buckshot Maxxis Mudders with the exception of the stock tire which is a Wild Country RVT. Formula used: (MPH x Gear ratio x 336) / (tire size ").

    Tire Height (advertised) Tire Height (actual) Gear Ratio Speed Engine Speed (RPM)
    31* 30.7 3.08 65 2191.11
    32 31.3 3.08 65 2149.11
    33 32.2 3.08 65 2089.04

    I am not sure how accurate the formula I am using really is. But on paper using that formula the difference between 31's and 33's is a whopping 102.07 rpm. Doesn't really seem like it would be noticeable. I have also thought about the tire weight as being a contributing factor as most increases in tire diameter also come with an increase in width. I could not find the weight of any tires to compare these figures so the weight increase may be negligible. Any thoughts?

    So, all opinions, thoughts, and personal experiences would be appreciated. Also, if anyone can devise a mathematical formula for calculating such things as tire change effects and seat of the pants performance I would like to see it.
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I went from smallish 35's to 36" TSL's which were about 2" taller running 3.73's. The RPM drop at cruise was about 150 RPM. I didn't notice any difference...

    A tick under 2100 on the freeway is acceptable...chances are you'll be doing 70 anyways./forums/images/graemlins/smile.gif

    I see 2207 at 65 with 33's (actual 32.2) and 2376 at 70.

    Speed (in MPH) x final gear ratio (3.08) x 355 / tire diameter (in inches)

    A manual tranny or lock-up equipped tranny would use 336 instead of 355. 355 is used to account for torque convertor slip.

    If you had a 700R-4 I'd say re-gear for sure. With the TH400 the RPM's are fine with 33's

    Rene
     
  3. Triaged

    Triaged 1/2 ton status

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    Math?...I'm too lazy
    Check out http://www.ring-pinion.com/calculators.shtml#
    I think you might have more problems with the torque converter than hwy. I think those 33's would kill your acceleration. I was spinning about 2350rpm when I had my 36's (which measured almost dead on 36") and 3.73's and it was something I could deal with...but not for too long...maybe I should put it this way...I think 3.08's and 31's would suck!
     
  4. thatK30guy

    thatK30guy 1 ton status Premium Member

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    </font><blockquote><font class="small">In reply to:</font><hr />
    I have even heard of ridiculous sounding gear and tire combos like 2.73's and 38's that seems to work for some people

    [/ QUOTE ]

    I resent that remark!! /forums/images/graemlins/tongue.gif /forums/images/graemlins/rotfl.gif
    Seriously, I ran 2.73's with 35's on my old '84 Blazer once. /forums/images/graemlins/laugh.gif Even had the 700R4. Never knew the actual RPM because I never had a tach in it.

    My '85 K30 runs 35's with 4.10's and it still puts out considerable RPM's on the highway. I have a brand new tach in my desk, just never mounted it. I need to someday just for kicks to watch the revs and see what I'm doing. /forums/images/graemlins/thumb.gif
     
  5. BigJohnson

    BigJohnson 1/2 ton status

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    Thanks everyone for the replies.

    Rene,
    Thanks for the info. 355 instead of 336, I thought something wasn't right. At one time I found a site that had all the formulas not just calculators but I couldn't find it.

    Triaged,
    I agree 3.08's suck but its really not that bad. Sometimes I think the whole ultra low gear thing is somewhat overated. It performs respectively off the line, not great mind you, but I get good highway milage. And even with a lacking axle ratio and tranny 1st gear ratio, the 208 in 4low makes for decent crawling. But I agree some lower gears would be best. I think 3.73's would work well for me.

    Thatk30guy,
    I was actually recalling an issue of 4 wheel and off-road that had a truck with a similar gear tire combo. I think the tires were even bigger than 38's might have been in the 40s, I can't remember. I think it was a mud only truck.

    So the general consinsous is that I can get away with it, right? For now I may just go with some used 31" BFG ATs I found. $180 for four tires mounted and balanced sounds good to me. They should last until I can find axles. I'll probably run a different size rim anyway.
     
  6. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    If you don't have one already install a tranny cooler to be on the safe side. The TH400 will take a lot of abuse...but no part is invincible.

    BTW my truck came stock with 2.73's and a 4 speed manual (31 x 10.5 tires) When I got it it had 33's on it.../forums/images/graemlins/smile.gif

    Rene
     
  7. TopOff

    TopOff 1/2 ton status

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    Check out www.4lo.com

    That site has the calculator(s) at the top.
     
  8. thatK30guy

    thatK30guy 1 ton status Premium Member

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    Funny, while you can run 2.73's and a 465, you can easily take the vehicle up to 80 mph, 3rd gear! /forums/images/graemlins/laugh.gif /forums/images/graemlins/thumb.gif
     
  9. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Yup...4th was like overdrive. 1800 rpm on the freeway in 4th. Sucked for towing though.

    Rene
     
  10. bablazer73

    bablazer73 1/2 ton status

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    my 87 has 3.08's and the 4spd stick. I took off the 31x10.50's and put some 235x75x15's on it! I had to leave it in 3rd gear running around town! anything under 35 mph in 4th would lug the engine! Gagh! i hate 3.08's!! but it's just a plow truck. maybe I'll do axels later.
     
  11. zakk

    zakk 1/2 ton status

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    well, with 39" tires, I am turning 2296 RPM at 65 MPH with 4.10's

    Personal expereince shows that steep grades, with afore mentioned vehicle specifications and the tranny being dropped into 2nd = overheating tranny.

    I am content with the 4.10's i have now, but i really see 4.88's as where i am going to end up, i can do about 75-78 MPH, but my engine runs outta wind at 3100 RPM, no idea on that one either.
     
  12. RedBrute

    RedBrute 1/2 ton status

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    I have a '87 ,700r4, 3.08 gears, 33's (actual 32") At indicated 65 mph the engine turns 1600 rpm in OD, 2050 in drive. I plan on going to 4.10's in the spring, be interesting to see what my actual rpm will go to. I hope it jumps buy at least 200 rpm. That would put me closer to the "stock" rpm range for my setup. Also, I never run in OD even at 65. When I run the "stock" tires, 30/10.50/15 the Blazer feels a lot snappier.
     
  13. K10ANDYKHAMNIC

    K10ANDYKHAMNIC 1/2 ton status

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    what is my 305s redline ? it hasnt blown up yet after hitting 4-5 k or so ...... /forums/images/graemlins/eek.gif it is a crate motor with and edelbrock intake dual plane performer , stock exhaust manifolds and true dual 2.25 exhasut with and H pipe behind the t case .
     

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