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Gearing and RPMs - What am I missing?

Discussion in 'The Garage' started by Shorts, Feb 20, 2003.

  1. Shorts

    Shorts 1/2 ton status

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    I'm confused on figuring out the rpms before and after i do the gear swap and tires. i have 3.42s and 265s stock and i went to 33x12.5s. after a couple weeks i went back to the 265s until i regear to 410s. city driving was killing me...wierd shift points, sluggish..etc etc..but what i can't figure out is the trucks performance if the numbers make sense. the truck felt really doggy and slow of the line, and i had no clue as to which way my speedo was reading...but other traffic sure passed me right and left! :wall anyway....

    according to the calculators:
    (Auto/OD tranny)

    3.42s + 265s = 1782rpm @ 70mph

    ok, that's about right, if memory serves me correct.

    then: 3.42s + 33s = 1706rpm @ 70mph
    32.8 = 1717rpm @ 70mph

    hmmm, why the big discrepancy on my seat-o-meter? i don't remember what rpm range the truck was cruising in with the 33s, but it took a big hit and i can't understand it.

    so, theoretically, after gears:

    4.10s + 265s = 2136rpm @ 70mph

    4.10s + 33s = 2046rpm @ 70mph
    32.8 = 2058rmp @ 70mph


    i guess my biggest beef is why was the truck such a weeny after going up a tire size. i weighed and measured the two tires. the difference was 1.2" and 5lbs for each tire. the only thing i can think of is i didn't weigh or measure them very good - soi can do that again. i just find it wierd :-?

    am i missing something? or am i thinking too much about the change, and therefore psyching myself out?
     
  2. willyswanter

    willyswanter 1/2 ton status GMOTM Winner

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    Welp, I checked the numbers and got:

    32.8" tires with 3.42's with tranny in O/D: 1716.6731707317074

    265 tires with 3.42's with tranny in O/D: 1781.8632911392403

    That is if your stock 265's are indeed 31.6" tall and your 33's are 32.8" tall.

    Now, with that little tiny bit of difference in overall height change you should have barely even felt a difference... After calculating it out I could only see you needing a 3.54 gear which isn't even made but is so close to your stock gearing that it doesn't matter...
     
  3. 6.2Blazer

    6.2Blazer 1/2 ton status

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    There are a couple of factors that I can see affecting your setup.

    1. Heavier tires - even though the new tires are only 5 lbs. heavier than the old ones (20 lbs. heavier total), and that is very small in terms of the total weight of the truck, the rotational force of turning that extra 5 lbs. is much, much greater than if you just put 20 lbs. in the bed of the truck.

    2. More aggressive tires (assuming the 33's are) - a more aggressive tread pattern typically has more rolling resistance, and therefore requires more power to keep turning.

    3. Wider tires (again, assuming the 33's are wider) - same as above, wider = more rolling resistance.

    3. Gearing - in my opinion 3.42 gears are way too high for even 265 tires, especially in a truck with an OD transmission. Running under 1,800 rpm at 70 mph is really low, my daily driver turns closer to 2,200 rpm at 70 and feels pretty good. Since the engine is already lugging even a relatively small change in tire size just magnifies the situation. My guess is that if you had 4.10's installed you wouldn't notice as much of a difference between the two tires.

    On the speedo reading, after installing the bigger tires it should read slower than what the actual speed is. The speedo is basically reading how fast the rear driveshaft is turning (driveshaft rpm) and than calculates that rpm into a vehicle speed based on the gear ratio in the axle and tire size. The bigger tires travel more distance with each rotation and thus meaing that for the same driveshaft rpm the vehicle is going faster. Installing lower gears will have the opposite affect, and if the gear change matches the tire size change than they should cancel each other out.
     
  4. Shorts

    Shorts 1/2 ton status

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    i was thinking the same thing you mentioned about the 3.54s/3.55s. Dad has those in his Ram. my effective gear ratio only(pfff..."only" /forums/images/graemlins/grin.gif) dropped to around 3.27 or somewhere around there(I remember checking that but can't remember exact #s).

    i just don't get it though??!?!! does my truck suck??? /forums/images/graemlins/crazy.gif /forums/images/graemlins/smirk.gif maybe next weekend i'll swap out tires again and work on numbers.


    edit: 6.2Blazer, i was thinking about the things you mentioned - about the resistance, added total weight and such. you hit a spot and now i remember that right after i put on the 33s, i got my front diff stuck in "locked". it was only last month that i finally fixed it...which of course, by the time i fixed the diff, i had already taken the 33s off and put back the 265s. i think i also equated the drag from the front end with the hit i was expectingto take with the 33s.

    hmmmm, now the lightbulb has gone on! i'll definitely toss my 33s on and see what happens.
     
  5. willyswanter

    willyswanter 1/2 ton status GMOTM Winner

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    Yep, I think it does suck and you need to do an SFA swap with 5.13's and 39.50's /forums/images/graemlins/thumb.gif /forums/images/graemlins/rotfl.gif. But then again I'm trying to figure out how to drive my truck now since gas is $1.93 a gallon /forums/images/graemlins/eek.gif

    But yeah I didn't think much about your actual RPM's. They do seem quite low. Mine are in the low 2k's at 70. Usually around 23-2400. Thats with the new 5.13's... Seems like your just not running in your powerband to begin with and the larger tires are just making it more apparent.
     
  6. Shorts

    Shorts 1/2 ton status

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    hehe SFA i definitely need. unfortunately, not for another 2 years /forums/images/graemlins/frown.gif school/work/life is getting in the way of my free time /forums/images/graemlins/grin.gif

    gas is $1.50-1.55 here. it keeps fluctuation in that range /forums/images/graemlins/angryfire.gif where did the times go when $.97/gal. was expensive???? /forums/images/graemlins/tongue.gif
     
  7. Shorts

    Shorts 1/2 ton status

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    yeah, the tires add another 20lbs, the lift kit brought around another 100lbs.

    i'm using 33x12.5x15 BFG AT/Kos, so the tread is fairly tame. although lately, i was thinking of selling them to go get a set of mud kings /forums/images/graemlins/laugh.gif

    my cruising rpms i keep around 2k on the hwy - that gets my mph around 73ish. rpms over 2k starts dropping my mileage on long trips, so i try to maximize that as much as i can.
     

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