front and rear axle different ratios?
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Thread: front and rear axle different ratios?

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    Isuzu reliability tester carcrafter22's Avatar
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    Default front and rear axle different ratios?

    So lets say you have a 4x4 truck of course you have 2 axles front and rear. The front axle has 3.55 gear and the rear has 3.50, will this work fine? What about running a 3.31 front ratio with a 3.27 rear ratio?

    Point is what is acceptable and what is not as far as a difference in ratios? I know some factory apps like chevy have ran a 4.10/4.11 combo but is it ok to be .04-.05 different (such as the 3.27/3.31 combo)? Do both axles have to be within .01 of each other?


    I have been told (and it makes sense) that you can run any two ratios you want as long as you leave a driveshaft out. Lets say you have a 3.55 rear gear and a 3.73 gear up front, all will be fine as long as you remove the driveshaft or leave it in 2wd, once you put it in 4wd you will cause binding and break something. Sound right? or is my logic flawed?

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    Randy
    1967 F100 isuzu 4bd1t 60mm turbo procharger d1sc, big shied injection pump, 100% injectors - currently building AWD chassis for it
    2001 Jeep Grand Cherokee isuzu 4bd1t, he351 turbo 370cc pump, 100% injectors, 4l85e trans, bw4401 transfer case, clayton long arms 4.5" lift - For sale

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    Isuzu reliability tester carcrafter22's Avatar
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    Default

    Ok so I just found a couple things about this (gonna leave this here anyway for the tech) Apparently its fine to have different ratios and most people feel its fine to be off up to around 1%, in other words a 3.27 rear will work with a 3.31 front, 3.50 rear will work with a 3.55 front, etc.

    Any other opinions or facts on the matter?
    Randy
    1967 F100 isuzu 4bd1t 60mm turbo procharger d1sc, big shied injection pump, 100% injectors - currently building AWD chassis for it
    2001 Jeep Grand Cherokee isuzu 4bd1t, he351 turbo 370cc pump, 100% injectors, 4l85e trans, bw4401 transfer case, clayton long arms 4.5" lift - For sale

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    Like you state, as long as you are in 2wd and don't have a full-time transfer case you will be fine with any ratio. Once you go into 4x4 you want to match to within 1% unless you keep the truck ONLY on surfaces with very poor traction. I can't verify 100%, but I have heard of ice racers or mud draggers intentionally putting a numerically lower ratio in the front axles, such that the front is always pulling more than the rear is pushing in an effort to keep the truck straight. But these are trucks run only on ice / slick mud, so they get away with it.

    It all actuality you can run WHATEVER ratio you want front and rear...as long as you are willing to do the math and make the front and rear tires smaller and larger to compensate!

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    personal opinion only...have the higher ratio in front.it's an old hill climb/mudding/sled pulling trick to enhance directional stability.if you intend to run at high speeds,in 4-high,i'd leave the ratios the same.i agree that a difference that small has virtually no effects on the drive line off road.
    currently modding an 86' m-1010 cucv ambulance,with factory 14-bolt/detroit in back and an open 60 in front.lifted 6 inches,running 37's on 17 x 8's.now swappimg in a mild 6bt,and an nvg-4500/np205 combo.hope to have it running by fall.
    link to engine build... http://www.4btswaps.com/forum/showth...n-engine-build...

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    the rule i learned was 1% on the road and 2% off road limits.

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    Generally speaking, we have found that the ratio's should be fairly similar on the road to provide full life potential. For instance, it is fine to run a 4.10/4.11 combination for on road driving. That close of a ratio difference will have no effect on the life of the 4wd parts. You can get away with around 1% difference on the road, but the life of the parts will be substantially lower. Off road, you can get away with as big a difference you wish. As long as it is in an extremely low traction environment (i.e. mud). It is common for mud racers to run wide variable ratio's, to aid in the front gaining more traction than the rear. This is merely to assist in keeping the nose up in the mud. As far as the minor difference in ratio's, it would be no different than having the same size tires, from 2 different manufacturers, on opposing axles. As some manufacturers measurements slightly differ.

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    From what I have found it seems like some manufacturers (like ford) actually ran ratios like 3.50 rear 3.54 or 3.55 front can anyone confirm this? I really doubt there will be anymore wear on things if you run around 99.8% of the time on the road with sticky tires in 2WD only and very rarely run 4wd off road. Thoughts???
    Randy
    1967 F100 isuzu 4bd1t 60mm turbo procharger d1sc, big shied injection pump, 100% injectors - currently building AWD chassis for it
    2001 Jeep Grand Cherokee isuzu 4bd1t, he351 turbo 370cc pump, 100% injectors, 4l85e trans, bw4401 transfer case, clayton long arms 4.5" lift - For sale

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    chevys came with 4.09s front and 4.11 rear on 203 awd trucks.

    its best if the faster gears are in the front like stated for driving control but it easier on the truck to pull the rear end along than to push the front end ahead.

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    Factory mismatches are pretty common with any corporate/Dana combos, it doesn't always translate exactly due to difference in ring an pinion teeth counts. For example the 9" uses 9 pinion teeth and 37 ring teeth to achieve what the D44 does with 11 pinion teeth and 45 ring teeth.

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    Go look up the difference in a Nissan GT-R. Its significant... But the difference is made up in the AWD strategy and setup of the "Transfer Case"

    Also, differences in tires sizes will change it. You could have a 2.5:1 in one end and a 5.72:1 in the other if the tire sizes were of significant enough difference. Different tire diameters equal different circumferences equal different relative ratios.

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    The real answer is what size tires are the front vs back, Are they exacly the same circumference?

    Factor in the exact tire circumference with the gear ratio and I would limit the difference between the two.

    example:
    smaller tires up front with a lower numerical gear and larger tires out back with a higher numerical gear.
    2500 04.5 6speed stock
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