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  • bigger tire ?'s

    I have a 79 cj7 and plan to lift it for 35 in tires. Does anyone know what gears i should install, it has the 304 with a 3 speed. Also is there any way to correct the spedo for the bigger tires?

  • #2
    By regearing to a similar-to-stock ratio with the new tires, you will restore your speedo reading to near-stock too.

    There are gear ratio to tire size tables out there on the web. Just google that term and you'll find them. Our host may have one in their catalog too I think.

    The place to start is by determining what your stock ratio is. A '79, I am going to guess at somewhere between 3.31 and 3.73 gears. The stock 235/75R15 tire is roughly 28.5 inches tall. A standard 35 inch tire is about 33.5 to 34 inches tall...unless you get an Interco brand, they tend to be true to size. Check the specs on your new tire of choice for the overall height.

    Now, on the table, look up your stock gear ratio and tire size. The table will give you an RPM reading for that tire size and ratio at a certain road speed. Next, find that same RPM reading at that speed (tables are usually at 60 mph) for your new tire size. Then just look over to see what gear ratio will give you that.

    Here is a table along with the formulas to figure the numbers out for your specific setup: http://www.4lo.com/calc/geartable.htm

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    • #3
      thanks for the all the info! i do plan to run interco tires and i was aware that they tend to be true to the size they say they are, i just hope they don't rub to bad! Thanks for your help.

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      • #4
        I would drop the three speed and pick up a t-176 before change out the gears in the front/rear axle.
        and if you find out you have 3.54 gears or 3.73 I would stay with them and change out the tranny as mentioned.
        unless you are planning on rock crawling only then I would move up to 4.56's.
        good luck,
        7jeeps7kings

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        • #5
          From my research for my rig, I was going to run 4.10's with 31's and a T5 5 speed. A bit deep for 31's, but my engine was weak and if I wanted 33's later, I'd already have the gears.

          A 4 speed is a good idea IMHO, but one you can get later. If you don't daily drive the Jeep, go for the gears you want and change the rest as you go...heck, thats why MB's could only run 45-50 mph: 5:38 gears! It's also why they could climb a tree with a gutless, teeny 4-cyl engine.

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          • #6
            Keep rocking your current T-150 tranny until it gives up the ghost. Just start saving for the replacement. Then your question will be, "the T-18 or the T-176?" But that is for another day!!!

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            • #7
              Autometer sells programmable speedometers. They will work with any size tires, or gear changes.

              Also available are GPS speedometers. I recently built a new dash with one of these programmable speedometers, but I would have bought a GPS speedo instead, if I had known about them.

              Rich

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              • #8
                Originally posted by rixcj View Post
                Autometer sells programmable speedometers. They will work with any size tires, or gear changes.

                Also available are GPS speedometers. I recently built a new dash with one of these programmable speedometers, but I would have bought a GPS speedo instead, if I had known about them.

                Rich
                I did install the Autometer pro comp marine GPS speedo, still have to install the "antenna" but it at least did away with gearing and ratio concerns. It's a little pricy. Look at rexmarine.com

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                • #9
                  i do plan to drive it as a daily and would like to put a 5 speed in it but havent found anything that would be a some what easy swap! I know that nothing will be a simple drop in installation but there is not alot of info on 5 speeds that will bolt up to the 304. I don't want to swap my engine out yet because it only has 65,000 orignal miles on it and there is nothing wrong with it. If anyone can point me in the right direction for a 5 speed that would be great!

                  I also plan to rewire the whole thing using a painless wiring kit and i see two types listed one says "complete harness assembly '75 & newer" and the other says "factory Preterminated replacement harness assemblyfor '76-'86" can anyone tell me the differance in the two they are both from painless i know my way around wiring but i'm no pro by any means! I was guessing that preterminated ment that it allready had all the conectors installed in the harness and that the other kit did not am i correct in thinking this or way off?
                  Last edited by mudbug; 08-10-2010, 12:45 AM.

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                  • #10
                    Not sure if there is really anything that will "bolt right up", but with Novak adapters, you can pretty much install whatever you like. However, new drive shafts etc are required.

                    Try to find a T-18 if you plan on beefing up your 304 or going with a 360. You can find T-18's without a granny 1st that are nice. However, most of the trannys that are "plug and play" will only see a 1:1 on your last gear. Anything with an overdirive will propel your Jeep faster than they should go anyway.

                    Just remember that most of what I say is opinion, and that is just a Pi and Onion.

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                    • #11
                      If you were sticking to the 304, I'd look into the sm465 swap-tough as nails transmission and has an excellent stock gearset- especially when mated to the dana 20 or 300, unless of course you were planning on doing 75+ mph in it. The ax15 is a decent trans for the 4.0, but I wouldn't run it behind the 304. Just my 2 cents. Good luck!

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                      • #12
                        I'm 33" BFGs with 4.10s in my 84 and loving it. My original gears were 3.31s and made it hard to run on the highway. I've got an H5 block 258 bored .060 over with .040 shaved from the head. I was able to bark the tires in 2nd before now I just try to save them. but, 65 is right around 2500 RPM in 4th gear. The t-150 has a little higher first gear, but that shouldn't be a problem. They are tuff little S.O.B.s the last one I had was mated to a dana 20, which is also tuff. Just my 2 cents.

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                        • #13
                          mine has the dana 20 in it and i'v found a good bit of aftermarket support for it so i know im keeping it though i am going to upgrade to the twin stick shifter for it. 4.10s are what i'm looking to put in it i figure that if i run 35's the rpm will probably be around the same as it is now which i dont have a tach but i'm guessing be the sound of the engine is prety close to 3000rpm so between gears and tires it should even out, I hope! I know for shure that the t-150 is geting swapedfor a 4 speed, i'm taking a long hard look at the sm465. Novak has a kit that comes with a fresh trany and all the adapters to make it work.

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                          • #14
                            I bought the twin stick set up through Novak- great piece and I believe the stainless option was only like 10 or 20 bucks more. AA also builds an adapter kit to run the sm465 with the dana 20- and the plate is only 7/8" thick, so it allows plenty of room for a driveshaft (50-4703).
                            Last edited by cfitz; 08-28-2010, 09:44 PM.

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