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  • does this sound right ?

    i have 4.56 gears on my 79 w/ 304 .and 33's , all new gauges , and tach. at idle its at 1000 rpm ( according to tach ) at 65 mph ( i know speedo is off allittle due to tires ) tach says 5000 , does this sound right ? could my new tach be off ? sounds and run great , but rpm seems excessive (on tach ) or do you think its correct ?

  • #2
    That sounds way off to me. I know a few guys that ran 4.56 with 33's on TJ's, and RPM's at that speed were never over 3000. Those were electronic tachs, so I'm not sure on yours (mechanical?) I'd say if you can get another one in there it should tell you.


    • #3
      the mighty 304...


      Fuzz probably has the only Jeep here that can turn rpms like that consistently and get away with it. The 304 should sound like it's about to take off for the moon if you're going down the road turning revs like that. Tach is way off.


      • #4
        thanks blackwater , i have new everything in this jeep including painless harness , a bad tach out of the box ? or could it be possible i have it wired wrong ( although it seems theres only one way to make it work) ?


        • #5
          You might want to make sure there isn't a switch on your tach that allows you to run it in a 4,6, or V8. I have heard of this happening and they are correct, it should sound like it's about to coming apart at 5 grand.



          • #6
            to me , the engine sounds normal enough , i didn't see a switch though , its new from for 4wd hardware


            • #7
              i have never installed a new tach, but my advice is to check and recheck the wiring and i would check the tach for the switch. i have heard that they do have the switch. or maybe you have a tach that is simply for a 6 cyl?
              with your setup, i would think that you would be turning closer to 3000-3200 rpms.


              • #8
                I run 4:56 gears and 33" tires also. My factory tach reads about 3700 RPMs @ 55 MPH(GPS). The speedo reads 75 at this speed. I'm not sure how acurate these tachs are though, I'm thinking about running another one just to check it.


                • #9
                  you guys really have me thinking now , maybe i have the wrong tach , but if i did have it hooked up wrong it just wouldn't work right , looks like have a little work to do !!.........


                  • #10
                    hey motorhead , just out of curiosity , do run a 304 ? and what mods do you have on it ?


                    • #11
                      It is quite normal for an aftermarket tach to be off a few hundred RPM, especially with the "less expensive tachs"

                      I think the problem is probably what High5 mentioned, the switch on the back of the tach.

                      A tach counts the number of spark pulses that the coil or ignition module produces. As a rule on 4 stroke engines, the cam makes 1 revolution for every 2 crank revolutions. That means that for a V8, 1 revolution = 4 pulses, 3 for a 6 and 2 for a 4 cyl.

                      I went to Novak's website ( ) and looked at their RPM caculator. it says that at 65 MPH, you should pull about 3012 RPM without an overdrive. If your switch were set on 6 cyl, then you would read 4016 RPM on the tach (factor of 8/6 or 4/3). We know that a 33 inch tire isn't exactly 33 inches tall and your speedo is most likely off a bit, and the fact that the tach may be off a couple hundred RPM anyway then your 5K reading could just be the result of the wrong switch position.


                      • #12
                        do you think running a hei ignitiion ( dui ) has anything to do with it ?


                        • #13
                          i would agree that i did run a " less expensive tach " a factory replacement from 4wd , but i still did not see a switch on the tach for a six or eight cylinder , even the part number is the same ...


                          • #14
                            Originally posted by jackson
                            hey motorhead , just out of curiosity , do run a 304 ? and what mods do you have on it ?
                            Nope...I have a Chevy 350. Not many mods, just cam, intake and headers.

                            Something must be way off on mine also according to the link above. I should have another tach laying around. I'm gonna hook it up and see what it tells me. According to the chart:
                            Miles Per Hour

                            RPM= 3700

                            Tire diameter= 32" (They are 33s but measure about 32")

                            Transmission's final gear ratio:=1 to 1

                            Transfer case high ratio:=1 to 1

                            Axle ratio: = 4.56

                            Resulting MPH's:=77.28!!

                            If that equation is right my tach is way off also. I know @ 55 mph on GPS my tach reads about 3700 rpms. Motor sounds like its spinning pretty good also.


                            • #15
                              If it is the same tach for a 6 and 8 cylinder, then there are only a few possibilities:

                              1. You have to open the back of the tach to acces the switch.

                              2. The factory jeep harness adjusts the tach signal.

                              3. You have a different wire for the V8 tach signal commng out of the tach.

                              If there were no instructions with the tach, I would call 4WD and ask tech support, odds are it is a simple fix.