Weird RPM to Speed changes.

[muddpuppy01]

the .01 is going to make that the difference

if you are trusting the gps units the I would say the tach is going

does the motor sound different when you are having the big swings in rpm on the tach 500 rpm should be audible difference

[1979C20]

Okay! I think I discovered the weak link tonight. I almost never drive over 50 anymore. But, last night my dad and a bunch of family friends wanted to go camping and its mostly highway to get there. My tach didnt go over 2000RPM past 50MPH in 3rd gear. I was cruising 70MPH and my tach said 2000RPM. But, in first and second it has full swing. I was "race shifting" from a stop to see if the tach would go past 2000RPM and it worked like normal. I hit 5500RPM and shifted to second, rode that to 4000RPM and then in 3rd it wouldnt go any farther than 2000RPM when it should have been at like 3000RPM at those speeds. The tach would stay at 2000RPM until my speed went under 50MPH. What would cause this behavior with the tach? Is it the tach itself, or something in my distributor?

[Irish_Alley]

since your distributor has no ideal what gear your in i dont think its the signal there. and your tach seams to be the problem. buy another one since you will probably buy one anyway for your "other" truck and check what it says

[kdime]

ok i have the same tranny and gears but i have 33's. i run 1750-1800rpm at 45mph and 2krpm at 50mph, its sounding like your tach is going south... sad to say i've tached mine out at 7k before on a slick logging raod in 2wd and it still works fine and accurately. good luck

[diphthong]

The changing of the tires and wheels could have started the whole problem. There could be a vibration coming from the new tires.  And, at a certain speed the vibration is affecting a wire, marginally burnt coil, or something else, causing an interference with the signal inside the distributor (harmonics coming from vibrations can do strange things.)  Also, Don't leave out the weather, rain can cause problems too. 

Moving on,  I'm assuming your truck is a 1979 and still has the HEI distributor.  If that is not the case, make sure your  tach is connected to the negative side of the coil.  It's been awhile sense I've worked of this year truck and, so I don't know if Is this an OEM tach?  If the tach is correctly hooked-up, and the tach seems to be the only device malfunctioning.  What I would do is, see if you can find a cheap tune-up tach (from a friend, garage sale, or buy one with a money back  guarantee :-) hook it up and see how it responds.  If it responds normal, you have your answer.  Your tach is a boat anchor.

Hope this helps...

[1979C20]

Its HEI, and the tach is hooked up to the designated "Tach" plug on the cap. There was originally a green wire plugged into, but I have no idea where that wire went. My truck has developed a decent vibration. The tires and wheels are all balanced. I think its from mud, or maybe my driveline. My motor mounts are going out again and my tranny mount is missing 2 bolts now. And my speedo cable broke last night. Along with an oil leak, coolant leak, header leak, power ateering leak, high temps, low oil pressure, and high idle. My heater core started leaking at the end of last winter so I bypassed it and now its getting cold. And since this is Arizona, it has random 100° days, and I havent put refrigerant in my AC yet. This truck is really getting on my nerves lately. I need to get my K20 soon so I can do a hit of an overhaul on my C20.

ANYWAYS, so vibrations could be throwing my tach off?

[diphthong]

It is somewhat of a stretch but,  it's certainly possible.  The replacing of the tires and wheels shortly before the problem started may not be a coincidence.  Glass breaks at a certain  frequency, right?  Well, vibrations are a frequency!  As I said, you could have a problem in the HEI unit or a wiring problem.  And a vibration coming from mud on the wheels, a bent drive shaft, or incorrectly balanced tire,  could be exacerbating the problem.  If I were troubleshooting this, I would splice another tach in to that " TACH" wire and see what happens.  Or you could remove the radiator cap and drive another truck under it :-)

[1979C20]

My drive shaft isnt bent, but it might be out of time. Ill try another tach soon. Theres one on both of my parents jeeps, so ill see if one works.