I just got a 79 f150 with an inline six. I bought a tach and it shows about 4000 rpms when I'm going 50... is the tach off or is the rpms really that high. Its a 4 speed too. Thanks

 

I have a 77 with a 300 but i have yet to have it on the highway. From what i've heard these things don't go very fast. Could also be high gear rear end. Also make sure the switch on the tach is set to "six".

 

there's no way it's spinning 4 k at 50...........


I drive my 78 f150 4x4 300 Np435 with 3.55 rears 75 plus on the highway all the time.

 

'some' tachs have settings for 4,6 and 8 cylinders...check to see what you have your tach set at

 

there's no way it's spinning 4 k at 50...........


I drive my 78 f150 4x4 300 Np435 with 3.55 rears 75 plus on the highway all the time.

 

That's just what the tach says

 

So do you think its wrong? The speed limit here is 70 and I don't dare go past 45... its a semi-new motor (3k miles) and I don't wanna break anything on it. Thanks!!!

 

check to see what it is set at, not what it says...if it is set for an 8 cyl you will get a different 'reading/display' than you would if it was set for a 6 cyl or 4 cyl...

 

what doe sthe engine sound like when you are driving it? You would know if it was revving at 4000 rpm from the sound. It would be screaming at you. If it sounds 'normal' then try a different tach.

 

Unless your truck has some seriously small tires, and a ridiculously low rear end gear, there is no reason to believe the tach.

My truck is a '74 and it has 3.50 rear end gears and is running on 235/75/15 tires. It has the 3-spd, but unless your 4-spd has an overdrive (which I seriously doubt) then your drive ratio in top gear will be the same 1:1 as my 3-spd. To turn 4000 RPM's in top gear, my truck would be going 98 MPH.

Change it to 4.11 gears, and I'd still be going 84 MPH at 4000 RPM! Assume some idgit put 4.56's in there and you're still looking at 75MPH.

Even if you keep the 4.56 ratio, you would have to cut the total tire diameter down to 20" to finally arrive at 52MPH at 4000 RPM.

What does all this mean? Quite simply it means that your tach is wrong. It is either a piece of crap that his completely inaccurate or, like others have mentioned it is configured to read a v-8 and is connected to a 6, thus making it read 1.33X what it should. That would instantly make your 4000 RPM's be an actual 3000. Even that, seems like an extremely high RPM for that kind of road speed.

Quite simply, the tach is wrong, WAY wrong. I can't tell you exactly how wrong it is though unless you can tell me your tire size, and rear end gear ratio.

 

Tire size is 235/75R/15 and how do I find out the rear end gear?

 

Well there are a few different ways. The easiest way would be to find the ID tag still hanging on the rear end housing. This would be good assuming nobody has changed the gears. Another way that I have never tried but should work is to jack up the rear end of the truck and make a mark on the driveshaft and a mark on the tire. You can then put the truck in neutral and figure out how many revolutions of the driveshaft it takes to turn the tire one full revolution, or turn the tire one full revolution and count the revolutions of the driveshaft. Either way should work.

There are other ways / things to look for as well but I am not sure what they all are. You might search for rear end identification. I know my '95 has a sticker in the door frame that gives me a rear end code that I could then just google.

 

I'm with the people who say check to make sure it's set for a 6! I wouldn't worry about what gears it has in it, because as stated if it was turning 4000 you'd know it!!!

 

True. Although figuring out what gear it has is mostly just academic at this point, it never hurts to know.

 

Alright. I will go check right now