I have a 75 f100 4x4 that not to long a go I turned my 360 into a 390. Engine: holley 750 vac. secondaries, performer intake, stock heads, 360 pistons, CR: 9.3-9.5 to 1, performer plus cam, Sanderson ff427 headers. The front chunks tag reads 3.50 gears and the door tag says it came with 3.50 gears so I'm assuming that's what it is unless they have been changed, which the PO said it had been swapped to 3.90 gears. That being said he's lied about the truck claiming the old 360 was of course a 390 which it wasnt. The truck has always been sounded like it was hollerin at 55 to 60 but I just installed a tach and it's reading around 800-1000 idling and 4000 at around 55-60 cruising down the highway. The Tach was gave to me from a member off FTE, it has no switch to change it from 4 to 6 to 8 cylinder. Anyone have any guesses? That seems like way to many rpms.

 

First, try to determine what the actual gear ratio is




If both front and back check out as the same ratio, and the ratio matches the tags, then another thing to look at would be if your clutch or torque converter is slipping excessively




Also, some tachs use switches, some use the clipping/not clipping of a certain wire on the back or under a cover to adapt the signal to different cylinder counts.

 

Man that is screaming. Thing must have 4.66 gears.


John

 

I'll check and see for sure the gear ratio and let you you know. It's a straight shift but when I dropped the motor back in the clutch didn't need to be changed it looked good, also the truck shifts fine. On the tach, I grounded the ground, put 2 wires behind one of my fuses then tied the last wire in to the wire running out the coil that says Tach.

 

It's been a awhile and all aren't the same. There should be a ground, a light, and a connection to the - side of the dist.

Not sure what the other wire is for, but as noted it has been a while since I installed one.


John

 

Good advise. Note the part about looking for cylinder settings. Tire size plays a factor too. Gear Ratio Calculator is a good calculator to find out what rpm you should be running at what speed. Here is common aftermarket tach wiring:

 

Agree ^^^^^^^^^^^^ that's some low gears!

 

4.56's = 1:1 transmission ratio = 3,600 rpm @ 70 MPH

 

Ok here is a REALLY STUPID QUESTION... do you have it in 4LOW?

 

I have a np435 transmission, it has 2wd to 4wd it doesn't say 4 lo or anything.

 

And sorry I kinda miss read your question. No it ain't in 4wd, I checked the first time I took it down the road.

 

I also forgot to add it has 33in tires on it.

 

Ok, have you jacked it up and checked the ratio yet? What kind of tach do you have?

 

I USED THE ONE ON THE LEFT THE BLACK ONE

I haven't had a chance to check the ratio cause I'm out of town visiting family for Christmas but I will do it asap.

 

Cant read the name on the face to id it