All reved up

[tod-hog]

The tach says about 4000 rpm @ 70 mph. This seems really high. I know the tach is correct and find it hard to believe the gears have been changed. The punkin tag says 3.25. Running 275x60x15 which is a larger diameter than stock so I don't think that's it. About to swap to a SWB that has .370 so not sure what to do from here. Have no idea how the gear ratio in the tranny works but isn't high gear (3rd -C6) all the same?

[flyboy2610]

High gear should be a 1:1 ratio.
I would double chech the ratio in the rear end.
It's possible someone swapped gears. That does seem pretty high for that gearing.

[Heavenlyfire]

Are you sure it's going into 3rd? That is way high for just 70 depending on the motor. What motor do you have? Fill out your "myinfo" button if you don't mid, it is very helpful. I have a 460 with a C-6 and I don't think I hit 4000 at 110 MPH.

[tod-hog]

Yes, I am absolutely certain it is in 3rd.

[DuckRyder]

Let us put it this way:

It is impossible for a 3.25 gear with a 1:1 high gear and a 28" tall tire to be turning 4000 rpm at 70 mph.

It should turn around 2700.

Either

The transmission is not going into high or is slipping VERY badly.
The tach is wrong.
The speedometer is wrong.
The gear ratio is more like 4.88.

Assuming the tach is right (How do you know that BTW) and the transmission isn't toast, that leaves the speedometer and rear gear.

Either get a GPS and check the speedometer or run it down the interstate at 60 indicated and time it between a few mile markers.

BTW, IF the tach is right and the gear is 3.25, you are running something like 102.5 @ 4000 rpm, so you might want to get that figured out before the troopers clarify it for you.

[tod-hog]

No speedometer cable and have judged by having someone else driving along side in another vehicle. Tranny has been rebuilt and doesn't display any indicators of slipping. The tach is correct so all that is left is the gear ratio.

[DuckRyder]

I assume being an F100 it is a 9" axle? (Locker?)

If it is a 9" it is a bit of a pain to pull the chunk, so marking the driveshaft and tire should be able to get you close.

If it happens to be a Dana, I would just pick up some fluid and a gasket and pull the cover and either count the teeth or find the engraving with the tooth counts (EX 13/46 = 3.54)

I don't belive there was any stock F100 ratio low enough to give you 4000 at 70 but someone could have put an aftermarket gear set in it.

I assume you have checked for something like possibly the tach being calibrated for a 6cyl?

[tod-hog]

Really find it hard to belive that someone would have put low gears in a limited slip 2wd long bed. The truck was owned by a buddy that had a 390/ C6. He claims no such issues with the rear end. I had my own 390/ C6 and swapped them out as his were shot adn mine freshly rebuilt. Apparently my motor and tranny were early 60's model and his was a early 70's. After installing my motor and tranny the ratio seemed weird. That leads me to belive that it may be something in the tranny but am 99% positive it isn't slipping as it has good power even wound. The tach has been checked and rechecked. Besides, I know what 4k feels like and it's wound up tight.

[68F250]

You say it's a limited slip? Piece of cake, check the ratio by jacking up the pumpkin and turn one wheel a complete revolution while counting the turns on the driveshaft. Do it on level ground with the front tires blocked.

[tod-hog]

Good to know. I guess of the ratio is not freakishly low I got other issues.

[68F250]

Yeah like maybe a torque converter going out.

Be careful in case the jack fails you don't get crushed.

[flyboy2610]

Always, always, ALWAYS use jack stands!
FORDification members are precious. We don't want to lose anyone!

[67effie]

So did you find out why the high rev? Just curious my 85 stang does the same thing.

[tod-hog]

Not Yet. Hope to get to it today maybe tomorrow, definitely this weekend. Will leave feedback.