Rpms at 70 mph

[IH444t]

So i just bought a 1994 idi and my dad has a 1994 idi and both manuals. When i drive his on the highway at 70 his rpm gauge is 2000 and mine at 70 is around 2500. how do i check rear end gearing? what other things could it possibly be?

 

[dunk]

Either yours does not have overdrive and has 3.55:1 gears or it does and has 4.10:1 gears. Your dad's truck has a ZF5 and 3.55:1 gears. To verify axle ratio check axle tag, count turns, count teeth, etc...

 

[The Warden]

4.11's and a ZF with stock tires should be doing 2372 RPM at 70 mph, and 3.55's with the same configuration should be doing 2049 RPM. Those are pretty close to your numbers, so my guess is that his truck has 3.55 gears and your truck has 4.10 gears.

With that having been said, it wouldn't surprise me if your speedometer's off. If you don't have a GPS, you can download an app for a smartphone that acts as a speedometer and, since it's using GPS data, it should be pretty accurate. Another possibility is the tach's off...

With a ZF, the only other option is clutch slipping, although it it were that bad, I think you'd be having different problems...

 

[direwulf23]

A lazy way of getting an idea of axle gearing is doing a VIN decoder check.

1994 Ford F250, 2wd, 7.3L IDI, NA, E4OD

2006 Ford Expedition Eddie Bauer, 2wd, 5.4L, automatic

 

[PwrSmoke]

Warden's nailed it.

Tire size will change overall gearing and what the tach or speedometer reads... assuming either was accurate in the first place. Don't know about '94s, but many (if not all) Bullnose trucks read about 10% fast with the stock tire diameter. About 2 inches more diameter makes it perfect. Anyway, I agree with the Warden... most likely you have 4.10s. The door jamb tag will have the axle code and, assuming someone hasn't done a gear swap, it can be decoded to get your ratio. There are other ways. In direct (4th) the truck should be around 3000 rpm at 70 if it has 4.10s

 

[franklin2]

VIN won't tell you the rearend ratios. The axle code on the doorjamb on the driver's side sticker will. Post what that is and maybe we can decode it.

 

[The Warden]

Don't know about '94s, but many (if not all) Bullnose trucks read about 10% fast with the stock tire diameter. About 2 inches more diameter makes it perfect.

That's been my experience as well...especially at freeway speeds, my '84's speedometer reads 5 mph fast. If it says I'm doing 70 mph, I'm doing 65 mph, for example. It's actually closer to accurate at lower speeds. I was surprised to find that the speedometer on a '91 F-350 was dead-exactly on...it wasn't even 1 mph off! First time I had ever seen a Ford speedometer that was accurate

By th way, this is the app I have installed on my phone: https://play.google.com/store/apps/details?id=org.mrchops.android.digihud&hl=en I've really only used it to verify speed...but, compared to the GPS that I use for work, it's exactly dead-on, and I would trust a GPS speed readout over any speedometer unless I've verified the speedo's accuracy.

VIN won't tell you the rearend ratios. The axle code on the doorjamb on the driver's side sticker will. Post what that is and maybe we can decode it.

One small caveat to that...this assumes that the axle hasn't been replaced or regeared during the life of either truck. The axle code will tell you what the truck had from the factory, but the best way to determine it for sure is to either pull the cover and count teeth or count revolutions. I remember hearing about a trick once...tape a piece of string to the driveshaft, and drive the truck straight forward exactly one wheel revolution (mark the tire and have a friend stand outside to tell you when to stop). If the string's wrapped around the driveshaft 3 1/2 times, you have 3.55 gears; if it's wrapped slightly more than 4 times, you have 4.10 gears.

Hope that helps

 

[IDIoit]

this post makes me jealous. neither my tach or my speedo works.
too many other items to address before i fix these.

 

[direwulf23]

One small caveat to that...this assumes that the axle hasn't been replaced or regeared during the life of either truck. The axle code will tell you what the truck had from the factory, but the best way to determine it for sure is to either pull the cover and count teeth or count revolutions. I remember hearing about a trick once...tape a piece of string to the driveshaft, and drive the truck straight forward exactly one wheel revolution (mark the tire and have a friend stand outside to tell you when to stop). If the string's wrapped around the driveshaft 3 1/2 times, you have 3.55 gears; if it's wrapped slightly more than 4 times, you have 4.10 gears.

Hope that helps [/QUOTE]

That's why I said lazy. I should've added that as well. You're right, of course. That would only let you know what it came from the factory with.
The string trick sounds interesting.

1994 Ford F250, 2wd, 7.3L IDI, NA, E4OD

2006 Ford Expedition Eddie Bauer, 2wd, 5.4L, automatic

 

[chris142]

Mines pretty close to 3000 @ 70.

 

[The Warden]

Mines pretty close to 3000 @ 70.

Your sig says you have a T-19 and 3.55 gears...3000 at 70 mph sounds like 4.10 gears to me. My truck used to have a T-19 and 4.10 gears and would hit the governor at about 75 mph in 4th gear. If you actually have 3.55 gears, you should be doing about 2600 RPM in 4th at 70 mph. So, either your speedo and/or tach are a mile off, or something's wrong...or you have 4.10 gears.

FWIW...

 

[direwulf23]

I have not confirmed by mine is supposed to be 4:10 gears with an E4OD and I run about 1800 RPM @ 60 MPH, but I rarely get above that.

1994 Ford F250, 2wd, 7.3L IDI, NA, E4OD

2006 Ford Expedition Eddie Bauer, 2wd, 5.4L, automatic

 

[chris142]

I pulled the cover and its got 3.55. The tach may be off,I never thought about it.that would explain my 4400 rpm redline.lol

 

[The Warden]

I pulled the cover and its got 3.55. The tach may be off,I never thought about it.that would explain my 4400 rpm redline.lol

Without hearing it, yeah, I'd bet your tach's off with a redline like that A couple of people have put 4500 RPM governor springs on their IDI's, and they really scream at that RPM...not to mention hearing the valves floating

 

[IH444t]

Which number is axle tag
Wb-155 type gvm x259 Trans c axle-35

Tag on axle hard to read
S129g or c last letter
10 10 2 3h24