First, I know I can look at my axel tag and get the ratio. I will do that this weekend. But until then, check this out....

I have a 2000 7.3L, 2WD.
I have Michelin LTX M/S - 265/75/16 = (31.8 Diameter)

I have used the 2000 RPM method many have posted here....

Posts here say at 2000 RPM

4.10 = 62 MPH

3:73 = 70 MPH

MINE = 65-66 MPH

Infact, I have measured my RPMs at 5 mile incriments. (flat highway, no wind)

Speed RPM
60 = 1850
65 = 1950
70 = 2100
75 = 2200
80 = 2400

Someone here linked to a website that calculates Speed to RPM for you.

RPM Calculator

Tire diameter = 31.8 (from Michelin website)
Tranny high gear ratio = .71 (per FTE)

If I plug in my info, and a 4.10 or a 3.73, I get exactly what others here have said they get. For me to get the speed-to-RPMs I am getting I have to plug a 4.00 rear end ratio. Then I get all my readings almost exactly!

So...Let's assume I don't have some "special" rearend ratio unique to my truck. What else could cause this fluctuation??

A 4.10 with a slipping tranny? (Did any 2000 7.3L Excursions get 4.10's?)

I am at a loss! Any Ideas?

 

Your tire diameter from Michelin is no load. You are probably 1 inch shorter with the tire mounted. Sounds like you have 4.10s as my 1ton has those exact rpm at speed comparisons. To answer the question about ratios available 3.73,4.10 and 4.30 as I know from posts here.

 

Are you going by your analog speedometer or digital from a GPS for your calculations?

 

My understanding is as follows for Excursions: (F250/350's slightly different story)

7.3L and 6.0L got 3.73's ONLY
5.4L got 3.73 or 4.10
6.8L got 3.73 or 4.30

Joe.

 

Analog. Didn't have a GPS. However, the other comparisons I found here (a lot) were both analog and GPS. The most I found variation on the ones who did both were 2 miles an hour.

 

Yeah, 3.73's only was what I understood as well. But my RPM to Speed ratio seems to be significantly higher then 3.73.

I noticed this before in discussion about towing. My RPM seemed to be higher then others I was talking with. Also was watching a video on youtube of a 7.3L pickup demonstrating 60 - 80 acceleration with a DP tuner on 80hp. I noticed they were turning significantly less RPMs at 80mph then I do. This got me thinking....

 

Interesting, I ran the numbers again with 3.73 at 60mph and adjusted the tire diameter until I got the 1850 rpms I am seeing. I would have to be 2.8 inches shorter to get 1850 rpms. Not possible with E load tires at 60 PSI. Good thought though.

 

The mystery continues....I just checked my Diff tag. L73, which means I have a Limited Slip 3.73. (at least I know I have a LS!). But why the big speed to RPM difference when compared to other 3.73's?

The Orig. Owner said they had the tranny replaced at 80k. Is there a chance I have a different type/model tranny that has different gearing?

Could a mechanical problem be causing the difference?

I'm at a loss.

 

Another explanation is that your tachometer isn't completely accurate. It's certainly not uncommon for Ford to put a gauge that isn't accurate in their vehicles. I'm working on a Taurus whose tach is about 300-600 rpm (low, in this case) off depending on the speed.

The only way(s) to really verify is to raise the rear axle and spin the wheel and do the count yourself or to open up the rear diff and count.

 

that speed/rpm/gear calculator must also be assuming what drive ration you're getting out of your transmission. who's to say you're at 1:1 at that speed range? i don't know anything about the gears in the tranny's, but odds vare you're not 1:1, could be 1:1.3 at fina OD cruising.

 

The transmission would be at 0.71:1 like he said in his first post at the top of this thread.

There is no chance that there is different gearing in the transmission. That's so expensive that it becomes impossible.

It is possible that the torque converter is not locked when you expect it to be locked. That can add anywhere from 200~600 RPM.

Neither the tachometer nor the speedometers are completely accurate. Your gauges could be off a little more than usual.

 

Wow! Really! That's unbelievable to be that far off. I figured 100 rpm would tops, and completely unacceptable in my book.

 

Thanks Mark. It sounds like a combination of poor Ford tach/speedo and/or non-locked Torque Converter is probably at play here.

I have a DP tuner coming. I know Jody's tunes change the behavior of the tranny. Do you think this might help if there a TC lock up issue?

 

It might help.

 

No, i wasn't assuming they were changed, just missed the fact he DID plug in the gear ratio. i was implying i didn't know much about these trannys so had no numbers to assume they would be myself.