RT15618? direct drive 18

Tacho or speedo could be out? Simple enough to find if it's direct or overdrive, jack up 1 wheel, put in 18th, paint mark driveshaft and dampner on crankshaft on front of engine, turn engine over, if marks are still lined up it's direct.

 

There is a section on Eaton's website that lets you decode your transmissions numbers to let you see exactly what you have.

http://www.eaton.com/rr/CustomerSupport/Support/LiteratureCenter/index.htm

 

According to my data, the older 18's (XX6-18) had 0.85 ratio in 18 (Direct in main box, overdrive splitter) 17 is direct.

The newer 18's have 0.73 in 18, overdrive in main box, underdrive in splitter. 16 is direct, 17 is 0.86, 18 is 0.73.

With tall 24.5's and 3.73 and 0.85 in 18, you should be closer to 1400rpm at 55mph, depending on actual tire size.
In direct rpm would be above 1600 at 55mph.

As suggested above, tacho and/or speedo could be out

 

I say no, go with the OZ! Not saying anyone else is incorrect but there is another web site that lists road speed based upon all the variables. You could go at it in reverse and play numbers to get your trans ratio.
No tag no idea, it could be anything with an 18 knob on top.

 

If your tach and speedo is correct (speedo-60 mph=60 seconds-mile marker too mile marker) know gear ratio and circumference of tire (diam. x 3.14) 5280 ft./mile you can find the unknown which is the finale gear ratio of trans. If I had the info would do it for ya.

 

Well, not very good for my first post, but I tried to paste this off of a spreadsheet that I have from Eaton. It did not come out comprehensible, so I erased it. It gives the gear ratio for almost all Eaton transmissions (for each gear) as well as the percent step per gear, and the torque rating. According to this there is no RT 15618.

I have an RTOF 15618 in one of my trucks, and the overdrive is the same as listed below; .85.
The way that I calculate RPM vs Speed is as follows, and works well if you can get the exact revolutions per mile from the tire manufacturer:
RPM x 60 = Rotations per hour, in your case 1500 x 60 = 90,000
Divide by trans overdrive ratio; 90,000 / .85 = 105,882.35
Divide by rear end ratio; 105,882.35 / 3.73 = 28,386.689
Divide by revolutions per mile (somewhere around 470 for most 11R24.5 talls); = 28,386.689 / 470 = 60.39 miles per hour.

If you have 24.5 low pros, I believe the Revolutions per mile is about 501 depending on the tire, which then would be calculated as:

28,386.689 / 501 = 56.66 miles per hour.

If you have low pro 24.5s that puts you pretty close to what you have stated for your specs with a .85 overdrive.

If anyone would like this spreadsheet, let me know and I will email it to you. It is in Excel.