Changing my axle ratio....update

Try this for me, let me try to predict your RPM's at certain speeds. Try it out. If they match up then it must be a .87.
At 1400 RPM with a .87 OD, 24.5LP, and a 3.90 rear ratio you should be at 50MPH
At 1550 RPM with a .87 OD, 24.5LP, and a 3.90 rear ratio you should be at 55MPH
At 1700 RPM with a .87 OD, 24.5LP, and a 3.90 rear ratio you should be at 60MPH
At 1830 RPM with a .87 OD, 24.5LP, and a 3.90 rear ratio you should be at 65MPH
At 1975 RPM with a .87 OD, 24.5LP, and a 3.90 rear ratio you should be at 70MPH

Remember, you're talking about a 29 year old truck. Who know the transmission didn't go out in say idk 1992 and they replaced it with a reman 15 from 1975? Last month I bought a refusbished 13 that had 2004 stamped on it. You haven't had the truck long enough to say that this is the original transmission nor the paperwork to show it. If the numbers I posted above match up with real life, it must be true. Or you have some 4.30's in rear diffs.

 

The speed you see on the dashboard may not be the exact speed that truck is doing, same for the rpm.

I think the best way to find out what the OD ratio is:
run the truck in DIRECT at a fixed speed, record the rpm and the speed. For example 50 mph @ 1750 rpm.
Change to OD, and the same speed and record rpm again. Make sure speed is exactly the same. For example 50 mph @ 1380 rpm.
Of course you have to pick a speed to you can run in both gears.

Divide the OD rpm by the direct rpm, and you will know the OD ratio.

This way you eliminate any errors in your speed gauge, and reduce the error in your tacho.


The data I have on eaton/fuller transmissons goes back 45-50 years, but I can not find a 15 speed with 0.87 ratio. All of them are in the 0.78-0.8 range.

Of course somewhere in the past somebody may have put a different gear set in your transmission, but if that is the case I would expect it to be closer to 0.73 than 0.87.
0.87 is what you will most likely find in the splitter part of a 13 or 18 speed, not in the main box.

 

It would be nice to have the 2.64 gears and cruise in direct then when empty, bobtail or in a hurry, you can have an extry gear to split so you're not running at 1750 rpm just to do 70. I have 3.90 tall rubber and .73 OD I can lug at 1200 at 50 mph but much over 60 mph just wastes fuel. But I look cool watching the world go by. Oh yeh CAT 8PN 3406C

 

That matches up with what I posted. Two requests. Please check one of the other speeds, and please try to find out who the manufaturer of the tranny is. If you can't find the manufacturer on the tranny to get the manufacturer of the rear end, usually a truck with meritor diffs has a meritor tranny and vice versa with eaton. I haven't been able to find any information on a 15 meritor other than that they exist

 

It might not be very common now, but when the rears were just rockwell it was very common to be paired with a fuller transmission. We currently have three trucks this way.

 

Not all connectors are created equal mate.
I use these on motor swap wiring harness', when soldering is not an option due to routing or location (solder joints can fail when bent etc).
http://www.cableorganizer.com/multilink-butt-splices/

Worth every penny for critical connections, like ECM wires, and so on.

Martin