Gear ratio Help

[mrf1002u]

Hey guys,

53 3800 1 ton DRW.

The truck will end up with a 250 engine
SM420 Trans
8R-19.5's
14 bolt rearend

Question:

Is 4.11 gonna be the desired ratio or will something numerically lower be better or OK. I.E 3.73 or even a 3.42 that I have already.

Probably see more back roads than highways and it will pull a trailer at some point. (3000 lb tractor on the trailer).

Thanks,

Larry W.

[Foot Stomper]

Here is a fantastic web site to help you figure that out exactly.
Plug in 3 of the 4 fields and then click "compute".
You can play around with tire diameter, rpm, gear set.

You will want to find out what RPM your 250 will want to cruise at (2000 rpm perhaps?) with the most torque.

http://www.advanced-ev.com/Calculato...ze/?ratio=3.08

[mrf1002u]

Originally Posted by Foot Stomper

Here is a fantastic web site to help you figure that out exactly.
Plug in 3 of the 4 fields and then click "compute".
You can play around with tire diameter, rpm, gear set.

You will want to find out what RPM your 250 will want to cruise at (2000 rpm perhaps?) with the most torque.

http://www.advanced-ev.com/Calculato...ze/?ratio=3.08

Thanks. I'll check it out. Don't have the engine yet either but should be pretty standard for the 250,s. Your 2000 estimate sounds bout right to me

Larry w.
Posted via Mobile Device

[mrf1002u]

Originally Posted by Foot Stomper

Here is a fantastic web site to help you figure that out exactly.
Plug in 3 of the 4 fields and then click "compute".
You can play around with tire diameter, rpm, gear set.

You will want to find out what RPM your 250 will want to cruise at (2000 rpm perhaps?) with the most torque.

http://www.advanced-ev.com/Calculato...ze/?ratio=3.08

Thanks for that site.

33.5 inch tire (My rears are recapped 8R-19.5's right now)
4.10 gears
60 MPH

Yields an engine RPM of 2,467.

250 seems to want to be at around 1,800 to 2,000 for peak torque, so might be twisting a little hard to maintain 60, but I'll probably go the 4.10 route and see where we are for a while.
Haven't driven a 6 in a while so I don't remember what RPM's they seem happy at for long periods. In the long run I might do the headers and the 390 cfm 4 bbl carb, but not right away.

Any trailer I'd pull with 3,000 lbs. would have brakes. Also, whether I stick with the HO72 or go with the 14 bolt, Disc brakes are planned on the front and probably the rear too.

Thanks,

Larry W.

[Foot Stomper]

The old 6's are long stroke engines. They are torque providers that don't take kindly to high revs. (The power used to pull a trailer is torque.)

Keep the rpm's in line, the engine will run forever.

Your challenge is a tricky one for sure.

[mrf1002u]

I'll go the 4.10 option and see how it feels.

3.73 seems like the right choice on paper given RPM and torque, but as with many things, seat of the pants may not agree.

Thanks,
Larry W.

[mrf1002u]

Good news!

Today I picked up this 14 Bolt 4.10 from a '78 G30 motorhome for $150.00.

Nice shape, clean inside and came with the wheels to roll it around on.

Bonus: The guy had a 75' tall crane in his front yard to load it with. That's a first for me.

So, I guess I'll be reselling the one I bought with the 3.42 gears (pretending to be 4.11's) LOL.

Maybe someone with an auto trans could use it.

Larry W.

[mrf1002u]

BTW...

I may be able to hook someone up with a person in Oklahoma (and maybe another) willing to make an even swap. Your 5.14 3rd member in your HO72 for one with a 4.10 out of theirs.

This is simply something I was doing for myself and got a better response than I thought. If both parties are willing, I will provide the contact info and YOU take it from there. Nothing in it for me.

Larry W.

E-mail or PM me.