I was searching around craigslist's auto parts section today, and stumbled across a posi-trac 4.10 geared 8.8 rear end from a 1st gen Ford Lightning. It seemed a little heftily priced at $450, but I don't have a lot of experience in the secondhand differential market. My F150 has a 4.9L inline 6, with the M5OD 5-speed manual, would this be a good combination with the 4.10's higher ratio? How much would you ask the seller for it?

 

Don't know much about the pricing on the rear Diff, but I had an 86 F150 St8 6 300 and 4sp with an unbelievable Granny gear. It also had a 4:10. I got like 10mpg with that truck. I was a great work beast but don't plan on trips. I finally sold it off about 10 years ago and the guy that bought it is still using it to haul his black top tank around with.

 

$450 for that setup is fair. It is a Trac-Lok setup and has the unique Lightning diff housing for better cooling.

 

Well I know the gear ratio in top gear on M5OD's was around .75 or whereabouts, where the granny 4's ran direct drive in top gear, so I'm sure that would lower consumption a little bit at highway speeds. And considering I drive a little spirited on the highway anyways, having less of a leadfoot might offset it even more.

 

What tire are you running? Your cruise rpms even with a 32" tire start to run into the 2500+ range at 70 mph with a 4.10.

Otherwise, the 4.10 and the 5 speed are a great combo.

 

I run 31's, I sit at 2500 going 75 right now. And if i find some lightning wheels I might eventually lower the truck a bit

 

4.10s are a bad choice for a 4.9 IMO. You'll be turning 2700-2800 RPM on the highway. That's not a happy place for a 4.9L.

 

Assuming the 0.75: overdrive is right, 2700 rpm with 4.10s and 31s would be 81 mph. 60 mph would be 2000 rpm, 70 mph is 2330 rpm. Not terrible, but certainly not a good place for decent mileage with a 300. And you probably don't need it going that fast for decent power either. I'd want taller gears.

 

Agreed, that's a tall gear for a 4.9. That rear end being out of a GEN 1 Lightning, it's likely that it has pieces of friction plates in the bottom of the housing. Nothing a simple rebuild can't cure though.

 

M5OD-R2 have a .80:1 OD.

 

In that case 60 mph = 2130 rpm, 70 mph = 2490, 80 mph = 2840 rpm

 

ok. I have 4.9 ZF5 s-47 with 2.73:1 rear end.
In 4th gear I want to be able to run 2500 rpm while towing 6000 lbs. flat ground. What rear end ratio do I need ?