Your original 5.10 gear was correct, this 7.89 is wrong. Here's the quick math:

1.00 / 0.70 = 1.43 or 43% reduction in gearing (Overdrive to 3rd gear, asusming lockup in both)
1.43 * 3.55 = 5.0765 effective rear gear to make 4th gear engine speed identical to 3.55 rear gear's 3rd.

 

I tow an enclosed car trailer with my F150 that tops out at about 6400lbs. Performance has been marginal with the original 3:55 gears. I have been towing pretty flat land up and down I95 between VA and SC. I am taking a long haul on I64 West from VA to MO in about 2 weeks. I had the gear changed from 3:55 to 4:10. Haven't towed yet, but I expect a noticable improvement. Driving around town normally, I LOVE the 4:10 gearing. Truck is MUCH more responsive to the throttle. Gas mileage is just not an issue. I monitor fuel consumption with every fill up and use a scan gauge. There is virtually no change in gas mileage in day to day use. Go with the 4:10s.

-steve

 

I considered 4.10's the other day. What rpm are you at 80? One of the reason's I wanted to go with the 3.73 is they are factory gears and my truck is brand new so I figured I'd have less hassle if I had a problem down the road.

 

80mph??!! This is my TOW truck !!
If I want to go 80mph+ I drive the 427 Shelby ac Cobra replica that rides in the trailer behind the tow truck!
For real, truck speeds are 64mph max towing, 74 mph max highway cruising.
rpm @ 74mph in OD are 2320 with the 4:10 gears.

 

Yeah, 80 MPH with 4:10s ia making that motor scream, especially if it is a diesel.

With 4:10s in my PSD, at 2k rpm, Im right at 65 mph. It takes a LOT of revs to get to 80 mph. And I think I can here the fuel tank draining itself at those speeds.

If I want to cruise fast, I drive my SHO Taurus.

 

here you guys go

http://auto.howstuffworks.com/torque-converter.htm

love that site

 

[edit] Lock-up torque converters
Because slip within the torque converter reduces efficiency and may generate excessive heat, some converters incorporate a lockup mechanism: a mechanical clutch that engages at cruising speeds to physically link the impeller with the turbine, causing them to rotate at the same speed with no slippage.

The first automotive application of the lock-up principle was Packard's Ultramatic transmission, introduced in 1949, which locked up the converter at cruising speeds, unlocking when the throttle was floored for quick acceleration. The demand for increased automobile fuel economy brought about a gradual but widespread application of the lock-up converter for automotive transmissions between the late 1970s and mid-1980s.






between these two my understanding is as long as your at a constant steady rate it is locked up but once you take your foot off the gas or apply more it unlocks

please correct if wrong(will still search)

 

I don't think it locks/unlocks with any acceleration but with a lot, yes. I think it locks out in 3rd when I'm towing and I'm going to hold off on gears right now. 4.10's are definitely out of the question because I want to be able to travel at nice highway speeds when not towing. I may still get 3.73's down the road but not for a little while.

Thanks for the replies.