Rear Gear Set Theory

[justinray]

So my problems with my tach have gotten me around to looking at my rpms and speed and options with Gear Splits. I'm currently running 305/70R16, 4.10 Gears, E4OD (OD is .71:1), and Stock Transfer Case (I assume Gearing in transfer case is 1:1 for 2H) which puts me at 1940 RPM at 65 Mph, With our stock torque curve putting peak torque at 1400 RPM, wouldn't i get better fuel mileage if my Cruising speed was at Peak torque? well for this to happen, i would need to change my Rear End to a 3.05:1, which seems really steep, so i figured i'd split the difference and catch the 3.55's, or something around there. Would this be a mistake? is there a reason such a torque based motor needs such pulling gears? how much would my towing suffer? Any thoughts or opinions would be helpful, thanks.

 

[LCAM-01XA]

Peak torque on these things is at 1800-1900 rpms IIRC, not 1400 - this ain't no 6BT. So you may very well be in the engine's sweet spot already, especially if you're n/a...

 

[justinray]

I am in fact NA, I just thought i read on here that our peak torque was at 1400.

 

[LCAM-01XA]

Pretty sure that number refers to ideal rpms in OD for best fuel economy with a turbocharged truck. My n/a IDI used to pull 20 mpg day in and day out while cruising at 1900 on the interstate, of course with 4.10 gears and factory-sized tires the road speed at those rpms was around 55 mph or so - it don't bother me, I usually leave early and allow myself plenty of time for the travel, but many folks find that road speed way too low.

 

[Brianedwardss]

Your 305/70-16 tires are 32.8" tall and 12" wide. Your truck is NA. Those tires on the highway are pretty wide, and probably have a fair amount of resistance. Your E4OD/4.10 combo is gonna be the best option for an NA truck with 305's. If you had a C6, I'd say go for 3.55's but you've got overdrive so you're set

 

[david85]

Peak torque is at 1400 as far as I know.
http://www.forgottendiesels.com/6.9_ford.html

The old 6.5/6.2 chevy V8s peak between 1800 and 2000?
Yes, lowering your RPMs when empty will help your MPGs, but not by as much as you might think. Anything at, or below 2000 RPM is generally expected to deliver good fuel economy.

Towing performance will suffer depending on your conditions, and you will find yourself having to drop out of overdrive more often to sustain hills. Getting a heavy trailer off the line will take a bit more pedal, and low speed manouvering on steep inclines (particularly in reverse) will be more difficult. Depends on how much you are trying to move, however. Its all relative, and if you are not planning to drag a 7000lb 5th wheel around most of the time, then you may like the truck better with taller gears.