So I have been doing a lot of thinking about a zf5 mated with and aftermarket overdrive, then I discovered the brownie or auxiliary transmission. So the focus has mostly been on overdrive, and underdrive would be nice if I wanted to pull stumps or something really tough. Anyway it occurred to me the other day, what if I swapped the gears in my axle from 4.10 to 3.55 and still had a 1st under of about 4.50, i'd still have the low gearing I was use to for when I needed it but for the most part take advantage 90% of the time and save on economy.

Thoughts,
Devin

 

My F250 4x4 has a ZF5 and 3.54 gearing, still u can start into 2nd when empty. MPG around 18-21 depends on driving habits.

 

You mind sharing whats your tach say when your going 55?

Devin

 

Look in my thread Romel´s truck, there is a video 0-70MPH and u can see the cluster.

 

65mph is around 1800rpm in 5th gear.

 

I made a bunch of rpm charts with different gearing scenarios. It includes my gear vendor overdrive which is a 'lil much on top of a ZF and 3.55's. But the chart shows you what rpm's it will have in each gear:



Edit: so at 55 mph the rpm should be 1628. This is assuming a stock 31" tire...

 

I know 3.55s, with 35s, and zf is a pretty slick combo for good mileage. Got stuck in a pilot line for 45 miles doing 45mph (1300 rpm in 5th) got almost 25mpg in a lifted dentside with 13" wide tires, mostly up hill. Also, not nearly as sluggish as everyone says. Could start just fine loaded and towed just fine up to about 5k, past that it wasnt as happy. So, about 11-12k NA, would pull most hills in 4th no prob.

 

im running 3.55's in my turbo charged 5 speed 88 on 33's. I like the combo, im turning about 2k at 60 mph. I usually get around 18 mpg. I would like some more low gears when im towing heavy. I need more speeds between 2 and 5th lol. I always wanted to find a US Gear underdrive to set up as a splitter

 

Unfortunately it's kind of expensive, but a GearVendors Overdrive is a nice auxiliary transmission.
I'm turning about 1600 @ 60 MPH in double-overdrive with stock tires and 4:10 gears. 75 is under 2K.
It's been pretty reliable, and the gear splitting is nice when hauling a heavy trailer up hill from a stop.

 

How do I make one of these, I know I've seen the calculations before and they were easy but how do I account for the tire size?

Devin

 

go here Gear Ratio Calculator Will tell you every/anything you could want to know.

 

^^^that's a neat calculator hairy^^^

I did mine the hard way... sort of... I calculated everything in Ecxel spreadsheets. I calculated tire circumference and converted it to miles per revolution and multiplied minutes X 60 to get the miles per hour. Of course for my chart I chose the mph first and back calculated the rpm based on the gear ratios. It's really easy with Excel formulas to drag and apply an operation to a whole column, etc., I made pages and pages of this stuff to compare different transmissions, axle ratios and tire sizes. It was a little confusing at first but once I got all the formulas in I could make them quickly and I had the satisfaction of knowing it was right because I did the math myself.