I am looking for input on swapping out gears. I have 34.8 diameter tires, a 6 " lift, 5.0 V8 engine and a pretty tall 3.31 axle ratio. I've reviewed some charts and have come to the conclusion that a 4.10 or 4.56 gear ratio would be best for overall balance between performance and fuel economy. Sixth gear goes okay but any slight incline like going over a highway overpass, she'll need to kick down to maintain 70 mph. I know its a heavy 4WD but would like to have a bit more giddy up off the line. I got an estimate at a local shop for the gear change. It's not cheap so I'd like to make sure I am making the right choice so would like to hear from those who have done similar. My truck is a 17 so it still has the 6 speed.

I plugged in some formulas into a spreadsheet and have calculated speeds in different gears and have adjusted gear ratios and tire sizes so I can see the estimated effects. What I can't tell though is just which works and feels best in the real world. My goals are to get some improved off the line performance, utilization of my 6th gear without having to down shift over small hills and maybe a slight improvement in fuel economy.

Here are some of my calc results:
Tire dia Gear Ratio MPH @ 1800 RPM
31.9 (stock) 3.31 75
34.8 (mod) 3.31 82
34.8 4.11 66
34.8 4.11 73 MPH @ 2000 RPM
34.8 4.56 59
34.8 4.56 76 MPH @ 2300 RPM

So, going with the 4.11 would get it close to the stock setup with about a 200 rpm increase & going with the 4.56 would require a 500 rpm increase over stock to get to same cruising speed of 75.

I am leaning more towards the 4.56 but wonder if that might swing the MPGs too far down to where I might regret that decision. I don't have anything to tow at this point but a small boat, jet ski or side by side could happen.

Which ratio would you choose and why?

One more thing - what's the best route to get the speedo / computer corrected for the changed tire sizes and gear ratio? A tuner or can this be done with by some other means?

 

 

Thanks for replying JKBrad.

I do have the 8.8. My shop did say that the 4.30 would be a good pick but apparently there is not a gear vendor that offers that ratio. Not sure if it that is just the ones he deals with or all of them but I really have no reason to doubt him at this point.

You have any recommendations on how the speedometer can be corrected for the different tires and axle ratios? I am not familiar with what are the popular tuners for the F150. I had used SCT tuner on my Mustang but that was back in 2011, maybe there is something better now?

 

I believe you can modify the tire size to correct the speedo using Forscan. Its a future windows product being developed and you can download a copy and use it on any Ford vehicle today for free, but one day it will be a commercial product that can be purchased. You would have to purchase a special OBDII connector to use it. I've used it and it works pretty good. Then of course most of the tuners allow you to perform similar changes.

 

Thank you for the replies. I bit the bullet and had the shop install 4.11s and I am very pleased with the results so far. I have only driven it about 50 miles but I can tell already there is not the strain it seemed to have and the giddy up and go is much better. I was also thinking about adding a torsen in place of the factory e-locker but I held off simply because that was more of a want than a need and momma was already getting upset after spending money on new dual exhaust and the gears.