MoDy150

After reading vague comments about needing to re-gear after a lift install, I decided to go and do the research.

Here's what I have found. All info is based on my truck, 2008 F150 SCrew XLT, 3.55 gears, 4R75E Trans.

Q) Do you have to re-gear after installing a lift and upsizing the tires?
A) Yes...and No. The stock differential gears installed by the factory were matched to the stock tire sizes. Increasing the tire size will increase driveline component wear proportional to the tire size increase. If you are willing to deal with the increased wear or decreased driving response, then leave the gears stock.

Seems average cost to just change a set of differential gears is ~1500.
That does not include the tuner to reset the computer and gauges to account for the new gears and tire sizes.

I personally only have 17k miles and only commute 3miles a day, about 40 miles a week. I'm going to wait to install my gears until I can afford all the components I'm going to add with the new gears. (Gears, Larger Brakes & Tuner)

----My evidence that lead me to my decision----

Ford 4R75E is a four gear transmission; gear ratios: 1st(2.84), 2nd(1.55), 3rd(1.00), 4(0.70). This means 4th gear is actually OD and 3rd is D.

RPM/MPH Gear Ratio Equations:
MPH=(RPM*pi*TireDia)/(12*88*DiffGearRatio*TransGearRatio)
RPM=MPH*(12*88*DiffGearRatio*TransGearRatio)/(pi*TireDia)

Examples;
Stock; 3.55 gears, 0.70 OD Gear, 31.6 TireDia, 60 MPH
60*(12*88*3.55)/(3.14159*31.6)=1586 RPM
At 60 MPH, the RPMs would be 1983.

37" Tires; 3.55 gears, 0.70 OD Gear, 37 TireDia, 60 MPH
60*(12*88*3.55)/(3.14159*37)=1355 RPM

Therefore the Engine and Trans are working ~14.6% less because the wheels are going 60 MPH and the engine and trans are going slower.

It would seem this is good. But, I have been using more gas because the response is actually sluggish. So, to get the same response as driving stock, I press the gas harder.

To get the truck back to stock performance and "normal" wear and tear, I would need 4.157 gears.

60*(12*88*4.157)/(3.14159*37)=1586 RPM

But no one makes 4.157 gears, they make 4.1 gears. These will get me back to within 1.35% of stock performance.

Motive Gear sells the 4.1 gear set.

(to be cont...)

MoDy150

Below is my spreadsheet, The light grey section is the setup of the truck when I bought it. They had changed the wheels and tires. The one they installed were very close to stock sizes, but not exact.

Also, when the % diff is positive, then the engine/trans are working less for the same stock output. When the % diff is negative then the engine/trans are working extra for the same stock output.

The below gear sizes are based on the sizes available from Motive Gear for the 2008 F150.

MoDy150

If my assumptions are wrong or I'm way off base. Please...correct my thinking/logic.

Thanks!

rubbinsracin

wow. youre sure doing the footwork for this decision. props to you on that. i think what it comes down to is this:

if you are doing alot of towing, off roading and/or city driving i would stick a set of 4.56s back there. i under stand you will be giving up some at cruising speed, but if you do mostly the towing and/or city driving is making it easier for the motor to turn, hence forth giving you more performance.

if you are doing mostly highway driving with little towing or off roading you would probably benefit from having the 4.10s because you will see lower highway rpms. sure you are making the drivetrain work just a tiny harder, but the lower rpms cancel that out.

also, from what i have been told, once you hit the 35" mark you should go with 4.56s because they will help extend all the driveline components, yes you are giving up good cruising rpms. but the extra gearing will help turn them big a-s-s tires that have added alot of rotating weight.

ak_cowboy

From all the reading I've done it breaks down to:

>30" = Stock gears (3.55 or 3.73)
31"-35" = 4.10s
35"-37" = 4.56s
<37" = 4.88s

You can generally get your computer reflashed by a dealer to take care of the gear/tire change for about $100.

jerrybizzle

I would go with the 4.56, since your commute is so short and that way if u get the fever and eventually add 37s the 4.56 would still be sufficient. Like was mentioned earlier the 4.10s would get u back to stock equivalent, but that isn't factoring in the added weight and rolling mass of the larger tires.

schissler

Just because your RPMs are lower does not mean the engine is working less.

There is actually a HUGE increase of load on the motor despite the lower RPMs.

Bennyv83

That's right. By reducing your final drive gearing, you're reducing the mechanical advantage the motor has in moving the (subantial) weight of your truck. Remember that a gear set is just another form of a lever. Where taller gears move the fulcrum closer to the load reducing effort but reducing the amount of work performed (slower top speed) and shorter gears increase required effort but increase work (top speed)
Increasing your unsprung weight and reducing your drivetrain's mechanical advantage by increasing wheel size will require taller gears.

Johnson06

After I put on my 35's and 4.56 gears, I went to my local Ford dealership to get the speedo recalibrated. Since I have a 4.6l, he said the law where I live states that for the engine size I have, they can only re calibrate it for 3.73 gears and 33" tires. This is because the motor vehicles branch say's that my engine will not pass emissions tests with that gear ratio and tire size. What a bunch of Crap!!!! I'm pretty sure your not allowed to run a vehicle with a speedo that is reading wrong either. Anyone else have this problem?

Bennyv83

Buy a programmer then! Sounds like that's your only option