I have a 4" lift with 35" tires on my truck. It is gas - 5.4L. It lacks the power I want for certain towing situations - the occasional pulling heavy trailers, and just pulling buddies out of muddy messes. I want as much gear as I can get on the low end, but I don't know enough to know how much I would be sacrificing on the high end. I told my friend that I was going to an 8" lift, and either 38 or 40" tires. He said the 513's are the right gears for the 40" tires. Is he right? I do most of my driving on the road, and I don't want to sacrifice gas mileage too much. I will probably only put 38's on my truck. Can anyone please give me good advice on this and steer me in the right direction. Thanks -

 

This web site has some great automotive calculators that you can input your data and see the results, so you don't have to rely on goofy friends that probably don't have enough sense to buy a Ford.

Automotive Calculators offer to you by Ford Trucks Enthusiasts | www.ford-trucks.com

OK, first we have to know what your stock tire size was and your stock gear ratio. I'll guess and say you had 33s and you have 3.73 gears.

Going from 33s to 35s is like taking out your 3.73 gears and installing a 3.51 ratio.

To get back to stock gear multiplication you'd need roughly a 3.95 ration. 4.10 would pretty close, and in addition give you a little more gearing.

If you went to 40 inch tires, you'd need a 4.52 ration to get back to stock.

5.13 is not too high. Running a 5.13 ration with 40s would be like running 4.13 gears with 33 inch tires. That is pretty close to a 4.10 stock ratio. With that the factory says you can tow a bit over 10k.

One thing to be careful about; Most of the larger size tires are D rated, not E rated. If you're planning on towing heavier loads be aware of this and check the actual load rating of the tire.

The 5.4L and 6.8L gas engines aren't really sensitive to gear changes when it comes to fuel mileage. Members report about the same fuel mileage unloaded after going from 3.73s to 4.10s or even 4.33s. What will kill your fuel mileage is the lift.

 

i installed 285 75r 16 tires on my 04 with the 5.4 and hatted the pulling power and slug like bottom end. had 4.30 installed and its now like it should have been from the factory with no decrease in mpg. i would like a four inch lift with 35's but i dont have the money to regear again so it looks like im stuck with what i have. and i would install 5.13 with 40's it would be just about perfect.

 

Hey - I never saw my stock tires because I bought my truck with the lift. My spare tire is only a 31 though. Would there be any disadvantage to having the 5.13's if I only put 38's on? I only want to do the gears once, so I don't want to limit myself with too small of a ratio, but I don't want to unnecessarily limit my high end speed either. I just want to know that I can still pass a car when I need to, or merge on the highway no problem at 70 mph. Also, I just want to make sure I'm not taxing my engine with too many rpm's at interstate speed. Thankyou for the info and your help! - Jeremy

 

With 38s I would lean more towards 4.88s, but 5.13s would be okay, it will just hurt your highway mileage a little more. Really though, if you're running 38-40" tires, you shouldn't be thinking about mileage

 

I agree, Although the gearing may be ok with the 40" tires i doubt your gas mileage will be.
Im not sure here either but i think you would have a problem with the 4" lift and 40" tires. I hope if you go that route you get the truck setup properly. I don't think "I" want to be the one you pass at 70 on the highway.

 

With a 4" lift, 38-40" tires would require cutout flares. Something I've been considering...

 

The OP stated that he was considering an 8" lift kit and 38"-40" tires.

 

Good catch, I just have cutout flares on my mine

 

Redford, you have such a way with words.

 

LOL! Me too... I'm going back to work on May 10th so it won't be long before I finally install my cut-out flares.

 

To the OP, you can use the RPM Calculator (link below) to determine which gears are optimal for any size tire you decide to install. Your Torqshift has a "Transmission High Gear Ratio" of .71. Just enter the data into the link to determine the RPM's. As for tires sizes, I have not seen many 38" tires but I have seen many 37" tires. Therefore, I used 37" and 40" tires in the examples below. I used an average highway speed of 65 MPH but you can adjust that to any speed based on your area of the country and driving habits.

As for the amount of lift required to clear the tires, you most likely will need a minimum of 7" of lift to clear the 37" tires and 10" of lift to clear the 40" tires. Now, if you use Bushwacker "cut out" fender flares, you can reduce the minimum amount of lift to 2.5" (leveling kit) for the 37" tires and 4.5" for the 40" tires.

As for wheels, virtually all 37" and 40" tires have a minimum width of 13.5". You will need either a 9" (minimum) to 10" (preferred) wide wheel with back spacing in the 4.25" to 4.75". Therefore, your OEM wheels will not be compatible with these tires.

Here are the results with 37" tires:

Miles Per Hour: 65
Tire Diameter in Inches: 37
Axle Ratio: 4.88
Transmission High Gear Ratio: .71
RPM's: 2,047

Miles Per Hour: 65
Tire Diameter in Inches: 37
Axle Ratio: 5.13
Transmission High Gear Ratio: .71
RPM's: 2,152

Miles Per Hour: 65
Tire Diameter in Inches: 37
Axle Ratio: 5.38
Transmission High Gear Ratio: .71
RPM's: 2,257


Here are the results for 40" tires:

Miles Per Hour: 65
Tire Diameter in Inches: 40
Axle Ratio: 4.88
Transmission High Gear Ratio: .71
RPM's: 1,894

Miles Per Hour: 65
Tire Diameter in Inches: 40
Axle Ratio: 5.13
Transmission High Gear Ratio: .71
RPM's: 1,991

Miles Per Hour: 65
Tire Diameter in Inches: 40
Axle Ratio: 5.38
Transmission High Gear Ratio: .71
RPM's: 2,088

I have a 2005 F350 with the 5.4L engine and 4.10 gears. I have installed a K&N CAI kit and have a SCT tuner running a 93 octane "Performance" tune. Even with my truck empty, it doesn't feel strong at highway speeds until I reach 75 MPH / 2,225 RPM's. So, if you do any serious hauling/towing, don't be afraid to select gears that keep your engine's RPM's well above 2,000 to maintain good performance.

http://www.angelfire.com/space/slr/RPMCalculator.htm

 

Also, don't forget in regards to towing. The weight ratings are based on more than just motor and gears. 40's weigh...what 150bs more than the stock 31's, so that's a lot of extra strain on the axles, drive shafts,and other drive train parts. Just something to consider if your really gonna be doing a lot of heavy towing.

 

Yes, consider your future repair expenses. You will have accelerated repair issues more often regardless if you tow or not especially if it is used regularly or as a daily commuter.

Furian,
Your way too good to us.

 

31s to 38s, assuming you start with 3.73 gears, puts your effective gearing at 3.04. If you went to the 38s and the 5.13 gears, it would be like using 4.18 gears with the stock 31s.