I have a 1999 f250 SD and im looking to swap out the stock 3.73s,, i have the 5.4, 4" lift, 35x12.50's, superchip progammer, flowmaster exhaust and an air-aid intake. im thinking of 4.10s but considering 4.56 or 4.88s cause after my tires wear out i want to go with either 37s or 38s with more power upgrades. any ideas on what gear ratio would be the best? would it help my gas mileage most of all??

 

I'm sure you'll be getting some great advice shortly from more experienced folks her, but for what you mention in your post (35" tires) I would probably go for 4.30. With your new 37" or 38" I would most likely go for 4.56. I'm sitting a bit higher with my truck running 38" (will be going for "39 or 40" later this year) and I'm pretty sure I'll be going with 4.56 to keep it a little more highway friendly. I think 4.88 might be a bit too much for what I'm looking for. I'm hoping it's a good middle ground for me with maybe a slight edge on the crawling side when off road and some torque off the line. I have the V10 with a few addons that produce some extra horses and addittional torque compared to your 5.4 so it would be wise if you waited to hear from some the fellows here with expeience with a similar setup you have.

I does depend on what your ultimate goal is. Better highway, city or off road crawling/torque performance.

Rick...

 

The 5.4L should really have 4.10s or 4.30s with stock tires, so with 35" tires I would go with 4.56 minimum and 4.88 if you don't run real fast on the highway or might go to 37s eventually.

 

thanks for the imput. i think im gonna go with 4.56 gears but i want to see what other people say first befor i make my final decision. i love spending time up in the hills on logging roads and when i get new tires i want to be ready to push them with plenty of extra power.

 

I would actually consider 4.56 with 35's and 5.12 with 37's. That will put you at and equivalent 4.3 ratio for those size tires if you had 31" stock tires. You can use the calculator below to play around but I would suggest you use an equivalent ratio of minimum 4.1 or really 4.3.

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

 

on the highway i tend to cruise around 60-75mph and the accasional 90+ passing slow drivers lol do you think 4.56 or 4.88s will help with my gas mileage? i get about 12 in town and if im lucky 15 or 16 on the highway

 

jump to 4:88's..

if you do go with 36" or taller tires down the road ur gunna want that deep of a gear set to help with that 5.4L

 

I would go 4.56.

 

With 37's or 38's and a 5.4, I'm totally with Monster and going 5.12 as a minimum just to maintain decent performance. You're talking some massively heavy tires.... Even with 5.12's your 0-60 times are going to drop at least 1 second.

And if you're the least bit concerned about fuel economy, the LAST thing you should be thinking about is a tire that big! You'll be lucky to get 8 mpg with ANY gearing you chose. Simple physics.
My experience is loss of ROUGHLY 1 mpg per inch of tire diameter. Lots of things affect just how much, like width, actual weight of tire, construction of tire, tread compound, tread style, etc.

 

As a fellow 5.4L man with 35" tires, I'd recommend you do 4.56 gears if you stay with your current tire diameter. I have 35s and 4.10 gears and it is barely adequate, this coming from a 5-Star tuned 3v 5.4L at that.

Your 260hp 2v 5.4L is going to need all the gear it can get, so 4.56 should be a good match with the 35" tire.

If you go to 37s and ever want to use 4x4 High without toasting an automatic or burning the clutch on a manual, I'd strongly consider the 5.13 gearset since you have a overdrive transmission.

For example, most Ford Super Duty transmissions have an overdrive ratio of ~.69:1
Add a 5.13:1 ring and pinion ratio.
Now add those 37" tires
Bring the vehicle speed up to 70mph
in 2wd or 4x4 high your engine is turning 2250RPM

That same setup except swapping to a set of 4.56:1 gears would put you at exactly 2000RPM at 70MPH.

Drop down to 4.10 gears and you'd be turning ~1800RPM with those 37s.


See how gearing is not making as much of a jump at highway speed as you thought?

 

I agree... Totally. Go with 4.56s and 35s. After researching for myself thats what im gonna do when I can afford it.

 

wow, thanks alot guys for the awsome info and your input. im gonna go with 4.56s. i still want to go with 37s but i think im gonna add power, and lots of it. motor is gettin a full rebuild this summer anyways so i plan to do some decent internal work. i plan to upgrade the fuel delivery/computer, comp heads and cams, all forged internals and a ford racing blocked bored .030 over

 

Do a search using "RPM Calculator" for a lot of information.

 

Great post Backwoods99. I read your post because I am dealing with a similar problem. I didn't want to start a new post because I felt it wasn't necessary. The only main difference I have is a 6.8L V10 motor. My question guys is does everything you recommended to Backwoods99 apply to my truck or because of the larger motor I don't have to gear down as much? Perhaps, gear one step up. I hardly do any highway driving with my truck. I am currently running on 35's but would like to perhaps later go up to 37 or 38's. So what should I expect if I gear down to 4.88 with my 35's? By the way I don't care so much about mileage. If I did I would have never bought a V10 pig. I am more interested in how the truck will behave with the 4.88. Will it hurt my engine for example?

 

It will not hurt the engine if properly maintained. These were designed to run at higher RPM's than the old V8's.

With the additional torque of V10, you can get away with lower (numerically) gears and keep the same performance.

My advice: If you are happy with current performance, use the calculators above to find out what gears keep the rpm the same at a given speed, and then go one ratio deeper (numerically higher) to make up for the loss in performance due to the (much) heavier tires. If using wide or really burly tires (like swampers), these add a LOT more weight so go two ratios deeper. adding rotating mass really kills performance, so you want a lot more power or gear advantage.