I have an '05 5.4L SuperCrew 4x4 XLT with 3.55 gears. I got the 3.55 mainly because I don't tow anything that a Ford Ranger couldn't tow. I eventually want to get a leveling kit and tires in the range of 32-33". Will a 33" tire be a problem with the 3.55 gearing?

 

Hi TXpitdog, this will start a ruckus (not with our own Ruckus, I hope).

Do you notice loss of power? Some do, Some don't. One guy says he can tow 12000 lbs and not even notice it, but I notice a 2200 lb toad behind my P30.

If you want facts, when Ford optioned the 32 inch tires, they pretty much wanted you to use the 3.73 gears. Makes it come out the same as the 3.55 gears with the thirty inch tires.

What do you have now? Size will say the diameter.

I have the 3.55 open, don't tow with the little F-150 at all. My truck came stock with 30 inch tires. I bumped them about half an inch, and I can feel the power loss. Can I still drive it?
Of course, I just notice that it doesn't launch as hard. With 32 inch it would feel like I was taking off in second.
Still can do that, it would just be too 'high' for me. The truck is already high geared.
Tall gears are OK on the freeway, but taking off at a stop sign, even the stock size was not as quick as I would have liked.

33 inch tires will work, you can drive it, but you are gonna know they are on there.
Chris

 

TX, I put 33's under my screw with 5.4 and 3.55. noticed no change in power or rpms. I did however loose about 4/10 mpg but i only checked one tank so not a real accurate test. Most of my driving is city and slow speeds and according to some posts here my mileage might be better if I had 3.73 but oh well not gonna spend the cash to find out. Didn't buy it for fuel economy anyway.
Hope this helps with your decision.

 

I had 3.55's and put 33's on and actually got better hwy mpg... but had a hard time getting out of its own way IMO!

 

You will get better mileage with 33"s on 3.55 ratio axle. It's probably similar to have 3.27 gears at that point, which will allow lower revving on the highway and theoretically a higher top speed. Not that people bump the speed limiter on these things, as watching the gas gauge go down while driving is depressing.

Let's not forget the issue of unsprung weight as well. A 33" tire is HEAVY. Maybe not as heavy as 22" rims with low profiles, but they are much heavier than stock. This will adversely affect off-the-line performance as well as braking, due to increased rotational mass.

 

Assuming stock 30 inch tires the jump to 33 would be the same as changing the rear gears to 3.21 gears. Adding the extra weight, about 30 lbs a corner and she will be way sluggish out of the hole. Speedometer would be off 9% as would Odometer.

The problem with all the answers is that NOBODY is saying what diameter they went from! Stock ranges from 29.5 to 32.5!!!
Please mention what you went from as well as what you put on, otherwise it's not useful information.

Ford's idea was 3.55 with 30 inch tires, and 3.73 with 32 inch tires, which equals the same gearing...

Chris

 

Excellent point chris. Stock tire size makes all the difference in this discussion.

On a side note, these ratio differences could account for why some people feel their trucks are dogs while others are happier with acceleration. I have a 3.73 LS with the stock XLT 17" AT's. I am more than pleased with the get up and go. But a Lariat with a 3.55 non-LS & 18" rims would definitely be slower than mine off the line, assuming all things were equal under the hood.

 

I think you are right, but I have to ask, what size are your tires?

The number on the tire would say the diameter. In this example, 255/65/17, it would be 30 inches.
That would normally come with 3.55 rear, if you had a 3.73 with that size, it would make you 5% quicker, or another 11-15 horsepower for 'free'.

For those who don't want to use the various size finders around the web, the formula is;
Divide the first number by 25.4 (this converts millimeters to inches)
multiply that by .65 (or whatever the second number is)
then multiply times two,
then add the third number.

This is expressed as 255/25.4 = 10 x .65 = 6.5 inches 6.5 plus 6.5 = 13.0 inches, plus 17 inches = 30.0 inches.

or 265/60/18 would be 10.4 x .60 = 6.26 x 2 =12.52 plus 18 = 30.5 inch tire



Tread variance from brand to brand, other minor differences can give you an actual measured diameter of 29.8-30.3 but that can be disregarded in most discussions.
You can calculate it in seconds, or find it in a web size finder.
Or just post your size, and anyone that cares can find the number.
There are nine different stock sizes on these trucks, all between on either 17-18 inch rim.
To measure the actual diameter, you have to lay the tire on the ground and ‘caliper’ it.
The difference from site to site on the diameter is more related to tread differences.
Chris

 

I changed from 30.5" to 33" and I can notice a major difference with my truck. Mine is the HD with the 4.10s. With stock tires around 30.5", it will pull a hole shot with the best of them. I put 33" tires on it and you can tell the truck is a little more sluggish. But 4.10 and 33" is the equivalent to 3.73 and 30". So even with the 33" I still have more low end pull than the stock F150s with 3.73 if the tires are larger than 30".

 

Ct,
i don't want to know what your highway mileage is.

Chris, i have the 255/70 or 75 (i forgot) x 17". it is really jumpy off the line. But that "extra" 10-15 horsepower you mentioned should theoretically disappear on the highway, as this tire/gear ratio setup favors lower speed acceleration.

 

Actually the highway mileage with the stock tires is not that bad if you keep to 60-65 mph, around 15-16. I did get 12.5 mpg towing a 6000# trailer though again keeping to the 60 mph. However due to the extra pounds my truck has its around town mileage is around 12.

One of the reason I bought my Mustang GT this year, help offset some of the higher gas prices, at least that is what I told my wife to help justify, (if we buy the mustang we can save $75/month in gas....)

 

eForder
Tire Rack lists you has having 255/65/17, but without looking inside your drivers door, I can’t be sure.
Those tires would be exactly 30 inches. That would mean that with the 3.73 it would be at the optimum on out of the hole acceleration.
You would have a huge advantage over a Crew with the 3.55 and 32 inch tires. Or even the 3.55 with 30’s.
As to losing the horsepower at freeway, no, it stays there, as it’s torque multiplication, not true horsepower.
The only place you would suffer would be at freeway speeds, and only in gas mileage. But how much?
With your lower gearing you would still be way ahead of the highest geared trucks of just a few years ago.
Super high gears are done for CAFE and EPA emissions reasons, and no other. Your truck came with a 5% lower gear set, thus at worst would get 5% lower gas mileage on the freeway. It could easily get 10 percent better ‘going up thorough the gears’.
So around town, you should get slightly better than most (and it’s the around town that kills these trucks) and very slightly worse on the freeway. If you were careful with your throttle, you would be able to get more mileage in either situation than the norm.
As to ctfordguy with the stock 30.5 inch tires and 4.10 gears he would have been net 7% lower geared than you from the factory, and after increasing to the 33 inch tires
He is pretty much back to where you were stock!
So baseline Ford tire gear is
3.55 with 30 inch tire equals that 1.00
3.55 with 30.5 inch tire equivalent .98 or 3.48 gears(more/less what I have)
3.55 with 32 inch tire is equivalent .94 or 3.32 gears (TOO TALL)
3.55 with 33 inch tire is equivalent .91 or 3.23 gears (hard to drive)
3.73 with 30 inch tires equivalent 1.05 Nice increase in gearing.
3.73 with 30.5 inch tires equivalent 1.03 just a tad better that baseline.
3.73 with 32 inch tires equivalent .99 gears, or very close to the baseline.
3.73 with 33 inch tires equivalent .95 gears or frankly too tall.
4.10 with 30 inch tires (not seen this) 1.15 nice power, in OD still not low geared.
4.10 with 30.5 inch tires equivalent 1.13
4.10 with 32 inch tires equivalent 1.08
410 with 33 inch tires equivalent 1.05

So ctfordguy, you have moved your gears back to exactly stock with the 3.73 and 30. inch tires.
Now the hypothetical tire of 33 inch on a set of 3.55 gears is going to be very tall, and hard to drive. Some won’t mind, but then, some drive low riders, some drive monster trucks, etc. but it will be noticeable, and it will be out run by about everything off the line.
Chris

 

I'm going from a 255/70/R17 to a 285/70/R17. Basically a 31" to a 32.7"

 

That tire seems to be an aggressive tread, I find it listed as anywhere from 32 to 33 inches in diameter.

So your gearing which is now .97 of 3.55 or equal to 3.44
will be going to .92 of 3.55 or equal to 3.26 gears.
5% loss in gear power, and about like losing 15 ponies off the bottom.

With the 5.4 it will still pull OK, but you will be feeling it out of the hole.

If you had a 4.6 in a SuperCrew, you would not be a happy camper.
With the 5.4 you probably won't mind the loss too much.
Catback, intake and tuner and you would net more than you lose.
Chris

 

Chris-
I have talked to several dealerships and they all say they willl not change the gear ration and if they did they would not warranty it. Why is this?

They all also said that doing this would throw the tranny out of whack, as it is programmed for the factory gears.

My question is, in changing the gear ratio what potential problems are there (this is a truck I plan to keep for quite a while)? Can they all be simply fixed by using a programmer to reprogram the computer?