I'm planning to buy an F150 FX4, but I don't really know the implications of a couple of option choices:

1) What are the pros and cons of getting the 3.55 vs. the 3.73 limited slip axle ratio? I assume it's a power vs speed choice, but don't really know. Recommendations?

2) Is there a downside (other than cost) of getting the 18" cast aluminum wheels instead of the 17" wheels? I generally like the look of larger wheels and tires, and a bit of extra ground clearance would come in handy at times, but is there some downside to doing this? Also, I'm not looking for the huge rim/thin tire look that is currently popular - I'm assuming that the 18" rims will mean more ground clearance - not just thinner tires, correct?

If I go for the 18" wheels, there are two tire choices that appear to be the same specs:

B.F. Goodric OWL All-Terrain P275/65Rx18 OWL All-Terrain

or

Goodyear LT275/65rx18C OWL All-Terrain

I assume LT means "light truck." Don't know what the "C" means. Don't know why the goodyears are a $225 upgrade.

Which should I pick?

Thanks

Doug

 

Moved to proper forum.

 

Let's take the axle ratio first. The 18" tires have a larger diameter, so the 3.73 gears effectively give you the same gearing with the 18" tires as the 3.55's do with the 17"ers. The 3.73's are required with the 18" tires, the 355's are only available with the 17"ers. That was easy enough.

I can't give any reasonable recommendation on the tires, other than to say the "LT" (light truck) tires are a must have if you do a lot of driving on dirt/gravel roads. The standard "P" (passenger) tires will get chewed up in short order. That being said, I'd upgrade the 17"ers (if I went that direction) to a good quality on-off road tire, if that type of usage was there.

Finally, the "C" indicates the load range. Light truck tires come in C, D, and E load ranges. You'll only see the letter ratings on LT type tires.

 

You will be happier with the all around performance of 3.73s. If you (and anyone else reading this), dont know what those numbers mean, Ill tell you. With 3.55 gearing, that means that your driveshaft will have to turn 3.55 times to have your tire turn exactly one revolution. I have run a number of different vehicles, with both 3.55 and 3.73, and I FAR prefer the 3.73s. They provide good around town, and are in the perfect power curve on the highway. I always found myself having to downshift on the highway, with 3.55s, whenever I would encounter an increased grade. The LT tires are a MUST! The P (passenger, if you didnt know) rated tires do not belong on a truck. I just had to get 4 new tires (coincidentally the upgrade Goodyears), because all of the P rated BFGs were crap.

 

Polarbear, 17"vs18" rim sizes and 3.55 vs 3.73 have no relationship. Now if you are talking about a 31" tire vs a 33" tire axle ratio means something.

Doug,

What tire/wheel/axle package depends on your intended usage.

1. Mostly pavement driving - P-tires are your best choice(longer wear, quieter ride, better gas mileage) with your preference of 17" or 18" rims.

2. Modest amount of off road - LT tires(more durable, better traction), the 17" rim and the 3.73 for better response. Reason for the 17" rim is that for a given tire diameter there is more rubber between your rim and that rock. Sidewall is bigger.

3. Mostly highway driving w/o a trailer 3.55 gears(better mileage). Alot of town driving or trailer towing go with the 3.73 gears.

 

Consider that the 18's may reduce your GVWR. Also not every 18 wheel and tire options are actually taller than all the 17" wheel options ... Some of this will of course have to do with dealer selection in your area. Regardless I don't think 3.55 gear ratio is a good option for most uses, unless you plan putting very small tires on and/or pump 500hp or more out of the 5.4!

Note my perspective ... I'm running 4.56 gears with just over stock size 285/70/17's.

Oh, and the C is load rating.

 

Also with 18" wheels, you lose approx 500lb worth of towable weight, I personally would go with 17" wheels and save few bucks and get more tow weight. Only if you plan to tow trailers on regular basis. Besides that lot more tire choices on 17" over 18". Most guys said 3.73 limited slip is better choice, more power and can tow bit more weigh. 3.55 gear would give bit better gas mileage, though.

 

Respectfully- and strongly- disagree. The 18" rims/tires have a longer circumference than the 17". The question is actually moot- the 3.55 is the only available axle ratio with the 17" combo, the 3.73 is required with the 18" combo.

Here's a nifty tool to calculate tire circumference. Here's fitment formulas to calculate the effect of a size change on the final drive ratio.

 

Wheel size alone is only relevant to gear size if you drive on your wheels with no tires mounted! It becomes relevant at the point of calculating diameter or circumference of a tire added if a standard formula is used!

My 04 book shows 18’s as an option for 3.55’s. Also one of the tire choices for the 18" wheel has the same calculated circumference as a common 17" wheel/tire option. About 1/4 of the f150's I see on the lot recently are not shoe'd with factory list option tires so additional options can be had.

There are many factors to consider when making gear ratio matchup or option choice. Tire circumference is a primary contributer.

MPG is better with 3.55's claim is not always true as it would be dependent on driving style and type of conditions. In the case of these F150's and optimal highway conditions it is likely that better MPG could be seen with tire size being equal. There is a point of falling off the MPG potential edge or curve and I found with 33 inch tires that edge draws near.

Here's my own little gear chart that might help?

 

Nice chart- I like that!

 

Hey.....that is a sweet chart. I do like that. Good work.

 

Polarbear,

A 265/65/18 and a 265/70/17 are both about 31.6" in diameter and will both provide the same performance with respect to gearing. So my original statement is true that 17 or 18 inch rims are meaningless when talking about tire circumference and gearing. I stongly and respectfully urge you to consider that.

 

You're going to make me measure that out, aren't you? Seriously, though- revised order guide shows the axle ratio driven by the tire size. 3.73 is mandatory with the 18"ers on the 4X4, optional on the 17"ers. Ford engineering obviously has their own opinion on the matter.

 

I would put money on engineering not having anything to do with it!!! Though from an engineering perspective and argument sake ... most 18's weigh more than 17's. ... Nothing about the new f150's is engineered to that near of an edge from my observations yet!

No... I'd look at other factors for revised “order option” reasons such as supply chain, politics and such...

 

As a counterpoint- given CAFE requirements, if the 3.55's got better mileage with the 18's, they'd be standard. The fact that they're not, while companies are spending millions to make fractional gains in mileage, is telling. Just my own .02.