Gearing Education??

[CaFordDude]

Can someone please explaing gearing ratios to me. I understand the concept from a theoretical standpoint but am looking for a basic explanation.

My 2007 CC F150 is listed as having "3.55 Ratio Limited Slip Axle".

I ask because I want to have a lift done and upsize my tires a bit and am checking to make sure I am ok. I already have factory 275/55/r20's on it if that helps explain it.

Is my gearing ratio and axel a good setup or should I look at changing it?

 

[blacksnapon]

over simplified...... The pinion gear will spin 3.55 times to once for the ring gear.

 

[CaFordDude]

That is the theoretical part that I understand Vince. I guess maybe I am looking for a real world usage type explanation. What is good, bad, better, worse etc for different uses.

It may be a lot to type so if there is a link to somewhere I can read about it that would be cool also.

 

[blacksnapon]

That is the theoretical part that I understand Vince. I guess maybe I am looking for a real world usage type explanation. What is good, bad, better, worse etc for different uses.

It may be a lot to type so if there is a link to somewhere I can read about it that would be cool also.

the higher the number (4.10-4.56) the faster the motor turns to do the same job. But, it doesn't have to work as hard at it. Typically higher numbers are pulling gears, where lower numbers are highway/mileage gears.

 

[Brian_B]

I would guesstimate that the 3.55 are highway gears. That is what dads 97 has too. I have not pulled with it, but I bet I would not be too impressed.

My old ranger had 3.73s and was ok. However, my explorer has 4.10s. It pulls well. Just not as good of gas mileage out on the road.

As far as my unedumacated opinion goes...its a trade off. One way or the other.

 

[d-kuzmen]

Depends on how big of a tire you want to use. If you go up in tire size you can also go up in gear from 3.55 to 3.73 or so. As your tire size gets larger and you don't change gears it'll be like going from a 3.55 to 3.27 and so on. If you put some big monster tires on it then yes I would change the gear. You'll be able to tell once the tires are on if you need a gear change, you feel like your lacking power but you'll go faster once your there. Hope this help a bit.

 

[Skandocious]

Depends on how big of a tire you want to use. If you go up in tire size you can also go up in gear from 3.55 to 3.73 or so. As your tire size gets larger and you don't change gears it'll be like going from a 3.55 to 3.27 and so on. If you put some big monster tires on it then yes I would change the gear. You'll be able to tell once the tires are on if you need a gear change, you feel like your lacking power but you'll go faster once your there. Hope this help a bit.

Pretty much hit the nail on the head.

Perhaps this chart will help to clarify Aaron:
http://www.4lo.com/calc/geartable.htm

Basically, as you increase your tire size you want to numerically increase your gears. I stepped my tires up from stock 29.5s to 33s and my truck was a DOGG with my 3.55s. Had the 3.55s swapped for 4.10s a few months back and it really woke my truck back up.

Here's the kicker. Gearing and tire sizes have somewhat of a 'sweet spot'. Changing your gearing to a numerically higher ratio is NOT DIRECTLY PROPORTIONAL to gas mileage, as some people think. You also need to take into account ENGINE LOAD. Take this scenario for example. You're running 39" tires on 3.55 gears, so your engine is not turning very high RPMs to get those HUGE rubbers moving; hence you're putting a lot of load/stress on your engine; hence bad gas mileage AND the truck will be underpowered. So you regear it to, say, 4.56. HOLY CRAP the truck is pulling like a sonuvagun and... WAIT A MINUTE! Your gas mileage just got better too!! How the hell!? You aren't loading the engine up as much because it's having an easier time turning those huge rubbers at the higher RPMs. But you do need to be careful because if you overgear it, like say using 5.29s on 33" tires you're going to be running an excessive number of RPMs and thus burning more fuel than you need to for those measly 33" rubbers

A real life scenario of this? Me! My 3.55s with 33" tires was yielding an average range of 12-13 mpg. Swapped my gears for 4.10s and the truck was pulling a lot better, and hey what do you know, consistent 12.5mpg average

Moral of the story is, math your gear ratio to your tire size accordingly. That chart that I posted above is a pretty good reference to where you want to be.

 

[CaFordDude]

Ok well by that chart it looks like my truck may already be undergeared from the factory.

Like I said it is currently set up with the 3.55 with 275/55/20 tires (From the factory) the discount tire conversion chart shows that tire to be 31.91 inches in overall diameter.

I was looking to go to a 315/55/20 which has an overall diameter of 33.64.


I dunno it all seems so confusing to me. I just don't want the store to tell me I NEED to regear when it is really a maybe/maybe not regear.

When I sit back and think about dropping $5000 into my truck on a lift, rims tires, exhaust, intake billet grills etc...I almost wonder if it would not be a better way to go to sell my truck and buy a new one built the way I want it from the beginning.

Now a few facts not already on the table. I currently owe $33,400 on the truck. I am set to come into some money pretty soon that is going to allow me to pay off the truck entirely and have money for the goodies I want. So I guess do I take the $38,000 and drop it on a new one built from scratch, brand new warranty etc...or build this one up....Trying to make the wisest financial decision with the best built truck. I don't want to get back into a diesel, I just don't need it anymore so no need for that extra expense.

If I have to throw regearing into the mix and worry bout voiding axle/drivetrain warranties etc...

Many dealers around here will build trucks and warranty them as built. The dealers around here use 4wheel parts so I know it is a reputable company doing it.


So many choices.

 

[Skandocious]

If regearing is going to void warranties then I'd say buy a new truck built the way you want it... That warranty is worth some dough on a truck that new...

As for NEEDING to regear... With 33-34" tires you should be fine with 3.55s. It's not going to be the best rig for towing but it'll scoot around pretty good. My truck did okay when I had the 3.55s on 33s, but it was pretty underpowered. The reason I bought my gears was because we bought a 7-8k lb boat/trailer and I wanted to be able to tow it; and 3.55s on 33s wasn't gonna cut it.