355 rears?

The numbers are the gear ratio...3.55 is a 3.55:1 ratio, in that for every time the drive shaft turns 3.55 times, the tires turn 1 time.

Tire size ABSOLUTELY affects engine RPM at speed, same as a gear ratio change. A 40" (LP22.5) diameter tire covers a certain distance with every revolution. A 42" tire (11R22.5/LP24.5) coveres more distance, so that the drive shaft only needs to turn 3.36 times with the 42" tire to travel the same road distance as the 40" tire takes 3.55 revolutions of the drive shaft. THAT is how it matters, and why it is equivalent.

 

Like I said myths! I ordered a new wilson hopper in 2016 and they thought I was nuts because I wanted lp 22.5 Michelin tires. I just asked them to build it and not worry about me wanting lp 22.5s. Another myth is running max psi. Its great for fuel milage, not ride or wear. I run 1200 bushel of corn in a 42' trailer with 90 psi in tires. They still look good. Tall rubber will wear longer in mileage sense but the added weight, rotational mass, higher cost, and low resale/casing credit make them pointless unless you have a very specific need.

 

All of that is pretty much irrelevant to the OP's desire to change gear ratios in the rear. Swapping tire sizes to put the engine RPM's close to where they'll be with a gearing swap will help determine if the engine likes to run at that new RPM or if he'll be creating other problems...and BEFORE he spends time and money doing a swap that is not easily reversed. The price difference between 11R and LP is miniscule, so if you're needing tires anyway it is pretty much a free test of a slightly different ratio. If he's happy with that change and wants to test a little farther, swapping from 22.5 to 24.5 and running 11R's will get him a little closer to what he wants. If at any point he decides he DOESN'T like running at the lower RPM (for instance, puts cruising speed below the meat of the power curve...gutless, so to speak) all he needs to do is run buy a tire shop and swap back...either at the next tire change, or sell the larger tires as "used" to defray the cost of going back. If, however, he's happy with the new gearing then he can commit to the more permanent gearing swap in the rear ends and put the LP22.5 tires back on his rig.

Rear gear sets for my Mack run around $2500 each for the cheap remanned drop-in units...ones with 1 year warranty on up...and then there's the labor on top of that. Not to mention, the truck is out of commission for a few days while the work is being done. Don't like the change? Want to swap back? HOPEFULLY the shop still has your old gear set and haven't sent them back...otherwise, you'll have to buy another gear set...plus more down time plus more labor costs to put it back.

 

In terms of revs per mile I agree with that.
3.55 turns of the driveshaft to 1 turn of the wheels is a product of gear ratio How are the numbers calculated .What causes the numbers to be 3.55 or 4.11 etc.
If you have a 3.55 turn to 1 wheel turn that ratio doesn;t change if you change tire size. you may go farther but that ratio doesn't change,
Now lets add in starting at the bottom of a hill. for eg 3.73 with tall rubber and 342' with short rubber. all else being equal the truck with the 3.73 will reach the top first.
Also the 3;55 assumes direct gear in the transmission .what about at say 10 mph

 

Lets do some math. I'm pulling these numbers out of my head FYI.

Lets say the truck moves 75k miles a year and at 6.5 mpg that will work out to about 11,538 gallons of fuel.

Now lets say swapping the diffs nets you a 0.25 mpg gain. That's 11,111 gallons. That's a difference of 427 gallons. At $3/gal that's $1,281/yr savings. That means it'll take over 350,000 miles to break even if it costs $6,000 to swap out 2 rear ends. For such a poor return on investment I'd stick with having more torque going to the ground to be honest.

 

Driver habits are were your largest gains are achieved.

 

No argument with the last 2 posts.

 

A 40" tire with 3.55 rears turns the engine the same speed as a 42" tire with 3.36 rears or 44" tires with 3.23 rears. Swapping from a LP22.5 tire to an 11R24.5 tire WITHOUT changing the rear gears will simulate running a 3.23 rear end with LP22.5 tires. The actual ratio doesn't change...only the distance travelled per engine revolution. So if the OP wants to test how the truck will pull with a slightly different gear ratio without the expense and down time swapping the actual rear end gears, a simple tire swap is the way to do it. Takes no more time or money than it takes to replace the tires...so if you're buying tires anyway, it's a free test.

 

bulldog is correct, however by the time you go to 11r24.5 tires you will on have a good test on were the rpm range is. your mpg will be null and void by the added weight and rotation of the tires.