355 rears?

Transmission Ratio RPM Calculator | Spicer Parts ok here is a link. punch different numbers in to see..

 

Changing tire size and changing gear ratio is not the same. 2 related but completely different concepts.
I agree with the distance travelled per rev of the tire but not the simulated gear change. The truck with the numerically higher gear will always pull better no matter the tire size.
It;s simple torque multification and tire size doesn't change that.
Nobody has answered the number question yet. How do you arrive at say 3.55 etc. Whats the math to arrive at that number.
It really doesn't matter to me either way. it's just a subject I'm interested in

 

Number of teeth on the driven (ring) gear ÷ number of teeth on the drive (pinion) gear = gear ratio.

 

were not really going to get into this debate again are we?

 

Was just typing that Amodel. Guess he has no racing experience, Drag, Sprint cars, late models. Guess I shouldnt explain stagger. Ive probably been doing it wrong all this time.

 

If you change the tire size you can go to any shop with Cummins Insite and they can change the roll out of the tire without having to download a new cal. It is a simple ten minute job if that. Just change the tire size. Without doing the math on how many RPM's it will change I am going to say that it may only be .2-.4 mpg change.

 

Ya

 

Who me. yes I knew that and I know the coffee cup theory but how does any of that change or relate to tires changing gear ratio.
I know a couple things about drag racing so if you want to go there OK with me.

 

Read what you're typing and think about it for a second.

When you're turning wrenches in the garage, applying torque to a nut...10 pounds of pressure applied on a ratchet handle 12" from the nut applies 10 foot-pounds of torque to that nut. If the specs call for 20 foot-pounds of torque, you can accomplish that 1 of 2 ways...either increasing the pressure on that 12" ratchet to 20 pounds of force, or by grabbing a 24" long ratchet and applying the same 10 pounds of force. Different means to reach the same end result.

That is all that is occurring when you change tire size vs changing the rear end gears, except it is the nut (axle shaft) driving the ratchet. Same amount of power and torque are coming through the drive shaft either way. The 3.23 gears with the LP22.5 tires gain advantage with the smaller tires, but lose it in the gearing. The 3.55 gears with the 11R24.5 tires find the advantage in the gearing but lose it with the larger tires. It doesn't matter which way you go, the end result is the same.

 

I have 3.91 rears, the truck came with 11R24.5 tires. Pulling 140,000 lbs it always felt like it was lugging the engine. DD16 600 hp.
Recently I switched out the rims and put 11R22.5 tires on it. I gained about 80 rpm at 100 kph (62 mph) and it woke the truck right up. It’s more fun to drive now and will hold in top gear longer on the hills.
I keep religious fuel economy records, and I can’t see a difference there. Mind you it is winter, weather and temperatures have a bigger effect.
But as a side note, I have done two oil changes since the switch and my oil analysis shows that the soot levels have dropped in half. So I guess it was lugging.