3.25 vs 3.55

All other things the same, the higher the final drive ratio number the higher the rpm at the same speed. In other words, higher is lower geared.

 

I have the CM-871, the previous generation to your motor. I have the 3:42 gears in the rear, with an 18 speed. More than likely the same ratio as yours. At 65 mph in high gear, call it 18 for ease, or 13 on your truck, I run it on the low side at 1650 rpm. 1650 is where the fuel mileage is. You, are wasting fuel and lugging the motor. Lugging the motor leads to a fretted liner, normally #5 cylinder, and in frame before it is due. Drop down a gear on the splitter, take it to the low side and get those RPMs up and slow down. Just because there is a high gear does not mean you have to use it.

 

In regards to the delete, who did it, and do you have a boost and pyro gauge? Did they leave all of the equipment on the motor? Did they empty ALL of the cans to the walls?

 

Your RPMS will increase and you'll pull the hills slightly better.

 

It will give you higher rev's with those diff ratios its all about take off and cruising speeds the higher the ratio the better take off you'll have for a standing stop however you'll also have higher revs at top speed the lower the ratio eg. 3.25 you will get less acceleration on take off, however you'll also get lower revs at top speed so you lose pick up but gain on fuel economy ideally you want to be running at say 1400 - 1500 at say 65 mph to get premium miles per gallon fuel economy. Once you go beyond that speed forget about getting good fuel economy. Here are some fuel saving facts: FACT 1. A typical 420hp heavy-duty truck engine consumes fuel at the rate of around two litres an hour when left idling and stationary. FACT 2. If tyre pressure falls below recommended figures, rolling resistance increases and fuel is wasted e.g. a 10lb psi fall in tyre pressure is likely to result in a 1% fall in fuel economy. FACT 3. Fuel is directly proportional to the speed your truck is travelling. A 22% reduction in fuel consumption can be achieved simply by reducing your speed from 56 to 50 MPH

 

Your rear end ratio is the number of turns of the pinion gear needed for each revolution of the ring gear, or, in simplified terms, the number of times your driveshaft needs to make a complete rotation in order for the wheels to make one.
3.25 means that there's one full revolution of your ring gear and wheels for every 3.25 revolutions of the driveshaft and pinion gear. 3.55 means that driveshaft needs to make 3.55 revolutions for each revolution of the wheel. 3.55 being a higher number means the driveshaft needs to turn more in order to complete a revolution of the wheels, ergo, you'll run higher RPMs at any speed.

 

I Know this is an Old Thread, but I was wondering if anyone had an opinion on my truck setup. I don't have the truck yet, but I want to be an owner operator some day. I'm going for the match of power and fuel efficiency.

Peterbilt 567
3.08 Gear Ratio
Cummins X15 525HP 2050TQ
13 Speed Eaton

 

What tire size?