Can anyone explain this to me and the advantages of a higher vs lower ratio? ABN

 

When you see the numbers spelled out, such as 3.92 to 1, that means that for every time the driveshaft turns 3.92 times, the axle spins around once. So for 3.23 to 1, that would mean that the driveshaft would have to turn less times for the axle to spin once hence less revolutions of the engine per mile.

The higher number will give you faster takeoff, better towing, and all around better performance but at a tradeoff for slightly less MPGs.

The lower number will cruise at less RPMs on the highway using less fuel but you will lose some of the snappyness on takeoff.

I believe it truely depends on where you live and what you are going to do with your truck as to what is best for you. In my area, we have a lot of hills so the higher number works out better overall since the tranny isn't hunting gears as much. This overall leads to better fuel mileage. But if you lived in a flat area and didn't tow very much, the lower number gears may be just fine for you and would probably cause you to get better MPGs.

Just a note, if you wanted the limited slip rear end on this truck (both wheels locked together), you have to order the 3:92s. Helps quite a bit if you live in the snow belt. At least in my experience.

 

The higher ratio (3.91 vs 3.55) wil give you more power but the engine will run harder and get less mpg and with the lower ratio (3.55) you wont have as much power but will get better mpg. Think of it as a sprocket on a 10 speed bicycle the higher ratio (bigger sprocket, 1st gear) will make more power for going up hill but you wont be able to go as fast and with the lower ratio (small sprocket 10th gear) you have a higher top speed and wont have to work as hard to get there (better mpg)

 

The higher the ratio, the lower the gear. 4.10 much lower gearing than 3.21. The lower the gear at the axle, the better for take off power and towing. Many people have the misconception that the higher gear gets better gas mileage and that is not the case in many situations. With the higher gear, you will turn fewer RPMs at a given speed which can get you better economy on the highway as long as you are not hauling or towing heavy loads. The higher gearing will usually get worse mileage in the city as well. It is simple, it takes torque to get the truck moving and to keep it moving. The higher gear can't make as much torque and therefore must use throttle to make up for it burning more gas. With 20 inch tires, you raise the final gearing over say 17 inch tires. To get the best performance on the 09 with the common 20s, IMO you should have the 3.92 or 4.10 gearing. I have a sport that I bought as a demo and it has the stock 3.55. I would have been happier with the 3.92LSD, but I didn't choose.

 

Axle ratio is all about mechanical advantage.

Most 4X4 '09 1500s come with a 3.55:1 rear end. Simply put, for every 355 turns of the driveshaft, the rear axle will turn 100 times. A larger ratio, 3.92:1, 4.10:1, etc, will increase torque to the rear axle, improving acceleration and towing. The downside is that larger ratios get worse gas mileage, simply because the engine will have to run at a higher RPM at the same speed, versus the RPMs it would run at with a lower ratio.

 

Wow, thanks for the help guys. I'm in north Florida which doesn't have hills to speak of but may relocate back to western NC in the next few years where there are lots of big hills. About how much of a decrease in mpg does the higher axle ratio cause? I got rid of my '03 hemi ram because I was only getting 12 mpg in city.

Thanks again for the help.

ABN

 

generally, city miles improve a touch, on the highway is where you eat it with a low gear ratio. 3.55s make the engine work harder to get up to speed, but once it does, it runs at lower rpms and drinks less fuel. 3.92s make it easier for the truck to accelerate, but will run higher rpms once it gets to speed.

 

I've got the 3.92s and 20" rims on my 09 truck and have been getting just under 18 mpg. That is mix of terrains and types of driving. On my 05 which also had the 3.92s and 20" rims, I consisitantly got 14.5-15 mpgs. So the MDS and VCT in my case is getting me about 2.5-3 mpgs better all hand calculated. On the readout (which I have found to generally be high), you can watch the numbers go down in city driving and up when the MDS kicks in. Once you start to go above 60 mph or so, the numbers start to drop probably due to drag.

 

Well said. Your gear choice depends on your needs.

 

When I bought my RAM, I was pretty pissed that it didn't have the 3.92 out back. I'd really been set on a red or silver Sport with almost all the options. With the factories being shut for so long, it look an act of Congress to find one that was close to what I wanted. The only drawback is it didn't have the 3.92. Now why the heck a very heavily optioned truck wouldn't have that option is beyond me, but I caved and bought it. After taking it on a 800+ mile trip for the 4th, I'm glad I ended up with the 3.55 instead. My MPG was through the roof at 19.4 there and then 19.8 on the way back! While I'd love a little more power, the extra MPG is well worth the trade off.