I have been reading alot of posts regarding changing gear ratios. Can someone explain what the benefits are?

 

chicks will love you after you change them.

 

Well I don't know much, but I'll try to help a bit.

There are of course different sizes of gear ratios, and my understanding is that the bigger the tires you have the bigger gear ratios you need. I upgraded tires on my truck from stock 265/70/17 to 305/70/17 and I was told I should look in to getting larger gears as I will have lost some low end power.

I think some sizes are 3.55, 3.92, and 4.10. If I remember right the Ram's with the off road package come with 4.10's

I know that's not much info, but that's all I got.

 

HA HA HA


one of the main reasons to re-gear is from putting on larger tires to keep the speedo correct, and also keep acceleration like it was.

also the taller the gear the less work and better pulling power the vehicle will have. although you do kill your top end somewhat, you overall acceleration will be greater.

my truck has 4.10 axels. the other option was 3.73's from me having 4.10's that adds another 2000# to the GCVWR of my truck.

hope that helps


3.54's and 3.92's are for the 1500's. you can opt for 4.56 though.

the 2/3500's have 3.73's and 4.10's. after May 05 you cannot get 4.10's with the manual. the gearing is too high from the output of the tranny. my truck was produced before this, thats why I have 4.10's with a 6 speed

 

To put it in the most simpe form, torque and horsepower are directly related to each other. Horspower is the amount of energy required to keep an object in motion and torque is a rotational force, in essence how much force something twists like tour axle in this case. We use gears in places like trannys and rear ends to change the speed of the output shaft. Your tires are in essence much like a gear, if you change that then you are changing the output speed. When changing gears, you are accomodating for the difference in tire size or the change in ratio of HP to torque. When you go with lower gears like a 3.92 or 4.10, you are increasing the amount of torque being applied to the rear wheels and relieving stress on the motor and tranny. This is why the tow rating in many cases increases with lower gears, however your top end has been sacraficed because what you added in torque took from the horsepower. The motor will turn higher RPMs at the same speed as well as fuel economy will go down. This is why not all ram owners will experience the same when towing, they dont all have the same tire, gear, motor, tranny combination. My truck has 3.92 rear gears, 275 tires, a hemi and 5 speed auto and should tow much better than the same truck but with a 3.55 or 3.73 rear end. I hope that this helps.

 

Here's more than you ever wanted to know below. The first link gives you a description and a chart that you can follow (first, look up your gear ratio or whatever's closest to it, then, look up the tire diameter that you are moving to). This will give you an idea of the new gearing and RPM at 65 MPH. The second link is a long description on gearing by some 4X4 website.

By the way, hope I didn't spook you in my comment on your post about the Mickey Thomson Classic's II's and my being on the fence between 305s and 315 tires because of concerns over gearing changes with the 315s. I'm no expert on this stuff, but based on my calculations my 3.92 would be dropped to effecting gearing of 3.70 - I probably wouldn't even notice the difference. But, I use my truck as a daily commuter, so every bit of mileage/gearing efficiency counts. On the positive side, I figured that the 315s would give me about 1 1/2 inches more of ride height, so I'd get a poor man's lift kit with those - I've already hung up my truck on a log - glad I had skid plates.

http://www.louisvilleoffroad.com/pages.php?pageid=2

http://www.4x4wire.com/tech/applications/gearandtire/

 

This one did it fer me -

http://www-5.dodge.com/vehsuite/towi...axle_ratio.jsp

 

thanks for all the information

 

I am shocked that I had to order a 3.92 gear in my new Ram. The dealer said given the 20”s, Dodge only uses the 3.92. I really wanted the 3.55 but it’s not available. With the torque and horsepower of the Hemi I can’t understand why they don’t offer the 3.55 or 3.21 with the 20”s. What gives? All the ratios are so close I can’t imagine any big performance loss and a 3:55 with 20”s would have pulled 244 RPM off the tach. at 75 MPH.

A Ram 5-speed auto. going 75 mph with 32.5-inch tires, 3.92 axle gears in Fifth gear (..67:1 overdrive) and high range on the T-case (1:1) would look like this:

20” tires

(75 x 3.92 x .67 x 1 x 336) = 66185.28/32.5 = 2068.29 RPM

(75 x 3.51 x .67 x 1 x 336) = 59262.84/32.5 = 1823.47 RPM

(75 x 3.21 x .67 x 1 x 336) = 54197.64/32.5 = 1667.61 RPM

(65 x 3.92 x .67 x 1 x 336) = 57360.57/32.5 = 1764.94 RPM

17” Tires

(75 x 3.92 x .67 x 1 x 336) = 66185.28/31.6 = 2094.47 RPM

(75 x 3.51 x .67 x 1 x 336) = 59262.84/31.6 = 1875.40 RPM

(75 x 3.21 x .67 x 1 x 336) = 54197.64/31.6 = 1715.11 RPM

(65 x 3.92 x .67 x 1 x 336) = 57360.57/31.6 = 1815.20 RPM

 

hemi dave, I think that maybe you should read the post up above by me. It might help you understand gear ratios a bit better. The reason is for multiple things. A higher gear makes the motor and tranny work harder and can lead to an earlier demise, if you go too low it revs too high and you wear the motor out earlier. The ideal is to find a suitable gear ratio for the given output of the motor, gear ratios and tire size. While a 3.55 will lower the rpms on the top and give better fuel economy, that kind of difference will kill the low end, towing capability and make the motor work very hard to get up to speed. IMO, the hemi is not a very strong motor at the low end, therefore you would not want a high gear ratio, peak hp and torque doesnt mean much when talking about gear ratios. I will agree that it is strange that Dodge doesnt offer multiple ratios with the 20s but I will also add that I think the 3.55 is too high for this truck with 20's. Maybe a 3.73 but after driving a ram with 20's and the 3.92, I wouldnt go any higher, if anything I would put in a set of 4.10s. Gear ratios are not like what some people believe as tools to make you go faster ot get better mileage, eventhough they do have an affect on these things. There are, however, proper gear ratios. It is simple, you can gve in one direction but you take from another. You dont want something completely devoted to one end, but a happy combination. You start to get negative or pricey side effects of going to far in one direction. A 285 is a very large tire, it appears that you have done some math but top speed is not everything. The motor has to get to that speed to begin with. A higher gear makes the motor work harder, and that equels fuel as well as extra wear and tear.