gear ratio topic

[Trailer Trash 2]

I know its for dodge but gears are gears, I found this artical and it sounded interresting enough to post... regardless of you trucks gender.

Axle Ratio and Fuel Economy

In the previous issue of Turbo Diesel News, we discussed the effect of rolling resistance on fuel economy. Similarly, your truck’s axle ratio can have a negative effect on fuel economy if not chosen properly.

Simply defined, gearing, gear ratio or axle ratio are all defined as the number of revolutions that the driveline has to make for the tires to make one complete revolution. Axle ratios are typically categorized as high or low. By selecting the correct axle ratio for your application, you can improve your truck’s performance and fuel economy. Most owners choose the axle ratio for their new Rams when they purchase them. Changing the axle ratio is a costly process, so it’s important to research what you want before buying your new truck.

The following axle ratios are available for your Cummins 6.7L Turbo Diesel-powered Ram truck.




3.42

3.73

4.10

4.44

4.88


Ram 2500 Pickup

Ram 3500 Pickup

Ram 3500 Chassis Cab

Ram 4500 Chassis Cab

Ram 5500 Chassis Cab

There are also specific benefits for highway gears and towing gears. Highway gears pertain to the higher axle ratio (numerically lower) such as 3.42:1 to 3.73:1, and allow for lower engine rpm at higher speeds. Your engine will get better fuel economy on the highway if it is running at lower rpm, hence the term “highway gear.” This range of gears should be selected if you drive your truck most of the time on the highway and want to maximize your fuel economy. A lower (numerically higher) axle ratio, or “towing gear,” such as 4.10:1 to 4.88:1, will provide more low-speed wheel torque and pulling power. If your truck spends most of its time hauling a load, especially in town, then you should focus on this end of the range.

Given the high cost of regearing a vehicle, you’ll want your new truck to have the axle ratio that best suits your needs. If you’re looking at used trucks or you just want to double check your current setting, here’s a quick way to test the axle ratio in your truck:
1.Jack up the rear of the vehicle so that the rear tires are off the ground.
2.Place a piece of tape on the tire and on the driveshaft to help you count rotations.
3.Rotate the rear tire 10 times.
4.Count the number of rotations the driveshaft makes.
5.Divide the number of driveshaft rotations by 10.

For example, if you count 41 rotations, you have a (41/10) = 4.10 axle ratio.

Axle ratio is often overlooked and seldom changed, but it can have an impact on fuel economy if not correctly chosen for your application.

[Art-n-Marge]

This is only one of the factors that determines a tow or weight rating for a vehicle. It's not always about the bad-a$$ diesel engine/trans combination.

GREAT topic! I have a 3.73 differential so this is one reason I get good mpgs when I'm not towing. But I can't tow anywhere near many of you. Fortunately I only have to worry about towing my old '06 Monty. Getting a newer (and heavier) Monty will mean I need a new truck, but the MOC gave me all the learning to pick what I'll need.

Beware that many times just changing the gear ratio might not cut it. The price to do that means I'd probably seek a new truck anyway.

[7.3Ford]

It is nice to see Dodge gives you a choice. My 2001 F350 SRW only comes 3.73 and the 7.3 diesel engine. The F350 DRW only cam with 4.11 back then. I am glad to the big three all went to 6 speed Automatic, 4 speeds is not enough.

My solutions to my old truck was to put US Gears Aux transmission (no longer available) between the transmission and the transfer case. Now I have 8 gears, but when pulling I use 3rd over, 65MPH is about 2200 RPM, and it will pull most grades. I get lower EGT on with 3rd over, vs 4th gear. Gas mileage is same, but 4th is only good on flat lands

When not pulling I can use 4th over, and run 70 MPH at 1650 RPM.

[jimr]

I used to have a 2000 dodge 2500 cummins with a Bully Dog chip with the 5 speed manual and 3.54 rear end. I really struggeld pulling some grades and the gear split between 4th and 3rd was too wide. I solved the problem with a Gear Vendors unit that gave me 8 speeds. I towed in 4th over at 1800 rpm and 63 MPH all day. Worked great because it also took care of the wide split between third and fourth gear. I would get 12 to 13 MPG with that rig. Empty I could run at 80 MPH at 1600 RPM in 5Th over but the wind resistance stillresulted in only 19 MPG .

[7.3Ford]

quote:Originally posted by jimr

I used to have a 2000 dodge 2500 cummins with a Bully Dog chip with the 5 speed manual and 3.54 rear end. I really struggeld pulling some grades and the gear split between 4th and 3rd was too wide. I solved the problem with a Gear Vendors unit that gave me 8 speeds. I towed in 4th over at 1800 rpm and 63 MPH all day. Worked great because it also took care of the wide split between third and fourth gear. I would get 12 to 13 MPG with that rig. Empty I could run at 80 MPH at 1600 RPM in 5Th over but the wind resistance stillresulted in only 19 MPG .

Gear Vendors (GV) is still around, and I had one on my 1983 Ford diesel. It wasn't turbo'd and only had 4 speed manual. GV have couple limitations, but at least they are still making them. The two limitations are, can't use in 4-wheel Drive, and can't use engine brake (not sure why).

But is still an option out there for older trucks that need more gears.

[Capt Kidd]

We have a 2007 Dodge 3500 MegaCab with the newer Cummings. We have 4.10 gears and 4wd. Fuel mileage towing is about 9.5 to 10 mpg. We generally tow at 1700 rpm which is about 62 mph. I have tried various speeds and don't really see any difference in mileage. 55 or 70 just about the same.

[DQDick]

We have the 3.73 rear end, I believe, and get upper 10's towing at just over 60mph. With the 6.7 cummings we have only used the tow haul feature once however. This truck tows the Monty without any type of strain. Doesn't seem to know it's back there.