Changing the gear ratio

[Graymist]

Is it possible to change the gear ratios on a truck, say for eg., from a 3.36 or 3.55 to a 3.90 or 4.10 ? How much would it approximately cost, and what other modifications would have to be made ? Thanks in advance.

[matcat]

Is it possible to change the gear ratios on a truck, say for eg., from a 3.36 or 3.55 to a 3.90 or 4.10 ? How much would it approximately cost, and what other modifications would have to be made ? Thanks in advance.

Why change from a 3.36? Unless you pull a lot of mountains it is a great ratio on an 18 speed.

[Graymist]

Why change from a 3.36? Unless you pull a lot of mountains it is a great ratio on an 18 speed.

3.36 won't be much good for hauling 140,000 lbs, especially over hills and dales .

[tracer]

Is it possible to change the gear ratios on a truck, say for eg., from a 3.36 or 3.55 to a 3.90 or 4.10 ? How much would it approximately cost, and what other modifications would have to be made ? Thanks in advance.

You're about to make a big mistake. If I were you I'd be changing gear ratios for FASTER ones, not slower ones. I have 3.73 in my 2004 International and put low profile tires and .. man ... even if I do 60 miles per hour my RPM is already at 1,375 (which is high for CAT). So I found a gear shop that can change my axle ratio to 3.58 or 3.42. They are in Canada, near Toronto, ON and the guy charges $2,000 cash plus you have to pay someone to take the diffs out and put them back in (4-5 hours of labour). If I have 3.42, I'd be able to do 62 MPH at 1,300, which is perfect.

Here's the formula to determine your road speed depending on axle ratio (AR), tires (REVs), and top gear ratio (TRAN).

RPM at 60MPH = AR x REVs x TRAN

To find out your rpm at a different speed, take the result from the formula above, divide by 60 and multiply by the new road speed. eg. 1,375 rpm at 60 mph means you'd be doing 1375 /60 x 62 = 1,420 rpm. So, at 62 MPG the engine will spin at 1,425 RPM.

[Maniac]

Front rear over the counter $1800, rear rear around $1300, plus labor, probably be $5000 all told, is it worth it.............?

Not to me.

[tracer]

Front rear over the counter $1800, rear rear around $1300, plus labor, probably be $5000 all told, is it worth it.............?

Not to me.

A gear shop in Milton, ON will do it for Cnd$2,500 cash (I asked and they looked at my truck).

[Maniac]

A gear shop in Milton, ON will do it for Cnd$2,500 cash (I asked and they looked at my truck).

Does that include removing and replacing the carriers? if not figure on at least 20 hours labor for both, thats in and out.

And don't forget synthetic oil at $125.00 or so for a 5 gallon pail, you will need around 7 gallons for both rears.

What kind of warranty did they give you? an over the counter rebuild ALWAYS has a 1 year unlimited mileage warranty.

All that for maybe 1 tenth of an MPG increase................not worth it, don't forget to add in 3 days downtime.

[matcat]

Does that include removing and replacing the carriers? if not figure on at least 20 hours labor for both, thats in and out.

And don't forget synthetic oil at $125.00 or so for a 5 gallon pail, you will need around 7 gallons for both rears.

What kind of warranty did they give you? an over the counter rebuild ALWAYS has a 1 year unlimited mileage warranty.

All that for maybe 1 tenth of an MPG increase................not worth it, don't forget to add in 3 days downtime.

If you redo your rears, and get the same fuel efficiency at say 65 mph that you used to only get at 58 mph, that is a huge benefit and well worth it. Reality is putting the proper rears in can give you easily 1mpg+ in improvement.

[Kranky]

You're about to make a big mistake. If I were you I'd be changing gear ratios for FASTER ones, not slower ones. I have 3.73 in my 2004 International and put low profile tires and .. man ... even if I do 60 miles per hour my RPM is already at 1,375 (which is high for CAT). So I found a gear shop that can change my axle ratio to 3.58 or 3.42. They are in Canada, near Toronto, ON and the guy charges $2,000 cash plus you have to pay someone to take the diffs out and put them back in (4-5 hours of labour). If I have 3.42, I'd be able to do 62 MPH at 1,300, which is perfect.

Here's the formula to determine your road speed depending on axle ratio (AR), tires (REVs), and top gear ratio (TRAN).

RPM at 60MPH = AR x REVs x TRAN

To find out your rpm at a different speed, take the result from the formula above, divide by 60 and multiply by the new road speed. eg. 1,375 rpm at 60 mph means you'd be doing 1375 /60 x 62 = 1,420 rpm. So, at 62 MPG the engine will spin at 1,425 RPM.

Graymist Wrote:

3.36 won't be much good for hauling 140,000 lbs, especially over hills and dales .

Good luck getting started on a hill with 3.36's and an 18 speed when you're grossing 140,000!!!!!!!!!

Assuming the truck is on 11R22.5 rubber 4.10's would be the better choice.

Yes, I know this will put the RPM's out of the "sweet spot" at highway speeds, but sometimes that's a tradeoff you have to make, because a truck that's geared so high it can't get moving when fully loaded is useless.

.

[Bugsy]

OK...Help me understand this...

The lower the gear ratio, the faster you can go per say,no power or torque. (3.00 eg. for referance). The higher ratios at about 3.70 ect.. have more power and no speed?

If I where to spec a used truck, what whould be the optimum ratio for fuel economy? Just general freight with a van or reefer...

[allan5oh]

If you redo your rears, and get the same fuel efficiency at say 65 mph that you used to only get at 58 mph, that is a huge benefit and well worth it. Reality is putting the proper rears in can give you easily 1mpg+ in improvement.

Won't happen. At most it will be 2-3 tenths, I've done the testing, speed makes far more difference. Don't believe the salesman's hype "spec it right and you can run fast with fuel mileage".

[YerDaddy]

You can gear a truck to run at 70 mph at 1200 rpm but what folks leave out of the equation is how much more boost you'll be using to turn the wheels. It takes fuel to raise boost. You'll be pushing a lot of air at 70.
You want your truck to cruise along around 5 psi boost on the level at most for best efficiency. How many times a day do you stomp that boost pressure over 20? It's just dollar bills flying out your stacks.

The key to more mpg is slow down. You can't change the laws of physics any more than Obama can control the Earth's climate. Although there are many fools who think otherwise.

The less air you push the less power (fuel) you use.

Gears provide leverage not power.

[no_worries]

Why is everybody talking fuel mileage? The OP needs to change for startability and gradeability.

[tracer]

Is it possible to change the gear ratios on a truck, say for eg., from a 3.36 or 3.55 to a 3.90 or 4.10 ? How much would it approximately cost, and what other modifications would have to be made ? Thanks in advance.

I just did my axle ratio change. Total downtime - 1 day. Brought the truck to the mechanic in the morning and it was ready at 10 pm. Paid Cnd$2,100 to the gear guy and $650 to a shop that did uninstall/install. I told them to use the same oil as I just replaced it a couple of months ago.

My old ratio was 3.73 and with the low profile tires I was doing almost 1,400 RPM at 60 MPH. With the new 3.42 ratio, the engine spins 1330-1340 RPM at 62 MPH and when I'm fully loaded I stay close to 1,350-1,360 RPM or 63-64 MPH. Most 2-lane highways here in Ontario, Canada have 50 MPH speed limit, so I just use 8 Low in my 13 when I have to use these highways.

Gradability has naturally decreased but at least now I can maintain 62 MPH without killing my fuel mileage. With 3.73s, the engine did 1440 RPM at 62 MPH so I never even ran at 62.