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.

 

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

 

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.

 

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).

 

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.

 

Graymist Wrote:
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.

.

 

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...