Changing my gear ratio, should I?

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Jayson Vagenas

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ok i was looking at the project that John Brancaccio, Jr. posted about larger tires and gear ratios. I think i want to change my rear end gears fro 4.1 to 4.56 b/c i have 285/75r16 and feel i the extra gear for tow and mpg. any thoughts on this subject plz let me know.
 
It is a matter of happiness. You will get a better off the line acceleration or power if your towing something for example. You will need to do both the front and rear which will double up the cost. Since your going to the power side you will get less miles per gallon as well.



Bottom line, if it is what you want to do and it will do what you want then it makes sense to go with it.
 
If you have 4 wheel drive you must change both the front and rear differentials. How much do you tow. Does the current setup pull the trailer in overdrive or downshift to a lower gear. How low a gear?? Keep in mind changing the ratio will not allow you speeds possible without the trailer.

If you go to a lower gear then your RPMs will be up always, even W/O a load. So your fuel mileage will always suffer. If however with the trailer you run in 4th (not overdrive), leave well enough alone. The downshift to one gear below OD does not harm the tranny and when you are crusing W/O the trailer you get the benefits of OD and the current higher gear.

The engine only has so much power. I pull a 2,000 pound trailer to Albuquerque and going uphill have to settle for around 65 MPH.

My advice, the cost is not worth the benefit.

Dennis
 
With a 33" tire, going to 4.56 gears will keep your engine rpm vs. road speed roughly the same as it was stock with 4.10's and 255/70-16's. The rear isn't too difficult, the front is a PITA I'm told.

Shop around, some shops will charge a ton of money, others are more reasonable. Two gearsets plus install can be as much as $1000 though.
 
It's not necessarily true that changing from 4.10 to 4.56 will reduce fuel mileage, and if it does, it will be minimal. There are relationships between the transmission and differential ratios and engine RPM that should be (reasonably) maintained. The drive train is designed to operate most efficiently in a RPM band. The change in tire diameter is about 5.4 % (ratio change from 4.10 to 4.56 is about 10%). Fuel efficiency is one concern. Drive train stress, tire weight and footprint are others that contribute negatively.



You’ll not notice over the many miles, but there is additional wear to transmission and differential bearings occurring, as well as additional heat generated due to more torque converter and clutch slip.



You probably do notice a reduction in acceleration and the shift points are a bit different. To the extent that you respond to the reduced acceleration (i.e., more throttle), there is more drive train stress – that energy has to go somewhere, and during the increased time to accelerate, it’s pent-up in the drive train, and not moving the vehicle (poor for fuel mileage).



All in all, the additional shifting and wear, and the loss of acceleration are reason enough for me to regear. Of course, the tire diameter change of 5.4% and the ratio change of 10% results in a 4.6% over correction – more acceleration and higher RPM at a given speed and gear combination. This may reduce fuel mileage a bit, but towing will improve. Choices …

 
Actually the stock 255/70-16's are about 30.1", if you go to a 33" that's almost a 10% increase, same as going from a 4.10 to 4.56 gear. A higher gear will not necessarily mean less mpg when the tire diameter is increased too.
 
So, if you had different ratios, then you’d have to play with tire sizes to get equivalent travel. Id' be like a Sport Tractor ...
 

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