2"-3" Suspension Lift + Larger Tires = Change Gears?

[Keith S]

From what I have collected the 07-10 V6 has 3.55 gears (please correct me if I am wrong). With a lift and larger tires should I change the gear ratio as well? Looking to get the Rancho QuickLift (2") and 265/70/17 tires.



I know Ronin from the Explorer Forum changed his from 3.73 to 4.10. What is the purpose for doing this? And what would be the benefit?



Thanks,

Keith S.

 

[blksn8k]

Keith, when you increase the tire diameter as you are planning you will effectively be changing the overall gear ratio as well. A larger tire takes fewer revolutions to cover the same distance. The reason to change the ring and pinion gears in the differentials is to return the overall gear ratio to something similar to the original setup. With a 3.73:1 ratio the driveshaft turns 3.73 times for every time the axle makes one full revolution. The 4.10:1 ratio speeds that up which in turn causes the engine to run at a higher rpm for the same vehicle speed. So, the end result is that while the larger diameter tire reduces the engine rpms compared to a smaller diameter tire, changing to a higher numerical gear ratio speeds it back up. Basically, you are attempting to get the engine back into its intended rpm range for any given vehicle speed.



If you had traded for that V8 you wouldn't need to worry about it so much. :grin:

 

[Johnny O]

Gen 2 V6's have 3.73 gears. Gen 2 V8's have 3.55 gears.

I would leave the gears alone and see how it goes with the bigger tires. If it ends up being a slug then change them. Ballpark parts and labor if you have a 4x4 and need two gearsets will be at least $1000.00.

 

[Yardsale]

The lift is not related to gear ratio.



The tire size change is. Assuming the same wheels, the increased tire size will weigh more than whatever you now have. A size increase will cause reduced engine RPM at all speeds, increased need to downshift to maintain speed on many uphills, reduced acceleration and reduced fuel mileage (mostly due to the reduced acceleration and increased downshifting).



A tire size increase also increases the stress and accelerates wear on the drive train.



For these reasons, you want to compensate for the increase by regearing the differentials.



As noted above, the target is the engine "power band" at cruising speed; typically somewhere in the 2500 to 3000 RPM range (the entire power band is much wider). Since ring and pinion gear sets come in set ratios, there is probably not a perfect ratio to compensate exactly for the change, but they can be close. For example, if the tire size change is 10% larger diameter than previous, a 10% change in gear ratio would be about right, except it won't compensate for the loss in acceleration (a guesstimate for this is ~2 or 3% additional compensation).

 

[Keith S]

Was reading around and found that since I have the tow package (Class III/IV hitch) the V6 comes with the 3.73.



@ Roger - Lol. Maybe, but I am happy with what I currently have and plan on keeping it for quite some time. (Unless I hit the lottery lol)



@ Jon - Thanks. I have seen a few others say that the 3.73 should be good to go. Was planning on getting the larger tires and seeing how the truck reacts. I was also thinking about putting in some air lockers down the road so will most likely do that when the gearing gets changed.



@ Yardsale - Current tire size is 245/70/17 and will be going to 265/70/17. I will also be purchasing lighter rims than OEM for use during most of the year. With this information do you think that the size change is significant enough to change the gearing?



Thanks all,

Keith S.

 

[blksn8k]

Something else to keep in mind is that with the larger tires your speedometer will read slower than your actual speed. Unfortunately, the only way to correct that is to reprogram the computer. It's not like the old days when you could just change a plastic gear on the end of the cable. A hand help programmer like the Xcal from SCT will take care of the problem. The setting that needs to be changed is the tire revolutions per mile. You can get that number from tirerack.com.

 

[Keith S]

@ Roger - already thought of that I am ordering a tuner from Unleashed Tuning after I get my tune up.

 

[Yardsale]

That tire change is about 3.5% difference in diameter. The next higher gear ratio (4.10 (or 4.11, depending on what you select)) is a 9% difference, so the gear ratio change might be more compensation than is needed (by about 5.5%). If you figure in a fudge factor of 2% for the increased rolling resistance, it's still 3.5% more compensation than needed.



In that it is so close, in this case, I'd probably do as Johnny suggests and wait until you try it for a few months.



If you are going to get a tuner that adjusts shift points, you can play with them to dial it in a bit, but this won't change the cruising speed engine RPM or the need to downshift for uphills. This was the biggest nuisance for me that made me change the ratios.



As a note, getting air lockers says "serious off-roading", to me. If this is the case, I'd definitely change the differential ratios; you're going to need the acceleration and control.



Eventually, one considers what the vehicle should do well - mild-mannered daily driver with the occasional off-roading or rough and tumble off-roader.

 

[Keith S]

Not necessarily a "serious" off-roader, I crawl for the most part. The reason behind the lockers is that some of the trails on base can get quite bad and lockers could have helped a couple times. I do the occasional State trails which would also benefit from lockers as well.



Keith S.

 

[Dreman]

I changed the tires on my 2003 from the stock 235-70R16 to 245-75R16. The speedometer was reading low, and changing the tire size corrected it! At least according to the GPS. Looks a lot better, too! It did lose some acceleration, though.