I have been going back and forth between 4.10's and 4.56's. I'm running a 315/75/16 tire right now with the 5.4L and stock 3.55 ls and i'm getting real tired of not being able to hold a gear in O/D while driving on the highway. It seems like unless I'm crankin about 70 (really doing like 78) and turning slightly over 1800 rpm's my truck is boggin out and really putting a big load on the engine. I have about 116,xxx right now on it and would like to keep it for another 100,000.

With the math that I have done, 4.10's would get me back to stock rpms. Right now with stock 3.55 at 60 mph im turning 1550 rmps and with 4.10's ill be turning around 1790 @ 60 mph. With 4.56's i figured ill be turning around 1990 @ 60 mph. I figured this by looking at my current rpm's and just doing some cross multiplication.

I was all set on getting the 4.10's installed, but then I started reading about gearing on this site and I can't find a single person who is running 35's and 4.56's and is not completely happy.

The last thing I want to is to fork out a bunch of money and be unhappy with my gear selection. I'm afraid that 4.10's may not make a noticable diffecerce to justify the cost and I'm also afraid that 4.56 will be too low and I'll be screamin down the highway. I also wonder if just running out of O/D during city driving would be comparable to having 4.56's

Those of you who have a setup similar to mine and have re-geared could you chime in and let me know what type of rpm's your turning on the highway so I can see if my calculations are reasonable.

I also do quite a bit of city driving and rarely pull a trailer.

 

http://4lo.com/calc/gearratio.htm

4.56's would be 2325 rpm at 75 in OD. Those are 34.6" tires.

 

Do you know a way to calculate how fast I would really be driving when my speedo reads 75mph with 4.56 gears? With my stock gearing I have calculated that my 35's are 12% (0.12) off using the circumference formula. So what ever my speedo reads I just multiply it by 0.12 and add that number to the mph reading. But with new gears that all changes.

I do not have a tuner, I used to but it was stolen after my truck was broken into.

 

Use the linked calculator and plug in the STOCK tire size, 3.55's, 75 mph, 0.7 tranny, 1 transfer case, and note the RPM's.

Reset the calculator, plug in your NEW tire size, 4.56, same tranny and TC, and plug in the noted RPM. This will give you a MPH figure, which is what you will actually be doing when the speedo says 75.

 

thanks, the readings i was getting were a little different than what the calculaters were saying, but that may be due to TC slippage, cause like I said it always feels like I'm putting a crazy load on the engine when trying to cruise at 60-65 mph.

 

i would go with the 4.56. i recently changed mine over, and my RPM at 65 is about 2100. the truck pulls MUCH better and hold OD with no problem. plus, on the highway, my MPG's have increased by about 2 or so.

 

Does anyone know of a good reputable place around the St. Louis MO area to have em done.