I went from 3.55s to 4.56s back in July and i only had 285/70/17 Toyos on up until recently. Honestly, i loved being over-geared. Truck didn't even notice there was weight behind it. Pulled a 3500lb car to the dumb at in August, put it in 4hi so i would tear up my dad's grass and from there it idled at 8mph or so with little bro steering the car. Cruising rpm weren't too bad but i mostly drive 60-65 on the highways and interstates anyways. With the 3.55s i was getting 18mpg on flat ground, 16 in the hills on long trips. Jump to 4.56s and I'm still getting 16. Got em installed in Knoxville Tennessee and drove back home to Sioux City Iowa. Then November 7th i left for San Antonio and got home on the 14th. Long drive but pleasant enough and i didn't have to fret about making sure i got my next fill up. A week and a half ago i put on 315s but haven't had a chance to compare mileage. didn't seem to really rob me of any power and i gained about 100-200 rpm back for cruising.

you won't regret 4.56s and they'll cover you if you ever decide on 37s OR theyll do stupid sick burnouts with factory tires

 

Heyyou - 4.56's but only because 4.88 for the chry 9.25 are tough to find at times.

I agree with dapepper9 about being slightly over geared. Especially now that my ram is no longer my daily driver im planning to swap in dana 60 axles front and rear and load them with 5.13's

 

Speaking from experience having switched from 3.55 to 4.56 with 35" tires, if you can find 4.88, do it. These are big heavy trucks with feeble gas powerplants, you need all the gear you can get. No, your gas mileage won't go down substantially and you shouldn't even be worried about that if you are considering this at all. Mine is my DD and with mostly city driving I am in the single digits FOR SURE. If you plan to tow with OD on (I do) you will want the 4.88 over the 4.56 for a little extra RPM on the highway especially for hills. Even unloaded my truck has a tough time holding top gear on steeper grades when driving slower at 60-65mph. Not awful by any means but a little extra gear would be ideal IMO.

 

About where are your RPM's at when OD is off towing with the 4.56 gears? I will stick to the unbearable 55mph law just to baby the transmission but if the RPMS will be 3500, I cant imagine holding that for extended periods of time being good on the transmission. I have 3.93's right now and with OD off at 55-60 im at 2500 RPM already

 

I have 4.56 and 33's, and towing over 55 mph just isn't going to happen, the engine would be screaming, and I could literally watch the gas gauge go down.... I am thinkin' about new tires here real soon, and bumping to 35's.... I think that will be a good compromise.

 

I went from 3.55 to 5.11s pushing 45in tires. Before I got 13-16 MPG now I'm getting anywhere from 9.5-12. the pic below is before the 45s. Problem is once I visit hey you I have to do it all over again because mine is a 2500 with 1500 axles.

 

So correct me if I am wrong, but in your case with the 4.56 gears and 33's, when you jump to a 35" tire your rpms will be lower compared to the 33" correct? because there is more tire eating up revolutions? So in a nut shell when towing with your rig, your rpms will be lower at 60mph with the 35's than with the 33's?

 

Correct. The 35's have a larger diameter, so, their circumference is also larger, which means, fewer revolutions per mile, which translates to lower engine RPM for the same speed as compared to 33's.

 

I tried this yesterday, at 60mph it revs 2750rpm per the tach. I do not tow heavy (normally just a 2000lb boat in OD) but if I did this would probably be the speed I'd tow at and where I'd want the RPM for a 6000-7000lb load to prevent downshifting on grades. Transmission is not really the concern with higher RPM, rather load.

 

Thank you for testing that for me, the trailers I am looking at are about 2000-2800 UVW so about the same as you and your boat. I think 2750 is a decent RPM to hold for extended periods. Ok so I think this makes up my mind, 35"s and 4.56 gears and a 3k or less trailer and everything (except fuel mileage which isn't my concern) sounds like it will be happy.

Thanks for the help i appreciate it.