What gears should I run with 37s (5.2 magnum)
 

Okay guys so I have a 2001 dodge ram 1500 4x4 with the 5.2 318 magnum in it. I've been spending the past year rebuilding my suspension with everything dodge offroad. I have a 4" lift from them and I just got my new 37x13.5 toyos on the truck. My question is what should I regear my truck to or if I should even regear to get it closer to factory. My truck is a daily and goes down the highway quite often. Dodge offroad told me I need 4.56 gears to get good mpg and still pull the 37 great down the highway at 70mph. But on the flip side I've been told by a few other people that 4.56 gears will make my truck run at a high rpm even with the 37s and drink more gas switching to 4.56. My truck has alot of mods most of the truck has been restored. I added a tuner, cold air, shorty headers, and catback exhaust for the performance side. I do know that with the 33s I had on before going 70 mph the truck would barely turn them in overdrive and going up hill the truck would just slow down to 65 and then snatch into 4th. Trans has been rebuilt with a upgraded torque converter as well.

I feel bad they you bought that overpriced lift kit. But to answer your question you need at least 4.56 (Hughes you can go with a Chrysler 9.25) but ideally 5.11 would be preferred. Yes you will use more gas on the highway.
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4.56 would be a good gear for 37's...... RPM vs Speed will be pretty close to what it was stock. If you have 3.55's in the truck now, you do NOT want to run those with 37's.... the trans REALLY won't like it. Doing gears front and rear is still cheaper than rebuilding the trans. :)

That said, half-ton axles, and 37 inch tires, on a seriously heavy truck, is going to EAT front end parts for lunch. If you off-road at all, you should expect to break stuff. :) All part of the game.

Welcome to DF!

What ratio is in there now? Guessing 3.55 or worse. I have 33s with 4.10s on my 2500, engine seems to do pretty good at highway speed and is good for pulling trailers. The speedo runs about 11% slower, so if I were to step up to 37s - it'd probably be 25% slower than stock.

I think 4.10s would probably work with 37s, but 4.56 might be a better choice if you do a lot of towing and offroad. You are not going to get good mpg, put that out of your decision criteria to maintain your sanity.

I think 4.10's with the 37's wouldn't be enough gear. Not like the 5.2 is a torque monster, so, compensate for that lack with a bit more gear.

I have 33's with 4.56 gears in my truck. (bought it that way.) And it is WAY too much gear for the 33's. Gonna step up to 35's when I replace the tires. :)

Lets do the math - assuming you have 3.55s... looking at the mathematical ratios:
gears... 4.56/3.55 is 1.284...
tire diameter ...37/29 is 1.275.

If you want it to be in the same rpm range as stock, you want the 4.56s. Keeping in mind, 3.23 is the ratio the newer trucks are running to get better fuel mileage.... however they do have more transmission mitigation for drive-abilty.

Thanks yall and yeah I was kinda hoping yall wouldnt say that about the mpg at high way speed. Also I do have 3.55 gears. I dont offroad with this truck a whole lot. Does anyone know what rpm range I'd be looking at when going between 70-80?With the 33s I drove 400 miles one way on a full tank of gas but I felt the trans wasnt liking it cause riding at 1600 rpms it was struggling to turn the tires. I'd want the rpms at that speed to be around the 2,000 mark if I can and it be good for the truck at other speeds.

Ok, tried a calculator.... Not sure just how accurate this is, seems a bit strange to me... but, should be good for comparison at least.

With 265/70/16s, 3.55 gears, in O/D, at 2000 RPM it is supposedly going 74 mph.

With 37" tires. (that is true diameter, tires are actually probably a bit less than that.) at the same 2000 RPM, in O/D, you would be doing 70. So, a net loss there...... around 2300 RPM at 80. Well into the engines power band.

Alright and that's using the 37s with which gear? .

That's in O/D. If you are towing, you won't be in O/D though..... In 3rd, at 60 mph, you would be turning about 2500 RPM. 2900 at 70 mph. Ouch. :D