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

Are you talking about regearing or keeping the 3.55s in the axles? I know keeping the 3.55s are bad for 37s but I'm on the fence about 4.10s or 4.56. Because My truck with previous tires which exact are 32.5 I think they are just 1.5" bigger than stock and I never hit 2000 rpms until i was going almost a 100. I wish it was at 2000 rpms at 75. I've always had so many issues with the truck keeping its speed down the interstate. Idk if all 2nd gens like mine have this issue but my O/D in my truck was useless on the interstate going 70 or up no matter the rpm it would not keep or gain speed unless it down shifted to a lower gear and was roaring down the interstate at 3500 or 4000 rpms. Or if it was going down hill or on flat ground so idk if I need to gear higher for it to be equaled out? Sorry if i sound confusing but I felt the truck needed to be regeared badly with those 32.5s. The truck has had the issue since we bought it back in 2002.

That is just how they operate. Mine is slightly better with 4.88s and 40in tires

Your truck should be able to maintain speed on reasonable hills, even in O/D. Think I would start looking at the plenum, tune up parts, and things of that nature.

I would run 4.56 or 4.88. Your adding a lot of rotating weight and raising the truck which kills aerodynamics. Your fuel MPG is going to drop. Here is a gear calculator https://www.rocky-road.com/calculator.html

Personally I'd lean to 4.88 or 5.13 with a 318 and not run over 70. Honestly we need more information to help you select gears. What driving conditions are you using the truck for most of the time? To and from work ? What speeds? Do you live in a city with lots of traffic? Will you tow a trailer frequently, occasionally, never? How heavy if you do? Is your drive to work 5 miles, 10, 20, or 50 miles? In town or on the interstate? Any off roading?

You need to gear for what the truck will do most of the time and make trade offs for things you don't do often. If your stopping and going in town mostly, a lower gear like the 5.13 or 4.88 is better. Mostly interstate use the 4.56 or 4.88. Be honest so you get the best result.

I think he is talking about the shift patterns
Highest you can go for a Chrysler 9.25 is 4.88

+1 The first performance degradation, with any vehicle, I typically notice is highway performance with overdrive. The indicator is the transmission won't stay in overdrive on even light hills. I usually start with a fuel system cleaner, and if that doesn't fix it, progress to cleaning/replacing sensors and tune up parts. You should probably do that stuff first so you establish a good baseline.

Alright so we own a custom cabinet shop that is at the house so I literally walk to work every day. We have a duramax we use for pulling most of our big trailers but every once in a blue moon my dodge pulls a enclosed trailer full of cabinets. Most of the time I'm only pulling ATVs to mud parks which is like once a month so I dont pull a whole lot of heavy stuff. That being said my truck is really only drove on the weekend to and from my girlfriends house which is 40 mins away on mostly interstate. I hardly offroad since most of the truck is in brand new condition. I live near a moderate sized town. No heavy traffic. Most of my riding consists of highway and back country roads. Its my daily I guess youd say cause it's the only truck I have that's 100% reliable. Like I said before I drove it to Florida 400 miles away one time. And as tunning up what do you mean by that? As of now I dont think there is much tuning left on my trans or engine. I cleaned the injectors, new spark plugs 30,xxx miles ago, new throttle sensor, new air intake, has a programmer on it, has high performance exhaust including headers, has a rebuilt trans, and brand new cooling system with radiator. Only thing on the engine that has issues is the freeze plug leaks and valve covers leaks a little. Is there anything else I should before swapping the gears or check?

the gear calculator you suggest said I need a 4.23. So I'm starting to lean more toward the 4.10. Idk if this makes much of a differnece with regearing but the truck has a 2500 suspension, steel offroad bumpers, standard size tool box, and 18x10 rims instead of factory. So with all that added weight I'd guess youd stt my truck isnt really a 1/2 ton but not a 3/4 ton either and I know the weight effects all of this.

You should check the plenum, and did you replace the distributor cap and rotor? You could also clean the throttle body and the sensors that plug into it. If the cats are still on it, make sure they are not restrictive.

For comparison, when Dodge added 33s as an upgrade (275 60 20) on their 1/2 tons - they required the rear gear be upgraded from 3.55s to 3.92s. I'm running the same tire with 4.10s, and if I wanted to go 10% taller tire (33 -> 36.3) I'd likely go 10% more gear (4.10s -> 4.5s). Seems like 4.56 would get you back to stock RPM ranges if you used a 37" tire. I think if you want to retain truck capability, 4.56s are the way to go.

Are you running 8 bolt axles (aka d60/70/80?) ? If you are, you would need to also replace your limited slip case - they sell those in 4.10s down and 4.56 up. Can't move from 4.10s to 4.56 without that change. I don't know what the caveats are on 1/2 ton axles, there may or may not be issues as you move to the more extreme gear ratios.

You have D60's under it??

No I didnt up grade to 3/4 ton axles still have dana 44 and Chrysler 9.25 but like my leafs in the back are off of a 2500 truck and the coils and such

https://cimg5.ibsrv.net/gimg/dodgefo...5bb69e54a8.jpg
Your 37" tires are probably shorter so I used 36. 2174@70 and 2330@75 with 4.56. We have 4.56 with 33s in my daughter's truck. If she ran a lot of interstate we'd put 35s on it. I think 4:56 would be close to matching the stock ratio. Someone correct me if I'm wrong on the trans ratio in overdrive.

O/D ratio is .69. :)

Don't get 4.10s not enough gear for 37s. I would go with the 4.88s. More rpms doesn't always mean less fuel mileage, you engine and transmission wouln't have to work as hard with 4.88s.

4.88s I think are a little big for 37s I'm running 4.88 with 40in tires and they put me right back into the factory RPM range.

You want to be higher than the factory rpm range because the bigger tires are heavier and have more rolling resistance.

I mean I'm close to factory range if it had bigger tires. I used a gear calculator and even talked to a shop and they recommended that size. It does have more low end getting because if the higher gears.

To my knowledge 4.88 gears is the lowest you can get for a Chrysler 9.25 so that was probably why they where recommend because you can't get any lower.

I'm running 4.88 in my 2500 HD and my 1500 with 40in tires

This.

That's the steepest they get for the 9.25 axle.

I upgraded my axles to a dana 70 rear and dana 60 front, but I have 37s and run 4.88 gears. Drives normal at highway speeds.