I have an '03 1500, 4x4 with the Hemi. No mods besides an AirRaid CAI.

Where/how do I determine my gear ratio? It came with the flower petal 20's, but I'm not sure they're stock. I've got another set of 20's on it now, with I want to say, 275.60.20's. A bit taller than what was on on the 'petals.

My issue is this: when I'm towing my travel trailer, it seems to want to downshift a lot. Even on not much of a hill.

So the big question is this, do I buy a tuner, like a SC 3815 (since I can't tune the trans) or re-gear, or both? Like everyone on the planet, my budget is a factor. And second, I'm not sure how long I'll have this truck.

Thanks for the input

 

The tuner isn't going to do much for the tranny "searching" issues, especially if you can't modify the shift schedule. Sorry to say, but the answer is re-gearing, which isn't cheap.

Going from 20" to 17" wheels will also make a noticeable difference, but not the nigh/day difference that re-gearing will do.

I'm guessing you have 3.55 gears, because if you had 3.92s it wouldn't be as bad as you're describing unless you had considerably larger tires. With your 275/60/20s which are basically 33" x 11", 4.10 gears would be ideal.

In a perfect world, the gears and the tuner would both serve you well, but if you had to pick one to BEST solve the issue, it'd be gears...

 

An easy way to determine your gear ratio is to get to 60 mph while in 5th gear then check the tack (RPM). You need to look up the revs per mile of your tire should be about 634 for 275/60r20. Axle ratio = RPM / (0.67 * 634). Ex: a. 3.55 should give a 1507 RPM. A 265/70r17 tire would get you upto 1569 RPM at 60. Normally for towing you run in 4th with the .75 ratio. If you have the 3.55 that would put you at 1688 RPM in 4th at 60MPH. Changing the gears would be quite an investment for that age truck.

 

The problem lies in the fact that the 545rfe transmission has in essence, two overdrive gears and the Hemi engine is "happiest" cruising around at between 2000-2200 rpm coupled with the fact that it's torque curve starts up around 1600 and doesn't really get to where you wanna see the torque numbers for pullin' until it also gets to around 2000 rpm.
Re-gearing gets you up into this "power band" faster (with less throttle too) and keeps it up there.

In this power band area the truck pulls much better and there isn't that lag you get when the rpms are under 2000 when you hit the gas, which IMO is a royal PIA when pulling weight and either trying to merge into traffic, passing or when pulling on hilly terrain.

The only real downside to taller gearing is you will be taching a bit higher when cruising on the open road and will yield slightly lower fuel economy at about 70 mph+. Of course since you need less throttle to get up to speed, the MPG numbers will be a bit higher when accelerating, so in-town mileage tends to go up proportionately.

Cost to re-gear starts at about $600 per axle (parts & labor) and can go up significantly based on your area and what local shops charge. Setting gears, especially in an AAM axle that has to be shimmed is not an easy task for the inexperienced and you shouldn't cheap out on the labor on this mod...

 

Check your diff and see if there is a small metal tag, or see if your sticker is still in the glovebox.

 

Thanks for the input everyone. In the not too distant future, I'm probably going to put 35's on it so that will make my problem even worse. Looks like a re-gear is in my future. And from the sounds of it 4.56's might be the way to go.

Who or where is a good source for the gear sets I need. I feel fairly confident that between a friend of mine and myself we can tackle the install, even if it is a bit tricky.

 

I agree, if you are going 35's then 4.56 for sure.

 

With an 03, have you ever changed the diff oil? If not now is the time to do so. While you are there you could also mark and count the teeth to determine gear ratio