Tire size / transmission question

I went to the 285's on my 2003 2500 Hemi, I lost about 2 mph too. I never had a problem with my transmission. (5 speed auto). It's allot more rubber on the road, more drag, less mileage.....

If you've got a 3.55:1 axle, you're lugging the hell out of your truck.

Trust me, I know where you're at - I got a 99 1500 2WD on 265's (225 stock). I get better fuel economy in 4th gear than 5th if I'm under 70 mph average.

It sure drives better on the bigger rubber tho. In my case, I should regear the truck (and tuck some decent springs under it too, its killing my back)...

Your choices are:

A> replace the ring and pinion gears in your axle for a ratio more in tune for the rubber you want to swing... With that much rubber I'd personally go with 4.10's for sure (I'm assuming a 318 or 360)... Aproximately 10% less stress on the transmission/gearbox/engine as 3.92, and 20% than 3.55 (correct me if my math is off, I calculated this a few years ago)... I was quoted about $650 for one axle rear axle, parts and labor. Actual parts cost appears to be about $300/axle to do the job 'right' (its a '99, you definetly want to go ahead and replace the bearings, Chrysler has a reputation for using junk)

This is also the moment you consider the addition of limited slip and/or air/electric lockers ;)

B> Cruise with OD off. This most likely will do nothing for your fuel economy but it will take the stress off your engine and torque converter (nothing for your transmission and gearbox tho)

C> Downsize the rubber. Prolly the cheapest solution but IMHO sub-optimal because your truck is somewhat 'gimped' with 3.55 axles even with smaller tires...

JS

Davenport wrote:

Just remember that if you go the regearing rout, you will need to do it to both axles. As for the transmission, when was the last time you changed its fluid? I would suggest an aux trans cooler as well.

Shouldn't there be an aux tranny cooler on his truck anyway.

I don't know, some do and others don't. I think that it has to do with the options ordered. Either way, if the trans is getting that hot, then what is there is simply not enough.

OK, thanks.

Reason I mentioned that, is mine has the tranny cooler (between rad and AC condenser. I must have towing package or some other option that requires the cooler.

It is a classic mistake that a lot of people make with tires and gears. ALso when they look are gear ratio, some are brainwashed into believe that you need 3.73 or taller gears for best MPG with a gas engine in a heavy draggy 4x4 that has a OD too. The bare minimum you should have in a 4x4 truck with a small block sized gas engine is 4.10 because when OD is in use it is less 3 to 1 effective. Throw in oversized tires and it gets worse. The Dodge hemi 4x4 suffers from two problems MPG wise. THe first is that that engine reachs peak efficency at 4000 RPM or so (torque peak) abd peak HP above 5000 rpm. This is not a good power curve for a heavy truck and tall gears and bigger tires make it worse. If you are running 265 or bigger with a Hemi you should be running 4.56?s at very least to better use power curve. The second probel is tha Dodge remove front axle disconnects and there is no provision for adding lockout hubs either and dragging the front axle can cost you 1 to 2 MPG depending on speed and conditions. THis is very poor judgement on Dodges part all to save a few bucks in production costs. 30 year ago a had a truck without lockout hubs and when you add them it does make a difference in MPG and smoothness too as I could always tell on hiway when they were not disengaged and you were dragging axle gearing.