>$1500, what size tires are you running?

 

if you get a change let me know the rpms your running at 65-70 and if you dont have the speedo corrected via programmer or other.

 

alright guys, to throw another curve ball in this, i am curious what i should run. I have the 4.6 with stock gearing (3.55s) in the 8.8 axle, and 38" tires (terrible i know) transmission was re-worked before i bought it and i have been really easy on her the whole time i have had it. but i know i am counting the days to my transmission. so what gear should i put in it? I have been told that 4.88s would be right where i need to be. but what will this do to my gas mileage on the highway? i know im never getting stock mileage back but right now im getting about 12 mpg on the highway. will 4.88s help my gasmileage (hopefully not...but possibly hurt it)?

at 70 mph i am running just under 2500 rpms...maybe like 2400.

 

Impossible. In overdrive that should be 1538 rpm with 3.55's and 38's.

Have you corrected your speedo for the big tires? You may think you are doing 70 but may really be doing 90.....................

 

possible. no i have not corrected my speedo but i have figured it out. speedo says 56.5 and im going 70. (checked it with radar trailers on the side of the road)

plus im not going to be doing exactly what the formula says...it doesnt account in the much added weight of the new tires and definte decrease in aero dynamics.

although i may be pushing it a little high with that suggestion. may be closer to 2000 rpms

 

Even in 3rd, it's only doing around 2200. Your torque converter must be REALLY loose......

The math doesn't change with tire weight or aerodynamics................................

 

but it takes more work to move the truck...thats like saying going up hill doesnt change the rpms...

thats what i have been told...and makes sense to me. it seems the math would work in a perfect world with no friction or anything...but there is always other factors..

 

Going uphill does NOT change the RPM's if you maintain the same speed, and it stays in the same gear and the TC doesn't unlock. The calculator I'm using assumes a locked TC.

With what you have now - at 70 mph in OD with the TC locked, you are doing 1538 rpm, plus or minus a small amount for exact tire height. With 4.88's, it would be 2115 rpm.

http://www.4lo.com/calc/gearratio.htm

 

ok so basically what your telling me is if i were to put the truck in second gear, pick a speed...lets just say 15 mph. and i go on a road that is first flat, then goes up hill, then goes down hill, my rpms will NEVER change? they will always stay constant?

havent you ever heard of a thing called gravity? it will put more strain on the truck when going up hill...it will try and pull the truck backwards...so in turn cause the truck to have to work more...hence higher rpms...right? then going down hill, the truck will have to work less to keep that speed. so lower rpms.

 

I think you are missing an important concept - torque converter lockup.

If the torque converter is LOCKED, the RPM's will not vary ONE BIT at a constant speed. If it's unlocked, of course the RPM's will vary according to load, but not a whole lot, probably a few hundred. Your concern needs to be with your RPM's at a cruising speed in overdrive with the converter LOCKED. That is the only valid way to compare the effects of tire size and gear ratio. In my example, where I said 1538 rpm with the TC locked, if it unlocks under load, it may jump up to maybe 1900. It could also unlock when you are coasting downhill - and drop to 1200. Also, under load, if the load becomes too much for OD with the converter unlocked, then it may downshift to 3rd - and the same thing applies there.

The torque converter CAN and WILL lock in 3rd and OD, I don't know about 2nd.

 

i come up with 1583 rpms in overdrive for your ratio and tire size and i doubt your getting 12mpg since your odometer and speedo are off.

as for a ratio. 4.56 would bring it back to factory ratio equiv. 4.88 would be better. you wont see much for mileage but at least you wont kill your trans.

 

I apologize as i didnt fully understand and i have found my problem..in OD i do run about 1700...but i dont stay in OD very long. it is in 3rd that i am running those rpms.

but...new gears will help that right? it will keep it from shifting as much?

I also apologize if i came across rude, it wasnt meant but i fear i may have come across as that.

yea i am getting 12 mpg because the route i drive twice a week has been measured out right at 60 miles (by another car) and i use right around 5 gallons everytime...some times more, some times less.

how did you figure 4.56? everyone else has told me 4.88 will be factory equiv. and as long as i dont see a huge drop in highway gas mileage, i will be happy. i dont want to kill my trans.

 

new tire/old tire x stock ratio= new ratio

38/31x3.55=4.35 so since a 4.30 doesn't exist for the front axle you have to go 4.56 to be close to factory equiv. with a 38 and stock ratio of 3.73 the 4.56 comes right in at 4.57 most guys will say to go the next step lower to help compensate for added tire weight rolling resistance drag ect.

so it is up to you a 4.56 would be minimum a 4.88 is a better choice with 37 and larger tires.

 

ok i see now. what i was told before was that if i had 4.88s it would bring me almost to stock and my spedo would be almost exact...off just a couple. but i see where you got that from. i think that if i am paying to get it done, i might as well get whats best for the truck so im gonna get 4.88s. im waiting to sell the tires to help pay for it. i am only going to do the back for now until i can save up enough to get the front done. i will disconnect my 4x4 switch so it doesnt engage. at this point, i never really use 4wd anyways as i am too busy to do much playing and i cant afford to break it so that keeps me on the pavement also. lol