I know all the ideas behind why changing the final drive gear ratio is not the most cost effective way to increase MPG. But doing simple math it seams very logical to do it. I have a 06' mega with 4.10's. I drive about 30K a year mostly interstate. I get about 15.6 hand calculated. My tires are stock BFG 265/70/17 (tires are 31.6" and 99.85" circ). My math comes up with 1923 gallons of fuel a year. At an average of $3.40 im looking at $6538 a year on fuel. Or $26,152 (scary number)over 4 years. If I spent $1500-2000 on a gear change and went with 3.56 I would "guess" I could get about 18mpg. The numbers would be 1,666 gal @ $5664 X 4 = 22,657. Would $3500 less the gear swap be worth it? If I put 200-300 k on the truck it would be double that. So guys I do see thier are some very smart people on this site..... so start your chainsaw's and cut this theory to pieces.Truly would like some objective opinion's. My engine is stock other than K&N.

 

Mine with the 3.XX has a lower towing capacity. I don't know if it because of the auto trans or the gears. But it is still rated at someting like 9500lbs. Other than that I don't see why it wouldn't be worth it. Since the heaviest load I carry is my 3200# boat I wouldn't mind going with something even more highway friendly if they had it.
I have 36" tires and at 70mph I am running approx. 1750rpm. It'd be nice to get that down another 500 or so.

 

I tow a 19ft bass boat but thats about it. I turn probably about 2250 rpm at 75mph. I drive pretty fast on the interstate, because im a country bow who works in the city about a 40 mile drive one way. I have thought about the tire upsize but that seams to be a debatableissue also. Its like every truck has to be specifically tuned to the drivers needs of power vs fuel consumption.

 

Your options for gears are 3.73 or 3.42:1. I have talked to a couple people who have gone with the 3.42 gears and they "said" it improved mileage, but I have not seen any hard data to support the claims. Everyone who changes something on their truck for better mileage "claims" better mileage, but very few provide the hard data to support their claim. I would like to go to 3.42 gears myself, but I don't have the scratch so it won't happen any time soon. IF you decide to do it I would be very interested in the data you record afterwards. Good luck.

 

Both of you guys could benifit from the highest highway gear you can get, plus the tallest tire you can fit.
Both of these mods will lower your eng. rpm, resulting in better mpg's. The CTD makes more than enough torque to pull your light loads, you don't need big gears.


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Thats not all true. Yes the CTD has more than enough power and torque to turn whatever gear ratio you put back there but I guarrantee you will lose mpg unless you increase the HP and torque.
I went from stock tires and no mods to 35" tires. I lost close to 3 mpg in the city due to the engine lugging with the taller tires. I chipped the truck and added another 60 hp and my milage went back up and I even gained a couple of mpg in the city.
I thought the same thing as you and that is why I wanted to go with 35" tires. I dropped my rpm's by 9% so that instead of turning 2200 RPM's @ 75mph, now I am turning 1850rpm's @ 75 . But I was wrong and milage in the city took a dump on me.
Not dissing your post but I know from experience that higher gears don't mean better mpg. Now highway miles are a different story, well kinda. My mpg stayed the same on the highway. But after I added the 60 hp, i finally saw my gain in mpg's, it also went up about 3 mpg.

 

I think im more confused now than I was....

 

I said tallest tire...not big fat wide tire, with lots of rolling resistance. Also, did you recal. your speedo after installing taller tires? And did you hand calculate? I am not dissing your info either, but I have a hard time believing that all other things being equal, if you lower your eng. rpm @ a given cruising speed, that you will not gain fuel economy. I am talking about highway driving


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It is not a fat honker off road tire. It has an agressive tread and is a little less than 12.5" wide. As for the speedo, I spent 3 days with 2 separate GPS units calibrating my speedo. I am able to change the speedo with my programmer.
I have it to within 1/2 mph @ 100 mph so at 55 mph it is almost dead on!
You told the guy to get highest gear he could and get taller tires. When you get in the 33" to 35" range, 11.5" - 12.5" wide is about the narrowest you'll find. I did basicly what you are saying and I am telling you my experience with it. I hand calculated 3 tanks of fuel with the taller tires and figuring the 9% difference in me speedo. I found a calculator online that told me what my new speedo correction should be and once I calibrated my speedo it was right on with the 9% it said I was off. I am confident that my calculations on fuel economy are correct prior to correcting my speedo. And now with more power and a corrected speedo I have burned 1 tank on the highway and 1 tank in the city.

All I know is what I have done to my truck and the results that I have gotten.
Like I said, when I got this truck, all I wanted to do was get taller tires and drop the RPM. I knew for sure if I did that , it would increase my mpg by alot. Well I was wrong and I actually lost mpg in city driving. I know this has been dicussed here several times and you can ask some of the mor knowledgable guys on here about taller tires and the truck lugging. No you can't feel it lugging but you are mashing the peddal just a bit more to get the same performance out of the engine.
Yes I know you are talking about Highway driving but what is the point if you are going to lose $$ in city driving. And as I said, where I lost mpg in the city, my highway mpg did not change at all. So there was no beniffit other than seeing the rpm gage drop about 300 rpm's.

This is my experience, take it for what it's worth. Thats what we are all here for to learn from others and what they have done. There are hundreds of combonations of mods you can do to these trucks and every combonation will have a different outcome.
I can tell you that I now enjoy an average 3 mpg increase in fuel economy with and added 60 HP and 35" tires compared to what I was getting stock. With just the 35" tires only, I lost 3 mpg in the city.

 

Ive been looking hard at the overdrive gear sets for my truck in a way to lower engine RPM's. . .only thing is you have to shorten the drive shaft. total cost is somewhere in the neighborhood of $3500 and you dont loose your stock gears.