Matt127

ive got 3.55s and so far i get about 18 or 19 mpg

newriverSpecon

Length of drives has a huge impact as well. When my wife usesthe 97 for short trips, less then 3 miles, the mileage drop a lot. Even running about 6 miles each way to work still drops about two MPG. Tires make no real diffeerence. I can run the 315 mudders or some 265/75 R17 (2005 OEM) and there is no change. Driving type (type of trip and pedel use) has the largest effect.

On the 99 I didn't see any milage change when I did the gear swap. I did gain 10 mph at my cruisin RPM. If I ran 60 MPH with the faster gears I might see something. I have tried running 65 vs 70 for a few tanks and seem to lose MPG.

Randy

Buckshotram

Over the summer I was running 305's w/3:55 and got 18-19mpg allmost every tank mixed driving. Ive got 315's w/3:55 and and taking short trips to campus 3miles from the aprt in city stop and go and the milage is horrible, and still not as good on the interstate as before with 305's. The two biggest thing ive noticed to help out with milage is slow down on the interstate and take it easy in the city. I think 4:10's and 315's would be a pretty good cobo.

strokerace

I get around 18-20mpg with mine and I have 3:55's, if you do the math it really doesnt take as long as you think to pay for the swap depending on how many mpgs you get after the swap. I have a reg cab 4x4 5spd w/ a flatbed but I have the stock height with 265/75 tires on it and drive her pretty easy out on the highway. Good luck, I hope you get her figured out, fuel is to high to be wasting it when you could be doing better.

C2O

it will pay for itself only if it help. some claim to get worse after a swap.

Luckyrogue

I have 3.55's in the rear, 265/75/16's with no lift on my '01. Stock everything except for the flatbed and I only get 14-15 mpg's. 14 if not on the h-way and 15.5 on the h-way, does'nt seem to change much if I'm in the pedal or not. I'm looking the any advice for better fuel mileage as well. Will a valve adjustment help?

colekicker

My 2wd 2500 auto has a 3.55 and running empty, cruise control, 70 mph, it is a consistent 21.4. The engine is completely stock. No chip or module. Stock injectors and IP.

Now, when I start to pull, the mileage is a different story. But it sure is good running empty.

Colo_River_Ram

Check this out:

http://www.miata.net/garage/tirecalc.html

Dhorn33

I have gotten fuel a few times since changing to the 3.55's but it also has gotten cold here and it snowed twice last week. One tank was 14 from during the storms and 4wd driving in stop and go - the next was 16.2 with somewhat normal highway/city driving in the cold. Temps have been from 0-20 degrees here so it is hard for me to compare with mpg prior to my swap since the day I did it it was the first snowfall of the year and it got cold!

I am much happier with the way the truck drives down the highway - at 70 I am now running around 1900-1950ish and there seems to be plenty of power. I just feel like the lower rpms are easier on the rig and hopefully the wallet!

74dart

When my truck was still an auto and after i added the 370s my milage went from 18 to 12 because I couldnt keep from doing full throttle launches. Now with the nv-4500 I hardly ever run WOT and my milage went back up to 18-19.

Sunir

good thread...I'm running the stock 265/75-16 Michelin HT tires with 3.55 and an auto tranny...I used to see 19-20 mixed (hw and around town) before I got the triple disk Dunrite Converter and valve body and am seeing 17ish...I drive it pretty easy, not hard on the throttle; just smooth daily driving...hopefully with smarty level 3, plus 4" exhaust and some mild injectors (RV275 or M1's) I'll get some better mpg and keep the same nice everyday drivability...

JasonblkZ06

So what is it about short trips that makes the mileage so bad? Is it just stop and go, or does it also have something to do with engine being cold etc?

That is interesting that your mileage is the same with the 2 different tire sizes. I have been contemplating getting some smaller tires since I need to replace mine. I have 315/70R17 H2 wheels and tires currently and get about 18-19 mixed city and highway.

infidel

A bit of both.
You have to remember it takes a lot more energy to get a 7000 lb truck rolling than a 2000 lb sedan.

ecbrewer2224

ive got a 2001 sport 2500 6 speed when i got it it had 265/75/16 on it and it had a small lift and really looked dumb with small tires so i got 315s and i added a edge juice chip with a a2 monitor when my gas light comes on my truck holds right at 33 gallons i can go 300-320 miles depending on how far i go with the gas light on so im getting 9 to 10 miles per gallon and it doesnt matter if its all highway on city i got the same mileage before the the 315s and chip ive tried running on stage one where its suppost to help mileage then i just shut it off i still get the same mileage no matter what other than the chip my truck is stock it only has 38,000 miles and i hate to sell it but i cant handle $100 fill up for 300 miles any help would b great

wcbcruzer

For city, 4.10s are best. For highway, 3.55's are best. For both, lightweight stock tires are best. Also, an automatic tranny has the lowest O/D ratio; hence, at 70 mph you turn the least amount of RPMs compared to any manual.

I don't buy that bigger tires give better mileage. The weight and drag of a bigger tire negates any benefit gained from diameter.

I was wondering if a triple disk TC would have a negative impact on mileage and it appears that it does according to Sunir. I suppose the added weight in rotating mass of a triple disk hurts fuel economy.

FWIW, using synthetic oil in the engine and differentials should help.