I am running a 4:30 gear with 32" tall tires and I am supercharged and I get 13.5-14 on the hwy cruising at 70 which is right at 2250-2300 rpms. Of course when I was NA i could only get 13.5 topps out of my truck and that was tuned.

Back in the days when I had a lifted 4x4 I ran 40" tires with a 4.56 gear and only a 3 speed non overdrive tranny talk about lousy gas millage I think it was 7.5 mpg but gas was only .86cents a gallon back then.

The more rotational mass you have the taller gear you will need to allow the transmission and engine to run in harmony and not strained. The rule of thumb in the fox body was one gearset higher if FI vs NA so if 4.56 NA then 4.10 SC.

 

That's all your truck gets? I figured it would at least pull 17 or so?

 

thats me driving. when the wife drives it she has gotten 17.5 on the hwy.

 

I drive like your wife:o

 

x2

 

I am nervous now after reading this post. I am planning on doing the 4" Rancho and 35's in the spring, and was just about to order up a set of Yukon 456's from Justdiffs because thats what everyone on here has been telling me are the gears I would need. I only have a S&D CAI and Magnaflow exhaust and plan on having my Edge changed into a Gryphon by PHP so I can add some custom tunes. I know alot of guys on here with 373's like I have run 35's and say they have no problems, but I don't want to blow up my trans prematurely running the 373's. Like I said now I'm really confused. Maybe I'll give Wayne a call at Justdiffs and see what he says.

 

Ive been running 35's with my stock 3.55 gears for over 4 years in my truck. get the 35's mounted and see how you like it.

 

And if you want height and big tire, you lose MPG.

 

And you lose power. It's a no-win.

 

I average 12-13mpg with my truck, leveled on 35" KM2s with 4.56 gears. That is driving aggressively with a 93 octane performance tune. On the highway it is typically around 14mpg. No better, no worse than it was with 35s and 3.73s, but the performance is significantly better. My odometer is also spot on and confirmed by gps and radar. And I'm turning closer to 2100RPM at 70mph.

Is your speedo correctly calibrated? If so, I think the transmission is your problem, especially running that RPM.

 

yes it's all calibrated correctly, I have an SCT handheld tuner that I use for my supercharger and I can compensate for tire size, gear ratio, and a whole lot of other things. I'll let you guys know what happens when I get my built transmission and can figure out the miles/gallon. I think that is the big culprit right now. There is a lot of slippage and noticable loss of power/performance in the last 2 weeks.

 

My truck has pulled 18mpg on a trip to dallas(all highway). That with 4:56s,35s, and a supercharger running 93 octane gas. Something has to be wrong with your trucks or something. My truck averages 15 mixed. Now I dont race the thing, but I dont drive like a g-ma either.

 

Perfect advice from Windsor.



OT to the OP: are you a cadet/upper classman at the Point? I was stationed at Camp Buckner for a while about 40 years ago. They figured that since I could drive an Army tugboat, I could run a ski boat and pull water skiers around the lake. Best TDY I ever had... we ate in the cadet mess hall too because they didn't have a separate enlisted mess. I enjoyed going onto the main post... plebes, you gotta love 'em. They salute everything in green... including Spec 4s!

 

sorry guys i have been off for a while jimckenny i went with a ouo 6" rear block with NO taper. the blocks are pricey but you can interchange with anything with 3" springs and they hold the u bolts from slipping also. do not get 5 degree blocks you will have bad vibrations and burn pinion bearings. i know from experiance. i went with the add a leaf because of the height. it barely made mine stiffer it actually took some of the bed bounce i had out. lomo2surf yea my milage dropped badly. but i have way more power and i can still run 120 if needed it just whines out od. powerbraking is not a problem at all doesnt take much gas at all.. and it will do some hella doughnuts.. haha
before i had 3.55 and was getting 18-20 on hwy at about 70 now i just took a 2 hr trip and its about 15 with 4.88 and 35's. i loved the 3.55's because i could run about 145+ but the tires felt loke they were about to let loose. only done that once. but it will save the trans with the 4.88's in town my milage has went up because it doesnt take much to get going anymore

 

If you wanted highway gas mileage then why'd you go so steep?

There's always trade-offs with gearing. I would've went with 4.10s in your situation. 4.56s would be better for 37s or a lot of offroading.