hey all-

well, i'm getting just about sick and tired of my truck getting about 9-10 mpg, so i've decided that the rear end has just got to go. i've been trying to kick around in my head what ratio would work the best for me, but i'm not exactly sure what my options are. what are the possible rear end ratios for my truck? right now the tag says i'm running 3.55, but that can't be right. the truck's smack dab at 2500 rpm at 65 and 3000 rpm at a little over 75! sounds like a taller gear setting to me. i need something that'll get me down the freeway at about 75-ish that'll keep the truck below 2500 rpm. i've tested it out and 2500 seems to be the breaking point between OK (13-14 mpg) and horrible mileage (8-10 mpg) on long trips. (i drive back and forth between san luis obispo and san diego all the time). also, would it be best for me to buy the gears and have someone else put them in, or should i get a used axle and just throw the whole thing in? can i switch to another axle to ge the ratio i want (like a 9" or something), or am i stuck with the 8.8? any help is appreciated.



Mike

1990 F150 Custom
4.9L, Mazda 5speed
3.55? rear, 2wd
K&N FIPK, Magnaflow exhaust
60K miles (i'm just getting started!!)

 

[updated:LAST EDITED ON 17-Jan-02 AT 10:19 PM (EST)]A 3.55 is probably right for what you have if you are running 2100-2300 at 65mph in OD. I have a 9" with 3.50 gears that run at 2500 in 4th at 65mph, because I have 31" tires and .05 less gear. 3.55's were about standard for 8.8's in the 4x4 300/4.9 trucks. It's not uncommon to fine them in the two wheel drive, though. I'd jack up the rear and count the tire rotations to the drive line rotation to see what you really have.

If you want to get better gas mileage you could go with a 3.08. I believe that is the next smallest ratio found in the 8.8's. Rebuilding a 9" transplant, heck any rear, is a very difficult task. Take it to a shop unless you're really handy. I'd keep the 8.8 since a shop isn't going to try to charge you more for the fact it's a 9".

Your gas mileage now is awful low at that speed. I get around 13-15 with a 20 year old 4x4, sloppy drive-train, and no overdrive. Then again there are a lot of different reasons for fuel economy issues... like living in California :P

Kerry

 

Mike , Thats really low mileage for a 2WD , I have 3.08 in mine , I have a 90 4WD & I get 14 Mpg city & 16 Mgh highway with 31 In tires . I wonder if its not something engine / Efi related . Look on the sticker on the inside of the drivers door , See what it says under the Axle Section , post the numbers or letters , that will tell you what rear axle ratio you have . Only 60K miles ? I just turned 180K on mine tonight .

 

I believe the '95 brochure for the F150 also lists a rear ratio around 3.33.

INLINE SIX POWER!
300 Cubic Inches of Low RPM Truck Torque! And twin-I-beams too!

 

With three F150s, think that I have experienced about any problems common to these vehicles. My '84 has 2.47 gears, and 4 spd manual trans. Gets, regularly, 18-19 mpg, with 65K on rebuilt engine. The '85 and '86 both get low teens (when it gets to double digit) gas mileage, and both have AODs. Haven't checked the rear gears on the latter two, but my '84 still pulls strong with no add ons (no headers, carbs, etc) - just normal bubble gum and bailing wire!!

 

I have a 4.9, 3.55 gears, AOD, 235-15 tires and my tach reads dead on 2000 RPM @ 70 MPH. On the interstate I get 19-21 MPG if I cruise @ 60 MPH.

 

OK, I did some homework. Here's what I found:

1) The rear end tag reads that I have a 3.55 with limited slip.

2) When I jacked up the truck and tried to find out the true ratio by turning the wheels, it was looking like I was getting 1.5:1, which is obviously not right at all.

3) The truck has a nasty intake / exhaust manifold leak. (see some of my previous posts). I was told on a I6 that this can have a huge effect on mileage.

4) A few specs on the truck I might have neglected to mention:

-P235/75R15 tires (Pirelli Scorpion A/T) Usually 35-40 psi
-K&N FIPK, Magnaflow exhaust
-The truck has a shell on it, and I carry about 200 lbs of stuff at any given time

5) 3 month old spark plugs have a lot of chalky buildup. (Autolite Platinums)

6) I'm planning on having a buddy of mine smog the truck tomorrow, so I'll probably get around to replacing the O2 sensor if it's gone. I'll also have him pull codes.


OK, so there's the deal. If any of this helps anyone think of some ideas as to what the is wrong with my truck mileage-wise, let me know. Thanks for the feedback so far.


Mike

P.S.: Compared to the same truck with an AOD tranny, my mileage is significantly worse, which leads me to think that it might the culprit. Anything I can do to help it along other than shoot it?

 

>1) The rear end tag reads that I have a 3.55 with limited
>slip.
-Limited Slip rear will tend to give you poorer gas mileage due to more components and drag.

>2) When I jacked up the truck and tried to find out the true
>ratio by turning the wheels, it was looking like I was
>getting 1.5:1, which is obviously not right at all.
-You may want to double check this. It should be closer to 3.5:1
I can't think of anyhting that will give you this bad of a gear ratio.

>3) The truck has a nasty intake / exhaust manifold leak.
>(see some of my previous posts). I was told on a I6 that
>this can have a huge effect on mileage.
-Ahhhh...I think I see the big culprit. Guess I missed your previous posts. I had a problem with a leaking manifold gasket and lost about 4-5 mpg. Change this, tune the truck, and your mpg should go up noticeably.

>4) A few specs on the truck I might have neglected to
>mention:
>
> -P235/75R15 tires (Pirelli Scorpion A/T) Usually 35-40
>psi
> -K&N FIPK, Magnaflow exhaust
> -The truck has a shell on it, and I carry about 200 lbs
>of stuff at any given time
The FIPK should give you about 1/2-1 mpg if you nurse it. Overall, it just helps with performance (stomp on it and it hauls a bit quicker). The canopy, AT tires, added weight will tend to drop you down in mpg, but not to 9.

>5) 3 month old spark plugs have a lot of chalky buildup.
>(Autolite Platinums)
What heat range? AP45 should be right. Sounds like your running it a little lean. This could be due to the manifold gasket problem.

>6) I'm planning on having a buddy of mine smog the truck
>tomorrow, so I'll probably get around to replacing the O2
>sensor if it's gone. I'll also have him pull codes.
Is that smog check? Will it pass with the manifold leak? The o2 sensor may be getting bad signals from that leak causing even more performance and mileage losses.
>
>Compared to the same truck with an AOD tranny, my
>mileage is significantly worse, which leads me to think that
>it might the culprit. Anything I can do to help it along
>other than shoot it?
I don't have an AOD, but generally you should notice how it shifts, any delays it has in shifting, or if a noise is coming from it.

I'm pretty sure that it sounds like your gasket is the culprit. I'm not saying it'll fix it 100%, but it will get you closer to 13-14mpg. After you get it fixed, make sure you tune it again. The manifold vacuum will have changed back to 18-23inHg and you will have gotten a different advance to the timing.

Hope this puts you back on the right track.

Kerry

 

Thanks Kerry!-


That sure puts some priority on getting that leak fixed. (Up til now, I thought it was just annoying!) As for your spark plug question, I'm running AP5125. That's supposed to be stock, according to my computer at work. (I work at Kragen). I took the truck to have it smogged, and all of the readings were just fine once it warmed up. I did forget to have him pull codes, though. So, basically I'm looking at having the leak fixed and re-gearing the truck. 3.08 is sounding awfully nice.

Mike

 

Hey Mike,

No problem... love to help .

>As for your spark plug question, I'm running AP5125. That's supposed >to be stock, according to my computer at work. (I work at Kragen).

I am a CSK loyalist myself. But, I don't always trust their computer. It may be that your 4.9L head has a different set-up since you have FI and a computerized ignition system. I am old school and prefer the Duraspark stuff. I've noticed that if you try something other than the manufacturer recommendations you may benefit from it. What's $1.69 a plug for six plugs? You may want to try AP45 plugs just to check results or compare the two plugs side-by-side. See what differences you notice. If it's a thread size deal or anything that will prevent it from fitting right then, obviously it's not a good idea.

>I took the truck to have it smogged, and all of the readings
>were just fine once it warmed up.

Sounds like the leak is worse when it's cold. Then as the engine heats up the metal expands to fill in most of the leaks.

>I'm looking at having the leak fixed and re-gearing the truck. 3.08 >is sounding awfully nice.

Wait on the regearing until you check the mileage after the leak is fixed. You may ultimately hurt your MPG if the gear ratios in the tranny aren't matched for the 3.08. It will cause more wear on the engine and drive train trying to pull the truck out of a deeper gear.

Just some food for thought.

Kerry

 

Mike,

My stock '95 F150 turns about 2250 at 75 in OD. The rear end seems very well matched to the Mazda 5 speed (3.90, 2.25, 1.50, 1.00, 0.80) and when I had to replace the rear end (junkyard unit for 250$) I had my golden opportunity to change to a higher ratio, but didn't. I really didn't want to turn higher RPMs on the freeway. My mileage is around 16.5 with city/freeway mix. But remember, thing are pretty flat here in SE Michigan. However, I hauled a 2000 lb trailer through Yellowstone and had no problem other than normal downshifting and once trying to start from a dead stop going up a pretty steep grade (there a 4.11 would have been nice). Without the trailer I don't see any problem driving a light loaded F150 with a 3.08 in the mountains. So, as advised above, fix the leak and see how it does, then consider the different rear end.

INLINE SIX POWER!
300 Cubic Inches of Low RPM Truck Torque! And twin-I-beams too!