Hey Guys -

Bought my first 4x4 a few months back and have been fixing it up. It's a 96 F150 Eddie Bauer with 5spd and L6 300. It runs great but needed some TLC.

The guy that owned it before me put 31x10.5s on it. Stock tire size was 235/75x15. I fixed the speedo constant for the tire size to correct the speedo reading. In doing so, it got me thinking about how the tire size has impacted the trucks performance.

It has 3.08 Non-LS set up or at least thats the code on the door sticker. The truck accelerates pretty good off the line and up through the gears until OD (expected), but it's a pig on gas. I do most of my driving in town and I'm averagin about 11-12 MPG. The MPG doesn't bother me a lot, it's winter and I have been driving around with the manual locking hubs locked in. I did a 100+ mile trip in it a week or so ago and with another 60+ miles around town averaged 14+ MPG.

Anyways, sorry to be long winded, just giving the whole scoop. I was wondering what effect the 31x10.5s have on my gearing? I don't do too much off road with it right now and don't plan on it being a serious off-road rig. It's my daily driver. Should I consider putting in 3.55, 3.73 or 4.10 gears? Or should I just leave well enough alone?

Rob

 

I would get some 3:55, I had a 95 F-150 2wd with the 5.0/5spd, and it had 3:55. it moved real well. I had 31" tires on it as well. 3:08 gears seem kind of high, especially for a 4x4. The highest factory gears I ever had in a 4x4 were 3:23's It might help your mileage, giving your engine a little more leverage so you don't have use as much gas to get things rolling.

 

235/75-15's are about 29" tall. The 31" tires effectively raise (numerically lower) the axle ratio from 3.08 to about 2.88, which is about a 6.5% change. The engine rpms are lower at any given vehicle speed in all gears by the same percentage with the bigger tires. To correct the 3.08 ratio for the 31" tires would require a gear change to about 3.28.

I wouldn't do a gear change though unless I was at least going with 3.55's. You could go as low 4.10's with it, but with the 6-cyl engine, 4.10's aren't needed unless you want to pull alot of weight. 3.55 or 3.73 would be good ratios with 31's and a 6-cyl with 5-speed/OD. Ford did make some F150 axles with 3.31 gears, which would be about right if all you want to do is correct for the bigger tires.

 

Thanks for the info. I'm thinking about this truck as a long term project. I decided to buy it over a newer (new) one cause the price was right and I really liked it. I plan on keeping it a for a while and continuing to fix it up. I got a decent deal on it ($3K) and have put about another $2k into repairs - some needed, some upgrades. So I guess the gearing is not an immediate concern. I don't want to change the tires though, because I will be using it occasionally in the woods (hunting etc).

Am I doing any engine or drivetrain damage or excessive wear due to the high gearing?

And I guess a follow-up question would be, if I am going to change the axels or gearing anyway, should upgrade to LS?

Thanks again for the help.

 

During a gear change is a great time to upgrade the differential. If you're having a shop do the gears, installation of the new diff is free, since the diff is out anyways. Could be 200-500 more for installation of the diff if you wait and have it done at a later time.

Since it is a manual trans, you're not likely doing any damge to it keeping the high gearing in the truck. That affects auto trannys more.

The engine will be working harder to move the truck with less mechanical advantage (torque multiplication) due to the high ratio (numerically low). If you live in the hills, tow anything, or do alot of stop and go driving, I would expect decreased engine life and gas mileage. Fortunately, you have the lower-revving, torquey inline-6. It would be worse with the higher-revving 302 V8.

 

Thanks again. I appreciate the help/advice.

Now I will decide if I want to spend a bunch of $ and put new gears/ls-diff in or if I am just going to go back to stock size tires and try to sell the 31's. Like I said previously, it's daily driver and I won't do much off-road with it, so I guess it's going to be a want/need decision.

The type of off-road it will see is mostly going to be logging roads and the like, so I'm thinking that the 31's are overkill for my uses. But on the other hand, nice have them when you need them.

 

Techno, let me clarify that I'm not saying you HAVE to change the gears to run the 31" tires on the truck. You asked about gearing and it would be better to change them, but if you are happy with the truck the way it is, then you can certainly run it as-is and the truck is not going to self-desrtruct because of it .

 

Didn't take it that way. Decided to go back to stock tire size anyhow. My goal is to get it working good and take my time making the long term changes (gears/ls diff etc). For now, the 31s are in the basement.

 

Sounds like a plan. Have fun working on the truck!