I will be buying tires later this year(if prices don't keep jumping) and am wondering about running 31's under my truck. It is an 88' 150 4x4 (300ci), m5od and currently has 30" tires on it. Truck has 122k miles on it, and last fillup I got 19mpg. I want a little better look out of the truck, but don't want to kill my mileage. Is there any real-world experience of running 31's behind 3.08 on the 4.9? I have thought about doing a 2" leveling kit, I can get one for $100 for front and rear, but am afraid of losing mileage with the 2" lift and 31's. Has anyone tried it, and if you have what are your mpg?

I do not usually tow anything, but use it for camping or pulling a boat to the lake in the summer (very occasionally).

Thank you in advance for your help

 

I have almost that same set-up. Mine is 4.9L, 3.08, 4-spd TOD, 2wd, 31x10.5x15's, and I just put some new coil springs on the front to lift it about 1/3/4". I have not changed the speedo gear yet (came with 215/75/15 tires). I get roughly between 16 and 21mpg.

 

I have a 92 with 6" lift and 33's 135,000 miles before my o2 sensor stripped out I averaged 18 now it's 12.

 

What gears do you have turning the 33's? You were getting 18 with 6 inches of lift and 33's? That is awesome. I would think that 2in leveling/lift kit shouldn't matter then, as well as 31" tires. When you say stripped out, you mean broken?

 

Good Lord...I wish I got 18-19 MPG even on the highway with my 2wd!!! I get around 13-14...

 

I try to drive with a soft foot and shift below 2grand. I am seeing more and more that the mpg from the 4.9 is either low teens or close to 20.

 

Yea, I've done that. Got around 15-16.

 

If it's the truck in your sig., I don't know why you're not getting any better than that. If it's got the engine in your name, then I can understand why.

 

My old man drives a 90' with a 460 and gets 10-12.. can pull past anything but the gas station... what would an electric fan setup do for me?

 

With bigger tires your speedo and odo will be off.. So you wouldn't be getting the mileage you think you are.

I couldn't imagine 3.08s with stock tires let alone 31s.

 

Lol, no...I made that name back when I was looking at a sonic blue 95 F350 srw 4x4 super cab longbed with a 460 in it. Had a lift/mud tires...was gorgeous!

But yea, Daisy has her original 300 I6, in pretty good shape with a 4-speed TOD manual and 3.55 gears. I figure the gears are killing me. Oh well, I'd love for her to do better but I love her just the same!

 

the 30's are 3% taller than stock, so I actually take into account the difference when filling up and doing calc. The drivetrain spun for 106.4 miles on the factory speedo gear, and it took 5.699 gallons to top off, so with factory size tires it would be 18.67 MPG. Being 3% taller, I multiply 106.4 by 1.03, to get 109.592. Might not be exact but closer to actual than just saying 106.4. So in reality, it isn't just traveling 106.4 miles. It traveled further because of the taller tire, so I didn't just travel 106.4 miles, it was more like 109.592 or thereabouts. Anyways it traveled further than with the factory size tires. You get the picture. adjusted for tire size is 19.003.
Unheard of with 302's. The 4.9 loves low RPMs so it doesn't need to be wrapped up to make power or pull, that is why it gets away with higher gearing.

 

Wow, I'm kinda glad to know it's the 3.55 gearing. I was considering swapping mine out, but not now. Especially with the price of gas going the way it is.

 

When my F150 still had it's original 300 Six in it, I had 31" tires with 3.08 gears and it pulled that no problem. It helped that I had a ZF 5-speed so I had a granny low if I needed it, but I almost never did. As stated, the 300 likes a lot of gear. I got about 16-18mpg.

 

-I have 3.08 gears brand new 33" Dura Tracks and my speedometer was corrected for the tires Now if I can just fix that darn O2 sensor to get my mileage back (the sensor falls out if i floor the truck drilled the hole a pinch to large a hose clamp is holding it now).... I also just bumpted my timming up to 12 so this may effect my mileage again we shall see lots more power thats forsure! I admit on freeway with 2 place snowmobile trailer overdrive does not have enough to keep a steady speed hafta use 4th. I hafta thow this in my 91 150 has a 2" leveling kit with 32/11.5 15 BFG M/T's and got 10-13 wasnt the best sounds like it depends on how healthy the engine is idk. I would say go for it they look great leveled out and its pretty easy!