ok so I have an 84 f150 that I think has an 351ho with a c4. Also guessing 4.10 gearing? I haven't had this truck very long and haven't had much time to dive into it.
My question is, the original tires are 31-10.5-r15. I need new ones. With those I am getting 8mpg. If I switch to 235/75r15 am I going to see a huge difference? Or should I go for a 265/75r15 for a similar tire size. 80% of my driving is highway. Thank you!

 

Welcome to the forum, UUARD!

For best overall gas mileage go with the size on the door jamb [factory equipped].

Many guys like to go with a larger tire for varying reasons, but larger will weigh more, have more rolling resistance as well as air resistance, plus it will lift the truck a little causing more air drag. On top of that, the speedometer/odometer will not be accurate without changing the speedometer drive gears.

If you are not driving off the road, you might want to consider a less aggressive tread pattern.

 

Without knowing your actual gearing and how your current tire size compares to stock, you cannot really say what your mpg is unless you are sure the speedometer (and thus odometer) is correct.

For instance, stock tire size was 235/75-15, or about 29" tall.
Stock gearing could have been 4.10 but it also could be 3.54 or something else.

With a 31" tire you are turning more weight than stock BUT you are also traveling further with each revolution versus stock. Could be a wash between the two.

This goes back to proving out your odometer for mileage calculations. Even if the gearing is stock, you are traveling about 9% further on the 31's versus stock than what the odometer says. So say you're getting 8mpg calculated.....we'll say 160 miles and 20 gallons.....really you went almost 175 miles so calculated mileage should really be about 9mpg. According to fueleconomy.gov the three speed auto '84 F150 4x4 351 is rated at 10-12mpg highway depending on options.

 

Sellers- I know what your meaning, and I do think my odo is a little off. I have calculated with a gps and how many gallons of gas I have used. This has come out to between 8-8.5mpg. Does this help a little more? So your saying that I should go for a bigger or similar tire?

 

The stock tire size will give you less rolling resistance than what you're running now. The question is will the savings there offset the shorter distance traveled? Quite possibly. When I went to tires a couple inches taller and somewhat wider on my Jeep, I changed out the speedo gear so it's right and I did still lose 1-2mpg. The problem is you're fighting a gas guzzling monster either way

At least there's this....if stock gearing was 3.55 then you're actually getting awesome mileage ha ha.

 

What is the axle code on the sticker in the door jamb?

 

I would bet your trans is a C6.