hey everbody,
I was wondering what some people experiance and opinions on my tire choices. i have a 91 f150 with straight six mazda 5 spd and 3.08 rear ends, its four wheel drive and gets 14 city 19 highway. How big of tires can i run without affect on performance and milage in a negitive way?
thanks for any help!

 

3.08 is definitely the highway gearing. My Bronco has that with the 300 and I get about the same mileage as you (it's an '81 so it's carb'd and has the 3.00 rear end).

What size do you have on there right now? That's going to be the determining factor.
With that gearing, I wouldn't go much higher than 31" or maybe 32". With 32s, you're probably going to start feeling doggy.

 

I wouldn't go any bigger than 30x9.50's. I have 4.11 gear ratio and my 31x10.50s feel like they're too large for my 300. Maybe my particular 300 is just tired. Runs good, don't get me wrong. Just doesn't run like it did when I had 27x11.00 tires.

It's all up to you. I would just stick with OE sized tires if I were you. OE sized tires can still look good depending on the tire!

 

I currently have 30x9.5s on mine, and they work pretty well. With the 3.00 rear end, it's already kinda sluggish with those tires, but it gets moving, maintains good speeds, and gets nice mileage.

I know I could go with 31s or 32s, and it would handle it, but I already know that it's going to start getting really bogged down with out re-gearing.

The best thing to do is to just do some math. Take your current tire size (it's better to actually measure with a tape measure than what's on the tire. Just measure from the center of the hub cap down and double it, don't measure from side to side). You'll have to know your transmission gearing, which you can probably find here:

http://fullsizebronco.com/forum/showthread.php?t=49569

Then enter in the different tire sizes you want and see how much your RPMs change, or speeds change.
You can download a tire/gear/RPM calculator I programmed here:

http://www.angelfire.com/id/sandmann/stuff.html

Let me know if it doesn't work.




As for 27s 6CylBill, I imagine that you were probably used to those tires and the torque they gave you. Off the line pretty quickly, but I can't imagine much top end. With your setup now, you're still geared better for low end than most 300/4.9s from the factory. It's all what you were used to, so if you're engine is running well now, it's probably back to "normal."

 

You are correct AB, the top end is poor with the low gears and big tires. She'll still hold at 70 MPH all day long but the speed just isn't there. I built my truck for work and it does that very well. Just need a part time rear locker now.
I have to say though, a 300 I6 with high gears runs like a beast in the top end. I always did think my truck was fast when I had a 3.08 gear ratio.

I'm off to check out that link you supplied about the tranny gearing.

 

hey thanks,
you guys answered what i needed to know and kinda thought the 235/75/r15 are what i need i wanted 31-10.5 but with as much town driving i have to do and hate i need the smaller tires, gotta be able to still pull the flatbed trailer with my little mower its sad not letting my six do more!

 

Smart man! Stick with those OE sized tires. You'll have the best power and fuel econ that way.

Just a tip. Go for larger rims and keep your tire size OE if you're wanting that custom look. It'll give you that look without hurting power and fuel econ.

 

I agree with the others. Stick with the stock tires. To ease your curiosity, you might have a buddy like me that has 32 inch tires on stock rims. I need to go back to stock tire size myself, for towing power. We could swap wheels and try each others out for a few days. Then you could really feel the difference. I even thought about just trading even out. I could give sombody a great deal as my BFG all-terrains are like new and a fantastic all around tire. The swap would have to be local. They look great on an F150.

 

OE tire size would certainly be better for gas mileage and probably for towing as well, but I have no problem towing my 7000#Bobcat and I have 35x12.5 BFG mud terrains on 17" rims. I don't have any prob at all doing 70mph either(not while towing of coarse). I did have to put in a 4.33 gear though. Can't say too much for my gas mileage...oh well, you give and you take. lol

 

Hey guys thanks for the help
i traded tires and wheels with my dad and his bronco for a few days it has 265/75/15 and i like the way they are working so i think i will get me a set of them, pm me about those tires i need a new set

 

Ive got 33X12.50X15's on my 89 150 and she goes and goes and goes totaly awesome on fuel man. My gears are 3:55's Heck I can go back and forth for a week on $30. And our fuels worth more then all of yours lol