I have a question that I hope someone can help me with. Although I've read several messages relating to this issue, I haven't found one that answers my question directly. At any rate, I recently purchased a 2003 F-150 Lariat FX4 with BFG 315x70x17s on it. The truck is also equipped with ranch hand type front and back bumpers. (I'll post a picture later). Needless to say, the truck looks tough. The only problem, it doesn't seem to run like it looks. It has a 5.4 L engine, Flow Master muffler and a K and N drop in breather. The way I figure it, I should be turning over 300 horses. This pickup doesn't seem to have near the power that my previous truck had (Chevy 4x4 with 5.7 L and 265x70x16s). Although I really like the way my truck looks, I also think that a pickup should have some utility as well (I plan on eventually pulling a horse trailer with it). At the time I purchased it I was also looking at the 250 Powerstrokes. I chose the 150 primarily because of the price difference. I know that I've been pretty long winded, but I wanted to get some background info. out of the way. Now for my question, does anyone have any advice on what way I should go? 1. Should I go back to the original 265s, or will going down to 285s give me enough power difference? Last, would I have been better off going with the 250 Powerstroke and paying the extra cash? As I stated, I know this is a rather long message, but I would appreciate anyone's suggestions whether it be to change tires, modify engine, or change trucks. Thanks ahead of time, Robin

 

Welcome to the forum!
I bought a 1997 with 740 miles on it and it already had the 285's on it....so I didn't really "notice" any power drop due to the larger tires. That was until I got my 1715 Micro Tuner a month or so ago. The 1715 made a "significant" difference to the performance of my truck. (just ask anyone here, they are probably sick of me raving and typing all about it.)
My 2 cents? Read the post Mike Troyer just did today in the computer chips section on oversize tires and then do a search(top of page) on 1715 and 9100 for more info. I am beyound happy with what my 1715 did for me and mine.
Good luck!
edit: The post is not too easy to find, go here, to the last thread by Mike:
https://www.f150online.com/forums/sh...hreadid=153609

 

I appreciate the advice and direction. In reading the other message that you linked me to I came away with the idea that the 285s should be no problem for me and that they won't require any change in gears. I was also imipressed with the 1715. Think I'll have to go that route. Thanks, Robin

 

Two main culprits, both related to the larger tire size. First, you effectively reduced your torque multiplier by at least 9% over stock. That is the equivalent of going from a 3.55 rear axle ratio to about 3.25, if you are famlier with with those effects.

However, the larger impact on performance is the significanty heavier tire, which has its weight farher away from the center or axle. More rotational mass farther from the center has a lot more inertia than stock wheels which affects performance more than the increase in tire weight alone.

 

I went from 255/70-16 to 285/75-16 and it really slowed my truck down. I changed from the stock 3.55 to 4.10 (3.90 would have been sufficient) and it brought back my performance and then some.

That FX4 probably had 3.55 rear axle ratio with 265/70-17 as OEM tires. I'm assuming the rear axle wasn't upgraded and considering your larger tires, below is a calculation of what rear axle ratio you would need to return to stock performance...

Original Tire 265/70-17 = 31.61 inches
Upgraded Tire 315/70-17 = 34.36 inches
Original Axle Ratio 3.55
Corrected Axle Ratio 3.86

I highly recommend you change your rear axle if you want to get your performance back because of the larger tires. 3.90 would be sufficient, but 4.10 would be funner!

If you don't want to pay the $1000 estimated to get the new gears and install done, just get the stock size 265s. 285s will be an improvement, but still a little sluggish IMO.

Link To Axle Ratio Converter

 

Ok I agree with gpaje, that the tires are to big to match what your driving as is...If your hauling a horse ,driving on highways you should look at going back to stock 265/70/17....I think your rig will handle better with the trailer as well...Check what your warranty says before you use a different chip in case it causes you a problem later...Any idea what your gas mileage is now?

good luck