I cut this from my reply to a similar post, and pasted here to start a new thread:

Well, I have 3:55 gears and 33's, but they measure out to 30" - 30.25", so they're not that much larger than the factory tires, but they weigh 65 lbs. a piece as opposed to the stockers that weight about 30 lbs. I believe. Don't know what effect that has.

I have a 2001 F-150 SuperCab FlareSide 4.6L 4x4 manual. My GVWR is 6,250 lbs. My vehicle weight is 4,651 lbs. & payload is 1,595 lbs.

As a general rule, does this mean I can basically carry an additional 1,600 lbs. in the truck with the "max performance" tune, including fluids, people, accessories, and junk in the bed without risk of melt down? I believe with both my girlfriend, myself, the dog, the added weight of the tires, the custom sub & box, two dirtbikes, fliuds, accessories, and camping equipment, that puts me right at about: 1,600 lbs. What do you think?

 

Hi Jackal,

Wow, that's *quite* a difference between what was "advertised" versus the actual measured tire height on those so-called "33's" - to have them measure out at only 30" is a *huge* variance. This is an excellent example of why we recommend actually measuring to see what you've actually got for tire height, as we *do* see some variance here - like 33's will be only 32", for example, or 35's will only be 34.2", and so on - but to have 33's only measure out at 30" is a real eye-opener.

Just make sure that you are also taking into consideration the compression of the contact patch - so add roughly about 1/2 inch to the height you measure on the *rear* ties when on pavement - but even so, if they measured 30.25" at the rear on the ground, another 1/2" still puts them not over 31" of total effective height.

Since you have tires of less than 31" actual height, the only limitation you have on the MAX performance tune is simply not towing - you can still load the vehicle up to it's rated on-board capacity for bed load & passengers, etc. Now I can't tell you if that is actually a full 1600 lbs. or not - all I can tell you is to double-check and make sure you are calculating your on-board weight capacities correctly (including the additional weight from the new tires), by using the info from your vehicle's owners manual, as that varies widely based on vehicle configuration, transmission type, gear ratio, etc.

The only potential concern here, and the reason for saying you can't tow with the MAX performance tune, is simply what happens to the EGT's (exhaust gas temperatures) when hauling a heavy load up a long grade, for example - the MAX performance tunes use A/F ratios intended for max power, not for towing heavy loads up long grades, the type of scenario when EGT's hit their highest point. And the MAX performance tune was designed to still allow you to load the vehicle up with it's normal full on-board rated capacity, and drive it on any type of public road, whether flat, or a hill, or what have you - the only limitation being you cannot *tow* anything with it, and that this applies to *factory* tire sizes - not "true" 33's, 34's, 35's, etc.

I don't see any problem with you taking your vehicle up to it's rated on-board capacity with tires that are shorter than 31", so go right ahead & use your vehicle normally with the MAX performance tune - just don't tow on that tune.

 

Thanks Mike!!! I believe my Pro Comp AT's are like 32", but since I installed them on the stock 17's, I have to inflate the fronts to 34 psi., and rears to 24 psi. to get an even tread patch. That makes them "squish" quite a bit. The 30" figure is the actual rotational measurement which I also put into the tuner for appropriate mph reading.

 

You're very welcome!

Just a suggestion in case you may not have already thought of this (and you probably have already thought about this) - just make sure that 24 PSI you're using on the rears isn't so low that it actually causes those particular tires you're using to heat up more than normal, or wear incorrectly/prematurely, etc. - just FYI.............

Have fun & good luck!

 

If you are going to measure the "diameter" of a tire, I recommend measuring across, side to side vs. top to bottom. You minimize the squish factor which still has the same perimieter as a properly inflated tire. Perimeter is what we care about which, but people assume circumfrence which is the perimeter for a circle. Tires are not circles. I suspect the 32" number is more accurate than 30". 2" is very significant amount, 6%. That would be like the differnce between a 238/70R17 vs. a 272/70R17. Anyway, I'd use a GPS to verify your speedo/odo anyway.

 

I believe the ECU "sees" the tire's "squished" circumference. When I first got my tuner I measured the top to bottom without subtracting the other 5/8" or so and was off by a few mph being paced by two different cars.

 

My point is that squished or not, it is the same perimeter. If you were to take a piece of string, wrap around the tire including the squished area, it would be almost the same length as if the tire had more air and were theoretically round. The squish does not effect the perimeter by as much as you think. Perimeter matters, not height of a squished or unsquished tire.

 

Well, side to side is about 31 1/2", top to bottom is about 30 3/4", and I'd imagine the actual functional rotating measurement is right at about 30 1/8" Are you saying the ECU "sees" a larger diameter? ...or just that I shouldn't consider my tires as being only 30's instead of 33's?

 

The vehicle still travels the same distance every time the tire rotates, even when the tire is deformed due to the weight of the vehicle. If the side to side measurement is 31.5", you should enter 31.5" into your computer.

If you want to test this yourself, jack the truck up and wrap a piece of string around the tire. Mark the string so you know how long it was when wrapped around the tire. Next, take the string off and set the truck down. Drive the truck over a similar piece of string and wrap it around the tire. The two pieces of string will measure almost exactly the same even though the tire is deformed due to the weight of the vehicle!

Grim

 

Thanks, Grim. That was a lot more clear than what I was trying to spit out.