Tire Size Change Question
 

2008 Screw 5.4 went from tire size 275/65R18 to LT315/70R17. I first used the formula in the owners manual to calculate the correct number and by doing this I got 2741 so i programed this in took a 25 mile drive to the parents and used two higher end gps to verify speed and basically i had to show 75 with the speedo needle to go 70 according to both gps. at my parents i did another calculation this time taking the overall diameter of 34.7 and multipling it by 25.4 to get 881x3.1415 and got 2769 and then programed that new number and the speedo was still off by 5 according to both gps

Am i not doing something right or is the programmer not making the correct modifications to the cpu. any help would be great

When you change your TS you must also check your GR. When you open up CO on the gryphon it puts in place holder values. Follow Jack's Q&A in the gryphon programmers section it's pretty good and straight forward.

Lars

Right now all i am running is the canned tune on tow so I can get better shifts and update my tire size until my custom tunes are done so I dont know if this makes a difference or not to your response but I will be sure to check that out and make sure its right. Thanks for the info I apperciate it

You're not accounting for "squish". Try subtracting 3% of 2741 to get a more reasonable "loaded" circumference. When I do this, I get a value of 2741 - 82 = 2659 mm.

Unless you compensate for squish, your PCM will think each revolution of your wheels moves further than you are actually traveling. That equates to to a higher speed than you are actually doing. Try it with 2659. If you're still indicating faster than you're actually moving, reduce it still more.

Oh, and Lars was correct too. Make sure your Gear Ratio value is correct when you do this.

- Jack

To add to that, don't use the speedometer to gauge if you have the right tire size or you will be chasing numbers all over ther place; you should be comparing odometer to your GPS' trip mileage counter. There are different ways you can make it work or you, but you will find it is much more accurate and easier to know if you need to go up or down on tire circumference. If my GPS is clicking off tenths faster than my truck odo, I need to raise my tire size...if my truck odo is ticking out faster than the GPS, I lower the number. So far on Toyo MT 35x12.50-20's at 43 psi I found that 2702 is about as close as I'm going to get.

Thanks for the info. i have never heard of the squish until you just brought it up but i guess it does make sense. Another thing I was wondering is what should I run in these for air pressure? I currently have 35psi in them they are procomp xtreme mt in lt315/70r17 i was debating putting 40psi in them but i dunno

Call Procomp and ask. Every tire manufacturer should be able to tell you what pressure to run for the aplication, even loaded or unloaded. For example, on my 06 Screw with 35" Toyo M/Ts, I was told by their customer service to start with 45 psi, and adjust accordingly by gauging tread wear. I now run them 43 psi front and rear. If I have a 6000lb trailer on the back (600 lb tongue weight), I have to pump up the rears to 65 psi. They should be able to tell you all that info at Procomp's customer service line.

Grabber gave you a good answer, I'd call Procomp too. Somewhere, on the tire's sidewall is a max pressure value. DO NOT exceed this! And, I'm sure you know to check your pressure when the tires are cold, not after being "rolled on" for a while.

Just as a "guess", I suspect 35 psi is way too low. The OEM tires I had on my truck were recommended to be at 40 psi. At the recommended pressure, you maximize load carrying ability and treadwear, and minimize rolling resistance, heating (due to sidewall flexing) and hydroplaning on wet roads. If you over inflate, you tend to wear out the center of the tire tread and you increase stress on the entire tire and wheel which can lead to blowout.

I think, at the correct pressure, the squish value I gave you will be just about right. I was actually coming up with a 6% correction given your speeds, and that tells me you are underinflated too.

- Jack

You bring up a good point, J&J...perhaps a 5% squish lets you know you're where you need to be for cold psi? A long stretch, but I wonder how accurate that theory could be? That would take a lot more math than I'm comfortable with...

I'm guessing here, but I think the 5% figure that Ford uses accounts for the tendency of the "average" driver to ride around on underinflated tires. I'm pretty sure most don't check their tires at all, and they depend on the dealership or whoever changes their oil to do it for them, every 3-4 months.

I was in Discount Tires a few months ago getting a tire rebalanced and overheard some poor old soul being told that he/she (I can't remember which now) was going to have to get 4 new tires because the ones that were on the vehicle were worn beyond safe limits due to poor alignment and no rotation.

- Jack

I feel absolutely NO sympathy for those people whatsoever. If you don't know how to maintain your vehicle, you shouldn't be driving it. Period. There should be an elimination process...in your scenario, the customer would buy the new tires and have their License revoked on the spot. They could have their car back but wouldn't drive it again until passing a written and practical test. I can't tell you how many cars I see on the road with a tire so flat that it's riding on the bead, and it makes me real nervous, especially when I have my 9 month old with me. If some ignorant puke injured or killed someone in my family, I would personally ensure that they would not make it to the hospital.

While I do feel your solution might be a little excessive (but only a little), I definitely agree with you about ignorance of vehicle maintenance. Now if someone intentionally hurt my family... well let's just say that there would be a few well-fed alligators somewhere. :eek:

I certainly understand that emissions inspections can be tedious, but there is absolutely no reason to not have a mechanical inspection of every vehicle on the road. For example, Utah inspections will pull the wheels off and even inspect the thickness of the brake linings and condition of other mechanical components as well as check headlight alignment, wiper functionality and other stuff. Yeah, it's inconvenient but like security checks at the airport, it's in the best interests of public safety.

Too many people are killed every year from failed tires, brakes, steering components, and other mechanical issues and there is simply no reason for it. Owner/driver ignorance is not an acceptable excuse. Heck, how many people don't even have the common sense to check there oil, much less change it? These people should not be allowed to drive at all.

One other thing that pisses me off, while on the subject of maintenance, is the sad fact that there are fewer and fewer gas stations that provide facilities to check tire pressure and the ones that do charge anywhere from 25¢ to $1.00 for compressed air they are producing pretty much for free. Yeah, it's a little electricity and a compressing station, but isn't it worth giving that stuff away if it saves just one person's life? Too many people seem to put profit over common sense, which is unfortunate.

Anyway... that my :2cents:

I just checked the setting and put your new number in. the gear ratio was correct at 3.73 so ill give you number a try and see how it goes.

One thing to keep in mind is that the gear ratio calculator is not always 100% accurate. It is a best guess as to diameter based on the tire size designation. Tires with the same designation can actually vary quite a bit when measured and I've seen differences as much as 5% between different tires of the same "size". Even tires listed as 35" can vary +/- ½".

The first place to look is the tire manufacturer's website and see if they list a "rev per mile" value. If they do, take this value and divide it into 1609344 (ie. 1609344/rev per mile). This will give you the tire size in mm.

The next best thing is to park the truck on a long, flat surface. Mark the ground and the tire where it touches the ground. Roll the tire forward (or backward) 3 rotations and mark the ground again. Measure the distance and divide by 3. This gives a really accurate tire size value. If using inches, remember to convert to mm by multiplying by 25.4.

Hope this helps.

From what I've observed in my own truck and from feedback from several Ford owners on the forums, the 3% reduction in circumference seems a good starting place. Then, it's pretty simple to calculate any remaining percentage error and apply that to the "squished" circumference. It's what I did.

stryker6040 - PLEASE DO find out from Procomp what pressure they recommend. Don't let it go below that value when the tires are cold.

- Jack

Thanks for all your help i apperciate it. i plan on calling them on tuesday to see what they recommend i did put more air in them just to be safe. the tire says 50psi max so i put in 45 and it seems to ride better and like i said ill find out for sure on tuesday

45 sounds pretty good to me, with 50 psi max. I think you're going to get better handling, less rolling resistance, less heat buildup, better tire wear, etc. etc., at that pressure.

- Jack

When we have a custom tune made for us, is OUR tire size part of that tune? If it is, can we change it if I were to go to a different size tire? I plan on lifting the truck not too far down the road. Or even if I add a leveling kit in the next month, I'll probably buy larger tires.

Yes to both questions. You give Bill your tire size and he programs it into the tune. If you change tires later, you can revise the size using the Custom Options menu.

- Jack

Thanks. So I wouldn't need to order another custom tune after the lift. That's good to hear!

Jack the number you gave me was almost dead on. just a little more fine tuning and everything should be as close to prefect as we can get thanks a lot for the help!

You're most welcome! Glad we could help you out.

- Jack

I just got my custom tunes, took it out for a 250 mile drive this weekend. I don't have a gps, but using the mile markers on the freeway, my odometer flips about 150 feet before each mile marker. I also feel "slow" driving the same roads I'm used to. Decrease tire size a little? I have 285/75/16 Terra Grapplers, max psi 65, and running them about 43. Is this too low?

Your 150 feet in 5280 feet is about 2.8% in error. You COULD try subtracting 2.8% from the current circumference you are using. The pressure seems all right to me - are you using OEM sized tires and wheels? If so, there should be stickers on the truck recommending the tire pressure. However, since the max pressure is 65, I doubt you'd hurt anything using 50 psi cold.

When I enter your tire size in Bill's Gear Ratio Calculator in Pegasus and use a 7% rubber squish, I get a circumference of 2525 mm. What are you currently using?

- Jack

These aren't stock tires. I usually run them around 41-43 then up them to 50 when I tow the boat.

The custom tunes came with the value 2550. I just dropped it down to 2449. There is one of those parked cop things that displays your speed, I'm going to drive by that a few times to get it where it needs to be.

Good! I just subtracted 2.8% from 2550 and came up with 2479. I suspect 2449 will be much too low. I'd try something in the 2500 range as a start.

- Jack

Argh, when I change the tire size and it goes throught the 4 stages, it doesn't save the change when I restart the truck. What am I doing wrong? I did the same thing before custom tunes and it stuck.

When I go into check the tire size or any other changes. I do the following.

Open CO menu.
Then load previous settings.
Then to tire size usually says 2484
Then to gear ratio usually says 3.73
Then save changes/done. Or follow this link:
http://dygytalworld.ehost-services13...newpost&t=1101

Lars

Follow the directions Lars gave you in the post just above this one. I suspect you are not "Loading Previous Settings". Follow the link he gave you for an explanation of all the custom options idiosyncrasies.

- Jack