Tire Size Change Question

[Grabber523]

Originally Posted by JackandJanet

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.

[Power Hungry]

Originally Posted by Grabber523

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.

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

[stryker6040]

Originally Posted by JackandJanet

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

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.

[Power Hungry]

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.

[Jackpine]

Originally Posted by Power Hungry

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

[stryker6040]

Originally Posted by JackandJanet

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

[Jackpine]

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

[rednoved]

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.

[Jackpine]

Originally Posted by rednoved

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

[rednoved]

Originally Posted by JackandJanet

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!