[FireDoug2001]

Okay I finally wore out those crumby Contitrac tires. 27,000 and they had enough tread to go another 3k to 5k before they were totally bald. But as I have gotten older I have gotten wiser and don't try get every mile out of a set of tires, that usually ends up in a blow out and more wasted time.

Any way I decided to go with Toyo Open Country AT 285 75 r18 E. I kept the stock rims got a headache reading about the debate over hub centric vs. lug centric

So once I got the tires up and running I got into the EVO tire programing. My EVO was setup for the stock tires @ 2559mm, per Edge formula I entered 2779 for the new tires, that sent the speedo to fast, ended up via trial and error to 2660.

The interesting thing was that EVO manual said that the tire size of 275 65 r18 should be 2559, that tire is shorter than the 70 series tire that my truck came stock with. So somewhere edge's tire size chart and calculations are goofed up.

Anyway the new tires are awesome and the speedo is right on testing it against the Garmin GPS.

 

[SmokeyWren]

So once I got the tires up and running I got into the EVO tire programing. My EVO was setup for the stock tires @ 2559mm, per Edge formula I entered 2779 for the new tires, that sent the speedo to fast, ended up via trial and error to 2660.

The interesting thing was that EVO manual said that the tire size of 275 65 r18 should be 2559, ....

Converting your tire loaded circumference numbers into tire revs/mile the real world uses:

2559mm = 628 revs/mile
2779mm = 579 revs/mile
2660mm = 605 revs/mile

Toyo says your 285/75R18 tires have 591 revs/mile and they are a hair taller than 35" diameter at 35.1".

And are you sure your speedo was calibrated at 2559mm (628 revs/mile) for the stock tires? If your stock tires were 275/65R18, they had 652 revs/mile. Ford says the LT275/70R18 tires on some SRW pickups have 627 to 629 revs/mile, or right on the 628 you mentioned.

Most 35" tires have close to 600 revs/mile. Since yours are supposed to be a hair taller than 35", I would think the revs/mile should be a bit less than 600, and Toyo agrees. So if 605 revs/mile is accurate on a GPS, then your tires are a bit shorter than Toyo intended. Manufacturing tolerances, apparently.

I know most programmers claim to be able to calibrate the speedo, but most will not calibrate the speedo on a SuperDuty pickup. I'm glad to hear yours will.

 

[FireDoug2001]

Thanks for your great info SmokeyWren, to clear up a few items. My stock factory tires were the Conti 275 70 R18 and my EVO tire data that must have been extracted from the PCM was the 2559mm.

When I was using the GPS to adjust the tire mm calculations, I set the EVO to display MPH and compared that to GPS. When I get the EVO and my GPS to agree my speedo guage shows about 2 MPH slow. The guage has always displayed low, even with the stcok settings right off the lot. Adjusting the EVO tire size does effect the speedo guage, when I had it set for tire size of 2779 the speed was about 5mph too fast.

Here is Edge manual info:

Tire Size
(Note: Tire size will only function correctly on 05-07 model years)
The Evolution has the ability to correct for changes in tire size. If you have
changed your tires to a different size the PCM will miscalculate vehicle
speed which can affect shifting as well as the speedometer. Correcting the
tire size will allow the PCM to accurately calculate vehicle speed and
odometer readings. You will not need to change the value displayed if you
are running factory size tires. After choosing to set your tire size, the follow
screen appears:
The numbers on this screen represent the circumference (in mm) of your tire
size. The default value the Evolution displays is the current value stored in
the PCM. Adjust the tire size by pressing the [UP] and [DOWN] arrow keys.
Ford 6.0 Liter Evolution

The following table will help you determine your tire circumference by
matching your tire size with the common sizes listed. For example, if your
tire size is 285/ 65-18, the value (circumference) you enter into the Evolution
is 2600. You may also use the formula provided to determine your
approximate tire circumference. Finally, you may also measure your tire’s
circumference by marking your tire and rolling it one complete revolution
and measuring the distance in inches or millimeters. If you measure in
inches, multiply the distance by 25.4 to convert to mm. (ex. 91.3 inches x
25.4 = 2319 mm). The Evolution supports tire sizes ranging from 2000mm to
3500mm in increments of 1mm.
Width Ratio Rim Circumference Tire Size
265 70 16 2441mm 33 2632mm
275 70 16 2485mm 35 2792mm
305 70 16 2617mm 37 2951mm
215 75 16 2289mm 38 3031mm
245 75 16 2430mm
265 75 16 2524mm Circumference Equation:
285 75 16 2618mm [(width * ratio * 0.02) + (rim * 25.4)] * 3.1415
295 75 16 2666mm Steps:
305 75 16 2713mm 1. Multiply Width, Ratio and 0.02
315 75 16 2760mm 2. Multiply Rim and 25.4
325 75 16 2807mm 3. Add Step #1 with Step #2
265 70 17 2521mm 4. Multiply Step #3 with 3.1416
285 75 17 2698mm 5. The result will be the circumference of your tire in mm.
315 75 17 2840mm Note: if your tire size is not listed use the above formula
275 65 18 2559mm
285 65 18 2600mm
285 60 18 2510mm
325 60 18 2660mm


I know that Toyo says they are 35.1" tall but, here is the metric conversion for that tire size.

Your Metric Size of 285/75 R 18
Outer Diameter: 34.83 "
Section Width: 11.22 "
Rim Size: 18"