Another tire recalibration thread...

[shotgun]

You're doing it correctly. And yes, it does appear that circumference is too large for a typical 35x12.5x17 tire.

2841 = (approximately) an uncompressed (no squish) 315/70R18.

Does BFG have a revs/mile or circumference on their website?

Did you use the door sticker decoder to get your GR? How To: Decode VIN Number and Vehicle Sticker - F150online Forums

Did you look at the metal tag on the pumpkin?

[jwmaier1215]

Originally Posted by shotgun

You're doing it correctly. And yes, it does appear that circumference is too large for a typical 35x12.5x17 tire.

2841 = (approximately) an uncompressed (no squish) 315/70R18.

Does BFG have a revs/mile or circumference on their website?

Did you use the door sticker decoder to get your GR? How To: Decode VIN Number and Vehicle Sticker - F150online Forums

Did you look at the metal tag on the pumpkin?

Never saw that thread, but that is extremely helpful. See I was looking for where it plainly just said 3.73, 3.55, etc. So I went off of what dad said, and well dad isn't always right, lol.

Anyway, I just looked it up, they're 3.55 LS. So now I need to re-do the process with the same tire height inputed, just input 3.55 now. Correct? I'll have to wait until later today to find out for sure. My GPS is down at my apartment (I used my sisters yesterday) and I'm headed back to my apartment for school tomorrow, here in a bit. I just wanted to make sure the "2400" wasn't a standard starting point. So now that the GR will be inputed correctly, the actual (GPS) vs. what the gryphon says should be close(r), right? Which would explain why the number shouldn't be over 2792?

[shotgun]

You need to make sure the GR in the Gryphon is correct. Once it is, I would use 2709 cm circumference as a starting point and confirm distance or speed with your GPS.

If it's off, then use the formula:

(GPS miles/Truck miles) X 2709 = New Tire Size (cm, circumference)

Note that your speedometer will ALWAYS read 2-3 MPH fast at 65-70 MPH, even if your tire size and gear ratio are exactly correct. My odometer is dead nuts, but at 65 MPH (on my CTS), my speedometer reads 67-68 MPH.

I know this process seems confusing, but once you get the hang of this, it's really not hard (in case you change tire size again in the future).

[Jackpine]

And, what Shotgun said about the dash speedo reading fast is correct too. It does that due to mechanical errors. You want the odometer to agree with the GPS.

- Jack

[jwmaier1215]

Alright thanks guys!

I think you both are saying what I'm meaning, lol. Yeah I re-programmed it to 3.55 GR (my actual GR that I confirmed this morning with the VIN sticker). I just made a 60+ mile trip back to where I go to college and didn't have my GPS on me (I used my sister's GPS last night). So I have a homework assignment to get knocked out today and I should be free majority of the rest of the day. So I plan on using this opportunity to do a little exploring (I want to map out an alternative route home anyway). So I'll grab my gps and reprogram the tune to read 3.55 GR and 2709 tire size. Then I'll do the calculations and update sometime this afternoon.

And I NEED to do this at highway speeds, right?

Oh and yeah, I notice a 2-3 mph difference from what the gryphon reads and my actual speedometer reads. But 9.9/10 I use my gryphon as a speedometer anyway. Only thing I look at on the dash is mileage and where the gas gauge is.

One more quick question about the performance tune. It says a min of 91 octane is required. I have not ever ran premium gas through it. Will it make a huge difference running it vs. not running it? I went ahead and filled up with 92 octane a little bit ago (gas is cheap here, $3.49 for 92 octane, $3.29 for 89 octane).

[shotgun]

Since you're using the distance correction, speed doesn't matter. It's more important that you use a long enough test distance (10+ miles) to help negate any small GPS distance travelled errors.

I have never had the need (or desire) to use the 91+ tune, but you should feel a slight hp/TQ improvement (IIRC it's 7-15 hp).

Was your gas tank empty when you filled with 92 octane? If it was, I would wait (to change to the 91+ tune) until you've used about 1/4 tank to make sure you have all of the 87 octane out of the system.

If it wasn't empty (or real close), I would wait until the next tank before I changed tunes.

[Jackpine]

Either I typed something wrong, or you changed your tires or maybe the "gremlins" altered something.

Anyway, your 35" tires should have a circumference of 2792-2793 mm if there is no weight on them. Once mounted, the effective radius is reduced due to weight by close to 3% in most cases, which would make the circumference about 2709 mm.

Now, the 2569 mm figure you were using is too small, which makes your odometer and speedometer read "low". I calculate it as a 5% error, so inputting 2709 would make the 11.5 mile distance on your GPS look more like 11 (actually, 10.97 miles). So, we're getting there.

Now, there's no way your tires are going to have a circumference of 2841, so I'm convinced you must be using the wrong Gear Ratio (GR) too.

The GR code is under the entry "axle" on the VIN sticker. These are the OEM codes that should apply to your truck:

19 = 3.55 open
H9 = 3.55 LSD
26 = 3.73 open
B6 = 3.73 LSD
27 = 3.31 open
18 = 3.08 open
25 = 4.10 open
B5 = 4.10 LSD

So, if the code is "B6" or "26", and you have not regeared, you have a 3.73 axle. Otherwise, you need to enter the size you really have.

See if this clears things up and get back to us.

Edit: We were typing at the same time. Try the TS of 2709 and the GR of 3.55. I'll bet you are going to be VERY close. The difference between 3.73 and 3.55 is another 5%, which brings the 10.4 odometer distance to 11.44 miles. That's about as close as you can measure with those devices.

- Jack