Another tire recalibration thread...

[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.