Another tire recalibration thread...

[jwmaier1215]

You guessed it, it's another "help me recalibrate my truck."

I did find this in another thread:

Originally Posted by Longshot270

The easiest way I have found to get the tire size accurate with a GPS is to set your tire size to something you know is wrong, like 2400, clear the odometer on your truck and on the GPS, drive 10 miles then input the numbers as shown.

(GPS Miles/Truck miles) X 2400 = New Tire Size

So I did this. However, I wanted to make sure of something.

Here is what I got:

11.5 miles on the GPS = 10.4 miles in the truck odometer.
65 mph on the GPS = 59 MPH on the truck speedometer.

But here's my question: I could use the formula above, but instead of 2400, I want to input whatever my setting was originally, right? So for me, I had 2569 inputed (JackandJanet tried helping on F-150online back when I first got the gryphon and I thought this was correct and never checked it with the GPS until now). Anyway, so what I want to equate is:

11.5/10.4 X 2569....which would equal a new size of 2841. But isn't that taller than a stock 35in tire? JackandJanet said the edge manual says a stock 35in tire is 2792. So how can it be taller?

Here is the thread I'm referring to:
Edge Evolution Programer - F150online Forums

Is there a simple formual I can use?

/Yeah I know I sound like a total airhead in that link (i'm 95'f-150). I was unfamiliar with the whole ordeal.

I guess I should give a detailed description of the truck:
2001 F-150 4x4 Off-Road package
5.4 triton xlt
Gryphon programmer (with custom tunes)
K&N CAI
35x12.5x17 BF Goodrich MT's
Flowmaster si/do super 40 exhaust
3.73 gear ratio (I think, I can't find on my vin sticker where it says what gear ratio I do in fact have. And this could be the problem as well.)

I don't think all that is necessary, but rather be safe than sorry.

Thanks for any help guys! Oh and btw, it's a wonder I haven't gotten a speeding ticket yet! I usually run 65-67 in a 60mph zone (so actual 70-72ish mph).

[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).

[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

[jwmaier1215]

Okay guys, I inputed JJ's number, 2709 and 3.55 GR. Alls I have to say is WOW, WOW, and WOW. What a difference inputing the right data makes! lol

It's sooooo freakin close, idk if it's worth messing with it. The mileage was dang near spot on. The MPH was dang near spot on. Here's the breakdown:

55mph truck = 56 mph GPS
15.1 miles in truck = 15 (getting ready to turn 15.1 miles) on GPS

In fact the miles may have been equal, I didn't reset my trip meter (anxious to see MPG improvements and couldn't wait for a full tank to be ran, lol). So the miles were virtually equal, and the speedometer is only off about .5 MPH on the gryphon. I think I can live with that!

But good lord, here I was 2 years later still not seeing a huge improvement with a gryphon vs. no gryphon, but man does this thing have some pep to it's step again! May all be just because the speedometer is virtually right now.

Is it even worth tinkering with again, or is that about as right as it's going to get? Because to me, I can definately live with that and am just impressed with the improvement.

@Shotgun, yeah I was almost empty. I had about 1/8th of a tank when I filled up with the 92 octane. I'm going to run a full tank of it, and then run a full tank of 89 and see mileage differences. If 92 gives a lot better gas mileage, then .20 a gallon isn't too shabby (It'd take 5 gallons to equate $1 and 25 gallons to equate $5....what's $5 if you get 2-3 more MPG).

[Jackpine]

I doubt it's worth messing with, but you COULD add just a bit to the Tire Size. Maybe try 2715, since your speed on the Gryphon is just a bit low. Trouble is, the odometer in the truck seems to be just a tiny bit high, which would say to reduce the TS.

You really need to test over a long distance (about 100 miles) to get a more accurate comparison (which is what I did on an interstate trip). And, if you want to base the correction on speed, try to get on a straight stretch where you can hold a constant speed for 10-15 seconds. That lets everything stabilize.

Realize too, that your Gryphon MAY have been seeing 55.4 mph and the GPS may have been seeing 55.5-55.6, but due to "rounding" it would look like a full 1 mph difference.

However, I doubt you're going to get much better than what you have now. And yes, by correcting the speed, the part throttle shift points are happening when they should, since they are based on both vehicle speed and throttle opening.

- Jack

[jwmaier1215]

Yeah, I think I'm just impressed enough right now that 1mph or a fraction of a MPH isn't going to make me much difference. Right on, goovey!

Thank you again shotgun and JackandJanet!