Variance between Gryphon and actual milage

[p40whk]

I've always been in the habit of checking my mileage whenever I fill up my tank and just got my Gryphon programmer a couple of weeks ago and am using the average and instant economy screens. I have noticed that the average economy for my Gryphon reads almost 2 MPG higher than what I actually get by dividing total gallons used by miles driven. I've run

I don't have any custom tunes yet so I'm running the canned 87 tow tune since I have an AFE CAI. My truck originally came with 235/70-17 tires and I just replaced them with 255/65-17 which is an option OEM size.

I am wondering if I need to set the tire size in my Gryphon or if my odometer is not displaying correctly. I've run 4 tanks though the truck and the results are pretty consistent.

Any suggestions?

[Jackpine]

Originally Posted by p40whk

I've always been in the habit of checking my mileage whenever I fill up my tank and just got my Gryphon programmer a couple of weeks ago and am using the average and instant economy screens. I have noticed that the average economy for my Gryphon reads almost 2 MPG higher than what I actually get by dividing total gallons used by miles driven. I've run

I don't have any custom tunes yet so I'm running the canned 87 tow tune since I have an AFE CAI. My truck originally came with 235/70-17 tires and I just replaced them with 255/65-17 which is an option OEM size.

I am wondering if I need to set the tire size in my Gryphon or if my odometer is not displaying correctly. I've run 4 tanks though the truck and the results are pretty consistent.

Any suggestions?

Those tires are almost identical in terms of circumference, so I doubt you're going to see much effect by changing the tire size. Your old tires were about 2317mm around (subtracting 3% for squish) and the new ones are 2324mm at the same squish. That gives you only two revolutions less per mile. However, I WOULD use the Custom Options menu and set this new figure in anyway.

Before you do, please take the time to read my Q & A on the Custom Options menu so you won't be confused by the numbers you will see for Tire Size and maybe Gear Ratio. Make sure both values are entered correctly.

Now, back to the difference between the Gryphon and a hand calculation. I actually get about the same results as you do. The Gryphon always reads higher than what I am actually getting. I don't bother to monitor the Avg Economy for that reason. If you do though, DO not reset it at each fillup. It takes about 100,000 seconds of operation to "level" out and that corresponds to several tanks of gas. So, it will always be VERY inaccurate just after a reset. You'll probably find that you tend to have a fairly consistent error, and you can just "mentally" adjust for it.

- Jack

[88Racing]

Running the canned tunes mine was off by about 1.5+--2.0+ mpg. With the customs more like 0.5+--0.75+ mpg.

Lars

[p40whk]

Thanks for the quick replies, I'll add the circumferences tonight.

I wasn't sure what was more accurate since my speed on the Gryphon reads about 3 mph slower than what my dash needle shows. I would imagine the Gryphon is what's accurate since I regularly go by police with my truck in cruise control set 9 over and have never been pulled over. When my Gryphon shows me going 79 my dash gauge says I'm doing 82. Since the readings are coming from the computer and not converting the speed to an anolog gauge I am assuming that the Gryphon is more accurate. I hope I'm right! Otherwise I may get pulled over real soon!

[88Racing]

Your needle on the dash will always read 1+--2+ higher than what a gps or gryphon will read out. That's with a correct value for the tires in the gryphon. Some weird thing Ford did?

Lars

[Savagetwelve]

I have a quick question:

on the Gear Ratio Calculator in Pegasus what does Squish reffer to?
I am running 285/55R20's so how much Squish is that?

Does anyone what stock Gear Ratio was on the 04F150 STX? How would I find that info?

[p40whk]

I just assumed squish was the weight of the truck pushing down and flattening out (squishing) the tire. But, you know what happens when you assume.

You should be able to find the gear ratio on the dataplate inside the door frame. I think

[Jackpine]

Originally Posted by Savagetwelve

I have a quick question:

on the Gear Ratio Calculator in Pegasus what does Squish reffer to?
I am running 285/55R20's so how much Squish is that?

Does anyone what stock Gear Ratio was on the 04F150 STX? How would I find that info?

In the gear ratio calculator, "squish" refers to the "flattening" of the rubber tire under the weight of the vehicle. The metal wheel does not deform, but, the rubber part does. So, if the "squish" value is 5%, the distance between the rim and the ground is reduced by 5%. That's a constant value as the tire rolls, so the distance between the axle and the ground (the rolling wheel radius) is reduced by that much tire deformation all the time. Ford uses this 5% rubber deformation value when they calculate the number of revolutions your axle makes per mile on stock tires (according to Bill). I've found, for my tires, that a 7% squish value in the calculator makes the Gryphon speedo dead accurate. Other people have found it works for them too.

But, here's where it gets tricky. You can "pretend" that the entire wheel-tire combination compresses under the weight of the vehicle to simplify calculations. So if the zero-squish radius to the edge of the tire is 32", then using a 7% reduction in the rubber part is about the same as a 3% reduction in the 32" value. And, if the zero-squish tire circumference is 2559mm (my tires) then subtracting 3% (about 77mm) gives 2482mm - almost exactly what the calculator gives at 7% rubber squish (it says 2480mm).

What I'm getting to in all this, is, if you know the "unloaded" circumference of your inflated tire, you can subtract about 3% for squish and you'll be very close.

Going back to your question though, if I were you, I'd change the "squish" value in the calculator to 7% and use the resulting 2511mm value for the Tire Size in the Custom Options Menu. (I tend to be long-winded, don't I?)

For the stock Gear Ratio, look at the sticker on the driver's side door. Near the bottom, you'll see the word "Axle" and below that, you'll see a "code". Mine is "B6", but yours could be two numbers. I did a quick internet search and found this for F150s and F250s:

Axle Codes (LS=Limited Slip / Traction Lok):

F-150:
19 – 3.55 Non-LS
18 – 3.08 Non-LS
H9 – 3.55 LS
B6 – 3.73 LS

F-250:
27 – 3.31 Non-LS
26 – 3.73 Non-LS

Hope all this makes sense!

- Jack

[Savagetwelve]

Jack it some worlds it does make sense! Thanks for that lond detailed explaination!
I guess you just have to play with that squish # untill you get it right!
What are you comparing the Gryphon to? The speedo?

I am running LT tires would that mean they are less squishy? Therefore less of a % (eg. 1-2) making Squishy 8-9%
The calculation in the user manual does not account for the squishy?
( I just like uing the word squishy! )

W/O the squishy I calculate manually 285/55R20: 2581mm

I guess this will have to be my starting point!

[88Racing]

Start out at 2500 and use a gps.

Lars