Tire Size
 

Should the tire size be set at actual size according to the chart or allow for a "squish" factor. I currently have the stock 255/70/17 tires inflated to 37#.
I have the tuner set with a 3% squish factor that I have read about in some threads on FTE. Was wanting to make sure the setting is correct to get accurate benefit from the tunes. Thanks.

Mine is set with "squish". You DO need to include this correction if you want the speed and odometer readings to be correct, and, it will have a small effect on your shift points which can effect the benefits of your tunes. I've subtracted about 3% from my tire's "unloaded" circumference and find that to be about the right amount. My tires are 275/65R18s and they are inflated to 40 PSI.

Whatever value you try, it's a "starting point". The way I first calibrated mine was on a trip to California on the interstate. Over a distance of 100 miles (using highway markers - which are quite accurate) I found I had less than 0.1 miles error on the odometer. Later, using a hiking GPS with WAAS, I found the Gryphon's and GPS' speed matched (while the dashboard speedo read 2MPH faster - it always will read fast).

My guess is, you MAY have to increase your "squish" factor more for the tires and pressure you use. Let's say you tried the "distance" test like I did and found your odometer read 101 miles after 100 actual miles. That would be a 1% error, so, you could subtract an additional 1% from your tire's circumference.

Hope this makes at least a little bit of sense!

- Jack

So you're saying that the squish factor isn't really related to the amount of road kill you encounter while driving .......yuk yuk yuk ! :hehe:

That COULD be more of a "bump" factor! :cheesy smile:

- Jack

Well....I don't know about this technolgy stuff...I am suppose to have my new programmer delivered on Monday, but now I am getting a little scared about installing the dadgum thing....all these postings about trouble with starting...camshaft settings ...running too lean on canned tunes with CAI (my customs most likely are gonna take another 2-3 months), battery needing a external charge if the thing stays plugged in....tire size....tire pressure (don't know, the darn things are filled with nitrogen is all I know) ...squishin $hit.....tweekin this.....torquein that!.....geez.....hell I have to type all this stuff with two fingers as it is !.....only mechanics I do is hand me a BFH and stand back, anymore! Looks like ole Bill may just get a call before I start connecting anything....I havn't talk to anyone on the phone at PHP as of yet.....reminds me of when I was just teenager.....DAZED and Confused ....oh well! Story of my life I guess

Thanks Corey...I have been putting off calling until I really get my programmer due to some info I needed to get to Bill so he can do my customs (ASAP!!!:cheesy smile:)....I am really not as overwhelmed as I seem....:lookaround1:....I did have my truck into the dealer after I first got it (4000 miles) over what they told me was a plug and coil pack problem from the factory ....they fixed it (after a couple of trips to the dealership...(22000 miles ago now)....I am pretty sure they reflashed it then ...and I am not sure after all the *****in I did that they didn't bump up the tune on it to compensate (runs pretty damn good, I was worried about the programmer maybe messin that up).....anyway...like I said ...I'll call sometime after I get the thing on Monday....wifey and I are headed to see her family down in San Antonio this weekend ....I was hopin for an early delivery to go play in the sand in Port A.....awwww well....next trip for sure ( I get down there often)...yesssss!

:censored1:oooooops sorry about the slip of language.......my wife (who is a high school teacher) gets on me all the time....but it's hard for us ole dawgs....

got a quick question when bill does the custom tunes, does it come already program it for the gear ratio and size of tires we state on the order form

We normally set up the tire size and gear ratio as part of the custom tuning.

didnt mean to bug u, was just curious. wife wont let me play with it tonight but got to have the computer on movie night

This is the first I have heard of this. Maybe I’m a little slacked on it. Haven’t been on the forums in about 8 months. Maybe I just missed it before I left. What is a ball park formula for the "squish" factor?

Maybe this is something you and Bluejay Thought about!!!!:smiley_roll1:

Also how much does tire size play in shift points.

I run my stock tires all the time I can, but I have a second set of tires for off roading and hunting.... Such other things. I used to switch them out for a weekend here and there.... But with money being tight lately I have been running them as they are the only set that has a any good tread for winter driving in Montana. :nonod:

Anyways my stock tires are 265/70-17's and I’m running 285/75-16's. There about 2 inches bigger on diameter.

Would it make a big difference.

I told bill to write my tunes for 265/70-17's.

Can I still mess with the tires size on my custom tunes when I get them for the times I will be running my Mudder's???

Thanks Jack!:thumbs up yellow:

Adjusting your tire size in the Custom Options menu on the Gryphon has two (three?) benefits. First, if you enter the right value, (and your gear setting is also right) your speedometer and odometer will be correct (that's two). And, since normal shifting is partly dependent on speed, your normal shift points will be correct (that's three!). With odometer values right, you'll know what your real gas mileage is, if you calculate it the old-fashioned way at fill up.

Since tires are compressed by the weight of the vehicle, the distance they roll in one revolution is somewhat less than their unloaded circumference.

Bill has thoughtfully provided a tire size calculator in Pegasus. It's under Calculators->Gear Ratio Calculator. Simply enter your tire size in the first box like this: 285/75-16 and right away you'll see the rolling circumference (the value you enter in "Tire Size" in the Custom Options in the "Old Circumference" window assuming a 5% squish (which is a good starting point). For your tires, I get a value of 2551 mm. 5% seems a good starting point, but you can go down to the "Squish" window and change that value too to see it's effect with different numbers. I've found my tires have closer to a 7% squish, using the calculator.

What's the effect of not including squish? Oh, maybe 3-4% error in speed, distance and shift points. Not really much in the great scheme of things, but still....

If it were me, I'd adjust the tire size anytime I changed tires.

- Jack

I have known all about th espped changes and such but never thought about the shifts. Having a stick for so many years, i didn't think it mattered.

I'd rather have a stick too, especially in a 4X4!

- Jack

It takes a ton of getting used to having an auto...:thumbsdwn: I would give an arm (the left one:evillol:) for my stick back.