Tire Size - Page 2

twintips_17 · #11 Fri, February 20th, 2009, 11:33 PM

Quote:

Originally Posted by JackandJanet

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

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!!!!

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.

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!

***Jackpine*** · #12 Sat, February 21st, 2009, 09:11 AM

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

twintips_17 · #13 Sat, February 21st, 2009, 09:33 AM

Quote:

Originally Posted by JackandJanet

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.