Differential calc

[WVZR-1]

Interesting!

I measured my BFG G-force some time ago (years) doing a single rotation for circumference of the tire and I really don't recall the answer BUT a good while ago I was told to use this for "rolling radius" and it's usually been very close.

Advertised diameter less 1/2 of the tread depth x .96/2 but I've never passed that on anywhere. I can't presently measure mine but it seems like it's actually a good tool.

Your F1 315/35R17 25.7 - .16 (1/2 - 10/32) X .96 = 24.52/2 = 12.26

It seems a match or very close! Air pressure!

[XfireZ51]

Originally Posted by Paul Workman

I'm outta here. Oh, wait!...I'm outta here except to trade friendly jousts with your sister: Being a math teacher in public school is no glowing recommendation . I've had to teach logarithms to high school grads that couldn't balance their checkbook. I made a good living for several years teaching math to HS grads that didn't have a grasp of the 4 basic principles of arithmetic. (Did your sister teach you math too??)

P.

Paul,

I missed this last part. First of all, Kathy is my sister-in-law and she teaches at a top girls' Catholic High School in Chicago, Trinity. Its run by the Dominican nuns. She teaches the math honors classes and Kathy has her math major and physics minor degree from UIC. Not too shabby, wouldn't you say? If Kathy was incorrect, its because of how I set up the problem. And honestly, the snark on teachers in public schools is unnecessary. I have a good friend who taught math and physics in public school. Pretty good guy who knows his stuff.
Now that that's cleared up, turns out I was completely blind and the
Oracle of Addison (not you Paul) straightened me out. I had same wheel/tire combo on Azzuro-1 and measured ODO v GPS at that time. Rear gears were 3.45s. ODO was off by 5% with correct trans gear so at 60mph, ODO read 3 mph low and at the time I thought that wasn't enough to bother with re: correct trans gear. If I replace the trans gear with the correct 3.45 gear, I will be off by same amount now as well. Marc agreed I have 4.10 trans speedo gear in there now. A 5% diff means the car runs like a 3.28:1. To get it back to a 3.45 would take something like a 3.66. Closest would be a 3.73. Marc however still encouraged me to go with the 4.10's (which BTW I found a killer price on for OEM Spicer gears) and they effectively would run like a ~ 3.90 with the same wheel/tire combo.
Now all I need is $$$$.

[XfireZ51]

Originally Posted by WVZR-1

Interesting!

I measured my BFG G-force some time ago (years) doing a single rotation for circumference of the tire and I really don't recall the answer BUT a good while ago I was told to use this for "rolling radius" and it's usually been very close.

Advertised diameter less 1/2 of the tread depth x .96/2 but I've never passed that on anywhere. I can't presently measure mine but it seems like it's actually a good tool.

Your F1 315/35R17 25.7 - .16 (1/2 - 10/32) X .96 = 24.52/2 = 12.26

It seems a match or very close! Air pressure!

Is there any accomodation for "growth" of the tire due to centrifical force and heating of air at x mph? I'm supposing its negligible or within the margin of error for the speedo itself.

[sammy]

dominic .not sure if you are still interested in the 392s that came out of the callaway car .if you are i gave them to bill boudreau. he will sell them to you for the price we talked about .if you arent they are for sale and are in great shape just contact bill boudreau .

[XfireZ51]

Originally Posted by sammy

dominic .not sure if you are still interested in the 392s that came out of the callaway car .if you are i gave them to bill boudreau. he will sell them to you for the price we talked about .if you arent they are for sale and are in great shape just contact bill boudreau .

Brad,

I'll contact Bill on this. Thx

[Paul Workman]

Originally Posted by XfireZ51

Paul,

I missed this last part..

Dom,

Yes, you did (miss it)...AGAIN!

Well, Let me apologize, Dom. I got tickled when you threw your sister in law (I stand corrected) onto your funeral pire. I was tying to inject my sick sense of humor into the conversation, and ... well ... that's the problem with these boards: without facial expressions etc, things can be misunderstood. Actually, I was poking fun AT YOU, Dom, or (more precisely) your damned Italian stubbornness!!

Originally Posted by XfireZ51

Now that that's cleared up, turns out I was completely blind and the
Oracle of Addison (not you Paul) straightened me out. I had same wheel/tire combo on Azzuro-1 and measured ODO v GPS at that time. Rear gears were 3.45s. ODO was off by 5% with correct trans gear so at 60mph, ODO read 3 mph low and at the time I thought that wasn't enough to bother with re: correct trans gear. If I replace the trans gear with the correct 3.45 gear, I will be off by same amount now as well. Marc agreed I have 4.10 trans speedo gear in there now. A 5% diff means the car runs like a 3.28:1. To get it back to a 3.45 would take something like a 3.66. Closest would be a 3.73. Marc however still encouraged me to go with the 4.10's (which BTW I found a killer price on for OEM Spicer gears) and they effectively would run like a ~ 3.90 with the same wheel/tire combo.
Now all I need is $$$$.

You may or may not be out of the woods yet, my friend... Cliff (Dynomite) made a very astute observation: The ratios you should be concerned with are the actual wheel and drive train ratios, i.e., direct measurements, and NOT your speedo or GPS readings, or phase of the moon or anything else (read: indirect measurement) unless you have no other choice.

Truth is where you find it. So, wheter it was Marc or Cliff, or me, or some others too that shone the light, in the end it's all good.

I believe my math was correct - provided mulitple proofs - but I'm willing to be proven otherwise. That said, in (one) example you gave, I too arrive at the same "3.28:1" ratio you (or was it Marc?) finally derived. Regardless of how you get there, as long as the wheel change ratio matches the gear change ratio, you're golden.

Sorry if you thought I was taking a shot at your sister in law. I wasn't. But, I was messing with you, Dom!

P.