question about cam/tire size relation for rich

[travisr1988]

As some know my engine is together, still need to find the time to button up a couple things and she'll be on the road. One thing that needs buttoned up is tires. Now I picked up 17x9 wheels for her and need tires...this is where my cam question comes in. My cam is a lunati voodoo 60101 (256/262, .454/.468, rpm range 1000-5500), will a 30", 29", 30.5" or different diameter work best? The rear is stock and it is a 77 c10 250/3 on the tree, if I recall it should be either 3.07, 3.40, or 3.73.

[rich weyand]

As some know my engine is together, still need to find the time to button up a couple things and she'll be on the road. One thing that needs buttoned up is tires. Now I picked up 17x9 wheels for her and need tires...this is where my cam question comes in. My cam is a lunati voodoo 60101 (256/262, .454/.468, rpm range 1000-5500), will a 30", 29", 30.5" or different diameter work best? The rear is stock and it is a 77 c10 250/3 on the tree, if I recall it should be either 3.07, 3.40, or 3.73.

Well, that's the real question, isn't it?  You need to know the rear-end ratio before you make that decision.

[travisr1988]

I guess I'll have to jack it up and check then. So does the tire size on there now effect the "turn a wheel and count how many times the drive shaft turns" trick? Its got 28" tires on there now.

[rich weyand]

If you have a tach you can just drive down the road at 60 and note the rpms, and we can go from there.

If you jack up one rear wheel and count the turns, no, tire diameter doesn't matter.

[Irish_Alley]

does this help?

[travisr1988]

Ok, so I jacked it up and in nuetral each tire spins freely, in gear they spin opposite of each other. Anyway, 2 turns of the wheel gets me just under 3.5 turns of the driveshaft...so that means it's a 3.42 open differential correct?

[timthescarrd]

did you lower one side back to the ground and spin the tire in neutral when counting revolutions of the driveshaft? If so then yes 3.42 open

[travisr1988]

No I left both sides up as they spun independently in neutral. Turned the right side left side stayed put.

[rich weyand]

Given it's a 3.42, I would go with a 29" tire.  With a 3.73 I would have said a 31".

[travisr1988]

Ok, so a 275/55r17 as opposed to a 275/60r17. Thank you Rich.

[rich weyand]

I would think so.  That's a 29 instead of a 30, which I think would be better for a 3.42. 

Another question, though.  What is on there now, and is your speedometer reading accurately?  If you change the tire diameter, it will change the speedo.

[travisr1988]

Now it has 275/60r15s, however the speedometer accuracy is unknown as I have not driven it yet...and I doubt it's accurate because it was a 250/3 on the tree originally and the PO put a thm350 in it.