tire width vs tread width
 

hypothetical question time- if i have 2 tires, (both same rubber compound/brand) one is a 275/40/17 with a thread width of 9.5 the other is 245/40/245 with the same thread width of 9.5 , does this mean both tires will have equal "mechanical grip" ?

Tread width is tread width, I don't know what tire width is. If you mean section width then yes, that is different than tread width. Now to your ? if the rubber is the same then yes you should get the same traction but you have to take into account if the height of the tires are the same.

ROD

245/40R17
Tire Weight 26 lbs.
Rim Width Range 8-9.5
Meas. Rim Width 8.5"
Sect. Width 9.8"
Tread Width 9.4"
Overall Diam. 24.6"

275/40ZR17
Tire Weight 25 Rim Width Range9-11
Meas. Rim Width"9.5"
Sect. Width10.9
Tread Width"9.5
Overall Diam. "25.7"

any point at all to running the 275 over the 245 ? the 245 is going to be the lighter rim , is "tread width" all that matters or am i missing something?

tire width is the size of the tire , ie 275 and 245

Like I said width is width. I don't care what it says on the side of the tire, its the tread width that gets you traction. The 275 tire you showed is only 0.1" wider than the 245 but it's 1.1" taller and that will affect the car. I run tires on my S that are 1" taller than stock. That will lower the rpm's at any given speed around 4.2%, so if at 76 mph you run 4000 rpm they will drop to around 3800 rpm with the taller tires. So you can go faster in each gear but that doesn't mean it's quicker in each gear. Quicker and faster two different meanings.

ROD

Yes in theory.

Gearing differences aside, also consider a larger overall diameter tire of the same width will have the same lateral contact patch area, but it will be elongated which also improves braking and acceleration traction. If you can keep it balanced with weight and engine output, your going to get better overall performance the bigger tire you go with. If your lacking in engine output and depending on the environment used, there will be a point of diminishing returns.