is your cobra a strip car or a course car?
if strip, I'd personally go with a smaller diameter tire looks be damned.. get you rolling and the engine at stall speeds quicker.. I'd get as much contact patch as possible on the rear because the grip overwhelms the concern of weight IF you drop diameter.
if course, I'd go with a reasonably narrow, like, in the 285~305 range (less weight) on a lowish profile (no more than 40%, no less than 30%) for less lateral wiggle, and on no more than 18" diameter.. that seems to be the perfect diameter for negotiating the weight/torque loss to usable RPM range/distance covered by circumference ratio.. if the course was wet, though, I'd not hesitate to NARROW the tires width, soften the compound, and select a tire with outstanding sipes or tread designed to displace water..
and yup, in any case, keep the majority of the weight in the center as much as possible..
anything from this point forward is just me ranting about a subject that interests me, reader beware..
I first started paying close attention to size/diameter/weight when I began competitive mountain biking.. several broken bones and many many cases of beer later, I'm not so competitive anymore... but, the same holds true for any wheel/tire set-up on anything that applies power to them..
26" mountain bike wheel w/ 2.2" front and 2.1" rear, inflated to 22psi on soft track for traction (conforming to obstacles and in effect biting on at least two sides instead of one where the small contact patch touches) is a helluva lot harder to pedal than inflating them to 40psi on hard track- and the harder tire maintains speed hella easier..
29" wheel/tire- hard to accelerate (you REALLY notice when it's YOU providing power), but easier to maintain speed- the angle of approach obstacles such as protruding rocks or limbs, ect, is much smaller, so they sap much less inertia.. getting them up to speed can't be understated though- but neither can the engines (me) ability to maintain that speed once achieved..
in terms of vehicles:
the larger diameter 'trail' tires require less work load from the engine (in terms of torque) to roll over obstacles, because of the drastically reduced angle of approach.. in simple terms, I would have to accelerate the engine to 2kRPM to clear a 7" stump with 32" tires, but I can easily clear it @ 1200RPM with 35" tires.. yup.. that much difference..
on the road, smaller diameter tires allow for hella lower driving ratio's.. it takes MUCH less energy to get them rolling.. great for towing or hauling.. larger diameter tires are really tough on the engine to get rolling, but once there and maintaining speed, don't work near as hard on the engine... unless those tires also weigh a LOT more.. which, they often do..
contact patch- the more narrow, the less traction- but the less drag.. no good for offroad, but very good for economy..
tread pattern- the less tread blocks (treads that impact perpendicular to road) the less rolling resistance, the less noise, and the less ability to shuck debris and water..
sipes- the little cut looking things on treads- the less sipes, the less venting and the more hydroplaning.. the longer lasting tread, though..
compound- the softer the better traction, the more heat, the quicker wear, and the heavier; the harder the less traction, the longer wear, and the lighter..
for a DD, and a working DD on our rigs, I can't understand using less than a E load rated tire with a harder compound, and pretty much devoid of sipes and blocks (like that dura grappler, or most commercial rated tires)..
for a truck that sees a lot of offroad, I can't see anything useful less than a 12.5" width tire.. the bigger footprint the better, so we don't sink in at the drop of a hat- wanna spread that weight over as much ground as possible- also, we produce enough torque down low to not worry much about drive ratio- so at least a 34~35" tire.. WITH A TALL SIDEWALL- so, 20" rims are out.. 17's are the way to go, so you can lower pressure and conform to the terra without rolling the tire off the rim..
if you play in your work truck, terra grapplers, cooper at3's, BF' AT's are good choices.. around 275~285/60~75 range... kinda hardish compound to get some life out of them, yet plenty of blocks to get decent traction in most undemanding conditions..
just my $2.. (i think i blew my .02 around the second sentence)..