Tire question, Mondial Cab with 360 wheels

With some encouraging from the boards here I replaced the original TRX wheels on my '84 Mondial Cab with a set of 18" 360 wheels. These came with OEM Pirelli's already mounted, and I just bolted 'em right on (new bolts from Hill in UK).

Left rear is now bald, right rear is well worn. So clearly time for new tires.

Questions: does the differential wear mean I need to get my suspension tweaked, or at least looked at, because of the wider tire/wheel package back there? Or is it just that the P Zero's are big ol' soft tires that wear out fast?

Any reason not to mount up some P Nero's (all weather) in the back as replacements? I'm putting down 235 HP, not 390.

And if there are any tire sizing experts, OEM setup was with 240/55 16" TRX all around. What's the biggest tire I can put on the 18x7.5 front rims, and the smallest I can put on the 18x10 rears, to most nearly approximate this geometry?

 

before putting new tyres on you may want to have the suspension geomtry checked

All mondial Qv's came with 240/55 390 milimetrics rather than 16"s this equates to around 15.4 inches

 

I just bought a set of 360 wheels for my Mondial Cab. What length lugs were required to "bolt them right on"?
Thanks

 

240/55-390 equates to a tire with 25.748 inch diameter. You would need to find a tire that comes close to that diameter to maintain speedometer accuracy. (Though it can be argued that the stock speedometer isn't exactly accurate to begin with.)

In an 18" tire size, you would need about a 245/40-18 (25.717" diameter) in front, and a 275/35-18 (25.57" diameter) or 285/35-18 (25.854" diameter) in the rear. The 285 might fit the rim better, but be sure to check for fender clearance.

Hope that helps...