What would be the correct polyglass tire sizes for a 71 340 cuda 15 inch rallye wheels and could i go a little bigger in the back?The tire size now is 235/60R15 THANK YOU!!!!!!!

Original tire size if a 15" tire car would have been E60.15

That said. I would go 225.60.15 front(maybe 235.60.15 )

And 255.60.15 on the back

Any larger than 255 will need a wider rim

235/60R15 is equivalent to G60x15.
215/60R15 is equivalent to the E60X15s that would have been factory on that car.

15X7 Rallyes on my 71 Cuda, I'm running 255/60/15 on the rear, 245/60/15 on the front......but you can't take that as solid advise unless you intend on running the same tires (I have GoodYear Eagle GT II's) as tires in the same size, but from different mfg's will have slightly different diameters, thread widths, carcass width, as well as sidewall/corner profile differences that may/may not rub/cause interference/fit issues.... the PolyGlas is a very "square" tire regarding the corner profile...

Ehhhh....you can squeeze 275's on a 15X7 Rallye rim Bill, they'd fit on a 70-74 Challenger rear, but never clear the qtr lip on a 70-74 Cuda...DAMHIK

Mike

most conversion sites & the math will show
245/60R15 is close to G60x15.
225/60R15 is close to E60X15

tire construction & manufacturer all have different spec's for the same size tire even Kelsey Tire's reproduction POLYGLAS tires are not all identical dimensionally to what was original equipment

215/60R15 by BFG 25.2" diameter & 8.7" section width
225/60R15 by BFG 25.6" diameter & 9.0" section width
E60X15 by Kelsey 26.1" diameter & 8.9" section width