Tire size help....

[allthrtl]

Just bought a 1983 short wide chevy scottsdale, haven't taken pictures yet, it is clean and very straight, although cheap paint job is flaking off... One owner truck with 305 and 700r4...Unfortunately, the tranny slips badly when in 3rd, hopefully it won't cost too much to fix, guess we'll find out next week. Anyway....Currently it has 215/75/15 tires on it which seem too small, height-wise, and the speedo seem a little fast...The door jam shows it should have LR60x15's...What does this convert to, p-metric size....ALSO, it has a GQ1 rpo code in the glove box for the rear axle....What ratio is that???

Thanks in advance....