BILL "Quik-Question". - We only answer your questions when you start the topic with, "I have a QQ!".
I don't know how long Carlisle has made GT tires, but I almost bought my first pair of 23-8.50 'Stones USED back in 1970. Neighbor was pulling his 122 CC in our 4-H club's garden tractor pull and he brought his Firestone shod 122 to pull. He would have sold them to me but he said if he did well he was going to keep them. Well, he won TWO classes of the three classes held. He didn't enter the third class. Only real competition he had were 3-4 Economy Power King tractors trailered down 100 miles from Rockford, IL. I wanted them for mowing with the 70 I restored this spring. The 6-12 GY Pizza-Cutter turfs would spin out if you as much as spit on the grass. CC wheel weights were really hard to find too, but they sold for about $10/pair if you found them. First pair I bought in '71 I paid $5.50 for. The carriage bolts cost more than the weights!
So it's possible there's 40+ yr old Firestone 23-8.50 tires around. My Firestone Ag tire engineering book has the 23-8.50X12 & 27-8.50X15 All Traction Field & Road tires in it and there's no published date in or on it, but I would guess it was published around 1964/1965 by the age of the tractors in it. So they could be close to 50 yrs old. I bet your old Firestone tires are in better shape than my similar aged Good Years!
My two pair of "Stones are about 13 yrs old and 10 yrs old, and you couldn't tell either pair from brand new.
Firestone invested MILLIONS of Dollars building, "The MACHINE", a towable draft load dynomometer for ag drive tire testing back in the 1960's, and their 23 deg. tread tires were the result of their first few years of testing. I bet Carlisle doesn't have a machine like that! Even over on the RPM forum, when tire selection comes up, Firestone always gets the most recommendations. I really didn't care for the 23 degree bars when they first came out, but it's hard to argue with the way they perform. When IH first released the 1206, first over 100 HP row-crop tractor, they ALL came on Firestone 18.4X38 tires. Anything else they put on those tractors would spin the rims inside the tires ruining the inner tubes from the sidewalls buckling. The only thing I would even consider other than a Firestone would be a radial, like the old BF Goodrich Power Saver radial, now the Michelin AgriBib, and they're WAY more expensive. But Firestone makes a great ag radial too.
Ohhh and proper hydro operation is to reduce/change speed with the hand lever. The foot pedal is reserved for PANIC stops.