Ancient Troy-bilt tiller

Hi,
I've got an old Troy-bilt tiller which still works, and I'm too cheap to replace it until it dies. The motor has a horizontal drive shaft coming out the front, which connects to the transmission with a belt. The transmission shaft has 4 pulley wheels.
My problem is that the tires are shot, and I can't find a replacement for them (4.80/4.00 - 7, 1" shaft). I can find a larger (8") tire/wheel with a 1" shaft, but it's about 16" in diameter, where the current tire is only 13 1/2".
My concern is that, with the larger tire, the tiller will go too fast. I can use one of the other pulley wheels, but don't know if this will change the speed of the wheels only, or if the speed of the tines will change too.
If anyone knows how this works, or can advise me at all, I'd very much appreciate it.
Thanks, Earle
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Replacement tires 4.00- 7 are available from Miller Tire. http://www.millertire.com/tire_catalog.asp?class  At about 28 bucks per it's a lot cheaper than buying a whole wheel
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If the size of the shaft is the same it will work The bigger the tire the slower it will go ..more space to turn Plus smaller tires are harder to find
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