Are the Firestone the same height as a Goodyear FI tire of same size? When we were looking for new tires on our drill the Firestone guy told us the bias Firestone was a different height then the Goodyear so we would need to replace all to keep drill level. Told us to just get the good years
Going off the data books the 11L15 Firestone FI shows 30.6" overall Diameter, the IF280/70R15 shows 30.50", and the Goodyear Highway special shows 31.1".
Based on the information in the data books the Goodyear will raise the axle 0.255" over the Firestone IF. But then again the static loaded radius on the IF280/70R15 is listed as 13.8", and 13.7" on the goodyear, so real world differences may not be as drastic if those numbers are correct
A Bias tire generally won't have a very flat footprint, so the overall diameter will be the largest in the centre of the tread area, whereas the radial will be more consistent across the tread face.
***EDIT*** couple other things worth mentioning, the firestone IF280/70R15 has a flat plate (how much tread touches the ground) of 78 square inches, vs 66 on the goodyear, and the IF Firestone carries 3960lbs @ 52psi with a D (65km/h) speed rating. The goodyear carries 3960lbs @ 90psi at what they call "intermittent highway speeds", so probably short trips of max 80km/h I would have to assume.
If you're changing all of the tires I would be willing to bet the Firestone IF would be a much better tire option, and cheaper overall than the 12ply FI bias tires from Firestone or Goodyear.