llesra and PI are correct.
Basically, in inches, you've just gone from a 28.95" diameter tire to a 29.29" diameter tire.
Let's suppose that your speedo was perfectly calibrated for the first set of tires: when it showed 100 mph, your wheels should have been turning at 581 rpm. At that same rpm (same indication on the speedo) the second set of tires will be travelling at 101.17 mph.
Your upgrade has, in fact, reduced the indication error.
Working it backwards, given that your speedo was NOT calibrated for the original set of tires (you mention the 57 mph real vs. 60 mph indicated), your speedometer will indicate true with a 30.47" tire (245/75).
If you like, I can break it down even further (formulae, radians and other unit conversions), but that'll have to be over a beer or three...
Cheers,
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MG
'95 Disco I, 3.9L V8i 5-speed
Discoverying Bolivia...
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