Mark,
If the original rolling diameter was 28", then the change from 4.11 to 3.55 would result in an approximate speedometer correction of about 16%: 30mph would really be approximately 35mph.
Jim Carter's sells an adapter. You provide them information and they custom build an adapter. Their price is very good.
Before JC listed an adapter, I bought an adapter from
The Speedometer Shop that resulted in my speedometer/odometer being within 1% of correct at 60mph. I used Method #1, but I measured 10 miles and I did it 5 times and took an average. I gave them my results and they calibrated a ratio adapter.
A 28" rolling/loaded diameter is a little larger than an original 16" tire. My 16" radials are about 31.5" rolling radius.
