Jon,
As stated above, with a 3.55 conversion to a 4.11 rear-end, the speedometer would be reading about 14% lower (55 mp would really be around 63 mph).
If you have original optional 15" wheels and original-style light truck tires, you might find that they are larger diameter than 16" tires, depending on brand and profile. The 15" original tires were larger diameter than the original 16" tires. As you mentioned above, the best way to check this is to measure the tires. However, if you used the same tires before and after, then this is not the reason for not feeling "a difference" after the conversion.
Also, I recall that someone once posted a link to a company that makes speedometer correction gears (adapters?) that work on our trucks. I will see if I can find the post.
Tim
Earler posts have recommended
United Speedometer Service for speedometer ratio adapters. I will be contacting them before the spring and will post my results here. Maybe someone else who has bought an adapter from them will post their experience?
Patrick's (the company who supplies all the 3.55 and 3.38 conversion gears) sells speedometer adapters; however, his adapter is for stock diameter tires. If you are going to spend the money for an adapter ($60-$100?), you might as well get one that matches the tires/gears on your vehicle.
