. . . but every one i;ve talked to says 55mph is about it . .
They are wrong.
My larger diameter radial truck tires (on stock 16" rims) get me about 9% better speed at same RPM compared to the 15" tires you have. Add the approximately 10% improvement from the 3.55 conversion and you are close to a 20% improvement.
I can (and have) cruised all day at 65 mph. Two people can talk at that speed at normal volume.
If you get funny noises now at 55 mph, you have problems somewhere (u-joint, transmission, rear-end, wheel and/or axle bearings)?
