With 3.55's the rpms won't be too high on the freeway. I have 4.10's and at around 70 mph I'm pulling about 2400-2500 rpms. At 80 about 2600 rpms. So even if you one step above 3.55's which would be 3.73's, the rpms shouldn't be bad. Probably 2300 rpms at 70. There's a calculator online that can help with this.