Tim, I bought an adapter from the same guys who sold me my 3:55 gear set and it's still off by 5 mph on the speedo. I did a two way 10 mile check using the odometer and found that I indicating 8.9 miles/10 actual (at least that's what I think I remember, LOL). Anyway, the "standard" adaptor from Patrick's (for around $35) still results in my speedometer reading 4-5 miles slower than actual. I haven't checked the odometer again as I'm in a sort of bummed out holding pattern with the truck right now due to "funny" noises in the engine.
I am running 16" tires so assume that if I want to get better accuracy I'll have to run the test again, without the adapter, and find someone who can provide the correct ratio.
Here's a site to help in determining the correct adapter:
Speedometershop.com Don
