If I'm using my phone with Waze or Google for navigation I get the GPS speed, too. Usually I look at the "GPS" speed, then look at my speedo and think, "Hmmm... That's interesting" and keep driving. Then again, I have a speedo in Kilometers and that's why I do the math in my head as Lane mentioned, so already there is a built-in error factor ('cuz I never was great at Math).
I've used the "doubling the tach reading" in fourth, from time to time, but mostly I just look at my speedo (in kilometers, for me), multiply what it says times 6 and extrapolate what the more-or-less speed is. I'm far too lazy to use the correction of 6.2 instead of 6 to increase accuracy by 2 mph, and I'm totally OK with the indicated speed being just a suggestion. I just drive carefully for the conditions and keep up with traffic.
If my speedo read a more-or-less consistent 15% high I would just factor that into the equation. Indicated speed is 75 MPH? OK 75 X .85 = 64 mph. There's your correction factor for highway driving - mentally subtract 10 mph. Got it. The gap will probably be less as you go slower. Use your phone for "true" speed at 45 and 35mph just so you'll know what the correction factor is down there and drive it. That's one helluva lot easier than messing with adapters or getting new guts in the speedo for something no one else will ever notice, and if you're worried about the odometer being off, NO ONE CARES unless you're looking at a car for sale with under 2000 miles on it (and wonder why it wasn't driven more).
I honestly can't remember, after 20 years, whether my speedo is running high or low to the true speed, but OTOH, I really don't care a whole lot. As long as I'm keeping up with traffic or not over-speeding down a back road ('cuz you'll never know when someone will be walking their dog or stroller out there), I'm fine.
Just drive it.