I finally gave in a bought a Speedohealer. The install was super easy, though I did enjoy the disclaimer
'Do not attempt to install the product if you don't have basic mechanical skills.'
Having barely passed this qualification, I plugged it in (right above the fuel pump)
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and put the unit under the passenger seat
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It will calibrate from -100% to +10,000% scale, plenty for what I need. I tested it out by a roadside speed indicator, so far I think around 550% is correct, I'll be fine tuning this over the next few days.
I'll recommend the Speedohealer, a bit expensive (just over $100) but it works, is easy to figure out, and looks durable and good quality, but time will tell.
Edit: Corrected to 570.4