According to the Tire Size Calculator here:
Power Hungry Performance Forum - FAQ: Utilities a good starting point for circumference would be 2652mm.
The calculator may still not open in Internet Explorer, but it works fine in Firefox.
But, you didn't say if your speedo is faster that actual or slower than actual.
If it is showing faster than actual speed, your Tire Size (TS) is too big, Try reducing it.
You also did not say if you are talking about the dashboard speedo or the Gryphon speedo. If you get the speed on the Gryphon to exactly match what a GPS is telling you, which is your true speed, the dashboard speedo will probably be about 2 mph faster at 70 mph due to mechanical errors. However, your odometer will be correct in that condition.
- Jack
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2024 F150 Platinum SCrew 3.5L PowerBoost FX4, Peragon Tonneau Cover, LineX Bed, 35% Window Tint on All Sides and Rear, Full Nose Paint Protection Film, Husky Mud Guards, Lasfit Floor Liners, VIOFO Dash Cam