Speedometer error?

[1978 NYB]

I was on a small road trip in Amish country Pennsylvania yesterday and I noticed that the speedometer was reading 50 mph and the GPS said my speed was 57 mph. The rear axle and 727 txsm are all original. The speedo gear doesn't look like it was ever out of the txsm or moved to the wrong position. I'm running P235/75R15 radial tires. Any ideas why the speedo is off by 7 mph?

 

[300rag]

My guess would be that your 235/75s are just a tad taller than the original tire (either 215s standard or 225s optional) were, and the speedo may have been reading a tad low originally as well.