I've read this post a number of times and just now noticed something odd. You indicated at 60mph you were actually traveling at 71mph. This would occur if your car was calibrated for 3:55 and you went to 3:08 (just an example). My 76 came from the factory with 3:08 gears. It now has 3:36. The transmission does not have a reducer for the speedo cable. That means when in 3rd gear (1:1) and 3K rpm, my vette is travelling around 71 mph. The speedo indicates 80mph ! So, I'm actually going slower than what the speedo indicates....
If your speedo is indicating 71mph, your actual speed would be
in the 60 to 65 mph range.

if you read the post correctly his wife was traveling 60 next to him and his speedometer was reading 71 while his c3 was traveling 60 also. this would be correct with 355 gears installed inplace of 308 without changing the speedometer gear.
the drivetrain would be spinning faster than the actual movement of the car. the speedometer gears are easy to change. pluge right into the end of the cable

Sorry, I have a bit of aixelsyd...

Your doing the same as I did. I have a '74 coupe L48/auto.
I went from 3:08's to 3:55's. To correct the spedo you have to change the gear and the housing that holds it to the transmittion. Your old gear is a cream colored gear (I think 36 tooth) the new gear should be a black gear with 40 teeth. But you have to change the housing also, the gear shaft diameter is larger. I did drill mine out to the new diameter, but you have to be very careful to keep it straight and a tight fit but not to tight. If you feel the old gear in the housing you can match the clearance. If you want to play safe the new gear is about $6 or $7 bucks and the new housing is around $15 to $20. To install it you remove the one bolt holding a "c" shaped clip and pull the housing out. To reinstall it grease the "o"ring up good and tap the housing back in. Watch you don't damage the "o"ring, then turn the housing slightly (it's eccentric shaped to ingage the teeth)and match it up to the "c" clip and bolt it in place. My car was reading about 8 or 9 miles fast, it's now right on.

Can't ask for help that's any clearer than that. I appreciate the information. ~ Joe

I changed out my 3.08 to 3.55 gears and had the local speedo shop recalibrate the spedo. The odometer racks up na extra .3 per mile but at 50 the speed is correct. Radar gun put me at 39mph when the speedo read 50 after the swap.They put it on a bench tester and reset it to read 50mph at 50. He said they could have made a reducuter to go in the speedo cable that would have made the odometer read correct if I wanted them to.