New to me car, while driving today found out my speedometer is off by 8mph. Speedo has to show 68mph for me to be doing 60mph. What needs to be changed to correct that?
Thanks

 

Could be the speedometer drive gear in the transmission. They are color coded and need to match the rear end ratio. In 49 years, the transmission or rear end could have been changed.

 

Has the transmission, rear end gearing or tire size changed over the years or a combination of them ? Like the previous poster said, change the Speedo gear in the tranny. Not hard to do

 

Yep, lower rear gears or shorter tires will make the speedo register faster than the actual vehicle speed. And a replacement transmission doesn’t necessarily have the same speedo gears as the original. I’ve experienced all of those with my car.

 

Thanks guys for the answers. Bob

 

So, haven't checked the rear end gear ratio yet BUT, the tire size I have on the car now is 235/60 R15 and from the info sheet I have on the 75 H/O the original tire size was 234/70R15! I can't seem to find any 234 size tires so from other threads it appears the 235/60R15 is a common replacement.
I suspect this accounts for the speedo being off a bit???

 

I'm pretty sure a 234 mm width tire was never made, surely its a typo and should be 235. However, going from a 70 series to a lower profile 60 series would result in a smaller tire OD and would result in your speedo error.

 

If you accurately checked the 68/60 mph ratio (by gps or by timing between mile markers), then the fix is usually easy. Remove the speedo cable from the transmission. Remove the gear cover from the transmission and take out the speedo driven gear. Count the teeth (or look up by color). Multiply the number of teeth by 68 and divide by 60. Buy the gear with closest to that new number of teeth and install it.

If that number of teeth is off the chart, then reply. There are additional fixes for that circumstance.

 

Thanks I'll have my mechanic do that as I don't have a lift to get under the car.