How do i adjust the speedometer, reading about 20mph to fast. Thanks in advance for any replies. Ken

What wheels and tires? It makes a difference.

It’s not the tires. Had the Speedometer in for Odometer repair and asked the shop to reduce the speed shown by 8mph, they increased the speed by 8mph for a total over reading of 16 mph. Hoping I can just fix it myself as shipping can get costly.

You would probably need to take it apart and change some gears. Maybe the shop can send you the gears you need.

Does your receipt say on it "reduce by 8 mph"? If so it should not cost you a penny. Call and get a return label from them for shipping.

unless you have the correct ratio gears... not something you can do yourself.

The speedo works by spinning a magnet inside of an aluminum (I think) cup. The magnet more or less pulls the cup around in a circle, but is resisted by a clock-spring that tries to push the cup back the other direction.

The needle is directly attached to the cup, on the same shaft through the middle of the gauge.

To change the speed, you would need to change the spring for a stiffer one (lower reading for same RPM in) or a softer one (shows higher reading for same RPM in).

Or get a gearbox that goes in between the speedometer cable and the speedometer.

--DD

Thanks for all the replies, looks like I’m sending it back.

During my second drive of my new 74 I noticed that the speedo is off by 5mph at 60 compared to the Waze gps reading. Waze says 60 and the speedo says 65. Skinny standard spec tires, no knowledge of any gearing changes. The car seems mostly stock.
I'm ok with it but what could be the source of this discrepency?

During my second drive of my new 74 I noticed that the speedo is off by 5mph at 60 compared to the Waze gps reading. Waze says 60 and the speedo says 65. Skinny standard spec tires, no knowledge of any gearing changes. The car seems mostly stock.
I'm ok with it but what could be the source of this discrepency?

Not original hires

What size is printed on the sidewall?
The original tires had an aspect ratio of about 78
Go buy some 80 series tires and I’ll bet they are taller than your current ones

Even modern cars aren’t usually gps accurate.

Waze or other GPS based speed app's are often 1-2 mph different from speedo even on modern vehicles running OEM tires. Modern vehicle electronic speedo is much more accurate than 914 analog, cable and gear driven speedo so not surprised by higher variance to 914. 5mph is a bit high but not outrageous.

1) Tire have OD (Revoltuitons per mile) variability based on manufacturing, load, inflation pressures, and even running speed itself.

2) GPS has location accuracy/resolution is limited and is not perfect either, that can affect speed calculations.

https://www.gps.gov/systems/gps/performance/accuracy/

For highly accurate positional and speed accuracy you need a GPS base station and a differential GPS arrangment.

https://www.nj.gov/dep/gis/faqgps.html#:~:t...n%20the%20field

https://en.wikipedia.org/wiki/Differential_GPS

I made several comparisons between GPS readings and my 205x65x15 Michelin tires on Interstates during a trip, and the variations averaged about 2~4MPH between the two. According to tire spec charts that tire size should be very close in diameter to the original tire size for which the speedometer was calibrated on my 73 mostly original condition teener.