Tachometer and Speedometer adjustments

[maguyvr]

I have a pretty much all original 1970 Z28. I have discovered that the tachometer reads 200 to 300 RPM's high and the speedometer reads about 2 mph slow. I now this doesn't sound like much, but while driving 60 mph the RPM reads almost 3500 RPM and it bothers me. The tach needle at rest is above the bottom line. Can I pull the needle off and re-index it lower? Or is there a way to adjust it somewhere?
The speedometer is off because the original tires wire F60x15 and are now one size larger at 245/60 R15. What is the easiest way to correct the speedometer.

Thanks for any help.

 

[Gary S]

Those old factory tachs usually read a little high. Chevy made them that way.
The speedometer is corrected by changing the speedometer drive gears in the tranny or if the tranny has an external ratio changer attached to it, you can get a replacement one with a different ratio. 2mph is such a small amount that it hardly is worth it. Most new cars aren't even that accurate.
The problem with a 70 Z28 is the deep gearing in the rear axle. With deep gearing you get high rpms on the highway.

 

[FlaJunkie]

I have 3:73 in mine. It reads correct speed with Radial T/A tires. The tach reads high.

Back in the days of F60-15 Polyglas tires, double the tach gave you accurate speed. 3500 RPM equals 70 MPH.

With radials 235-15, tach reads really high. Like 4000 RPM 60 MPH.

 

[Green hornet]

The tach needle is not removable to be adjusted. You would have to remove your tach and send it out for recalibration to have it corrected.

 

[70lt1z28]

For the speedo, do you have one of the Stewart Waner speedo reducers inline with the cable:

 

[maguyvr]

The tach needle is not removable to be adjusted. You would have to remove your tach and send it out for recalibration to have it corrected.

Thanks for that. I didn't want to ruin it by forcing it off. Not sure where to send it if I wanted to get it calibrated though.

 

[maguyvr]

For the speedo, do you have one of the Stewart Waner speedo reducers inline with the cable:

No I don't. The cable screws right onto the transmission. I guess that is for sure an option though.
Thanks.

 

[maguyvr]

Those old factory tachs usually read a little high. Chevy made them that way.
The speedometer is corrected by changing the speedometer drive gears in the tranny or if the tranny has an external ratio changer attached to it, you can get a replacement one with a different ratio. 2mph is such a small amount that it hardly is worth it. Most new cars aren't even that accurate.
The problem with a 70 Z28 is the deep gearing in the rear axle. With deep gearing you get high rpms on the highway.

Very true. It has the original 12 bolt 3.73 axle as well as the original 350 LT1 engine. That's why I want the tach to be correct as well as the speedometer. I drive it to car shows on occasion and don't like running the RPM's up too high and I want it to be accurate.

 

[maguyvr]

I have 3:73 in mine. It reads correct speed with Radial T/A tires. The tach reads high.

Back in the days of F60-15 Polyglas tires, double the tach gave you accurate speed. 3500 RPM equals 70 MPH.

With radials 235-15, tach reads really high. Like 4000 RPM 60 MPH.

I guess I should have put 235's tires on instead of 245's. Then the speedometer would have been accurate!

 

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Daniel at GaugeMarks can fix the tach, or TachMan can as well