When I bought my GS back in 1999, it had the stock tachometer in the dash. Months later, I noticed the indicator would bounce around. Then finally, it quit altogether. I ended up buying an Autometer 3 3/8" in dash tachometer. I used an old GS clock housing mounting the Autometer in that housing. When I was done, it bolted in to the back of the dash panel like stock. It was an 8000 RPM unit, and I used it for many years. It proved to be reliable and accurate. Some years back, I was on V8, and saw one of the members, Scott Moody, screen name 87GN_70GS posting that he could repair stock tachometers. I responded to his posting and sent my tachometer to him. When I received it back, it indeed worked, but I found it to be very inaccurate mid scale. I asked Scott about this, and he more or less told me it was the nature of the beast, and that if he corrected the mid scale, it would be inaccurate at the extreme ends of the scale, which are more important. I elected to put the Autometer tachometer back in. About 2 months ago, Scott contacted me on V8 with a private Message. He told me he had come up with a way to install a more modern mechanism in the stock tachometer housing. He wanted to know if I was interested in being a "Beta Tester" for him. He would convert my tachometer for free. All he asked was that I give him a full and honest evaluation of it. He had 2 options for me to choose. One option was a mechanism very much like the Autometer that required an ignition on 12 volts, a good ground, and a separate signal wire. The other mechanism used just a signal wire exactly like the stock tachometer. In addition, the first option included an adjustment screw that would adjust the entire scale up or down. The adjustment screw would be hidden below a removable plug. I opted for option 1. I sent my tachometer back to Scott on August 26. Scott had the tachometer back to me on September 7th, 10 days including the Labor Day holiday, not bad. I rigged up some wiring so that I could run both tachometers at the same time. I duct taped the stock tachometer on top of the dash pad and to the front windshield. I then ran the engine between idle and about 3000 RPM so that I could compare readings. Surprisingly, I noticed the Autometer would read about 50-100 RPM low at some points during the test. I questioned Scott about this and he told me that his bench was more accurate than any tachometer, but that I should feel free to adjust the scale up or down as I saw fit. I opted to leave the calibration as delivered. I would much rather have as much accuracy as possible. I put the tachometer back into the dash and I am thrilled to have the dash stock appearing again. I asked Scott if he planned to offer this service in the future, and he said he did. Preliminary prices are very reasonable, $45 for converting the circuit board for points and HEI ( no external power needed) and $65 for electronic ignition (extra 12v power needed). He can also convert hood tachometers. Scott also sent a calibration sheet along with my tachometer, I highly recommend this service. I am very pleased to have my dash more stock appearing. Thanks Scott.
My particular tach was inaccurate there. I know what my RPM should be with my tire height and gearing. With 28" tires, 3.73 gears, with the GV engaged, RPM at 60 MPH should be 22-2300 RPM. My stock tachometer was reading more like 2800, so it was off quite a bit. Now it reads 22-2300 RPM like it should. 336/tire height X gear X MPH = RPM 3.73 X .78 =2.91 (Gear Vendors Overdrive) 336/28 X 2.91 X 60 =2095. Add 150-200 RPM for converter slip 2245-2295 RPM.
Wow that is off quite a bit. Probably most factory tachs are though. Not sure of a Buick tach but in my 70 442 I felt like it responded like it was vacuum. It must be so nice to have a responsive accurate tach. I put a Super Tach II mounted in the same spot as my factory tach. Fit perfectly. Much better.
By the way. Thank you for posting the math for everyone. It's not as confusing as it looks people. Again. Thanks
Maybe not. Do what I did. Figure out what your RPM should be at 60 MPH. Then drive the car and see where you are.
Many timing lights have a tach built in. This should be pretty accurate in order to compare with the cluster tachs.
I have an original 1963 Wildcat tach that I mounted on my 67 dash. Its awssome, however, it sticks sometimes and needs a slap. Is this conversion similar to D & M tach restoration which replaces the coil spring for fully electronic movement? I need to upgrade my tach. The dial is still perfectly sharp colored, just want needle accurate and smooth.
Gary, contact Scott and ask. That's why I posted this thread. Got to be a lot of tachometers with problems out there. http://v8buick.com/index.php?members/87gn_70gs.1236/
When they work they are usually pretty accurate. I would leave it as is, unless you're curious about it's accuracy.
If yours is working but sticks, it's normally debris in the meter movement. It will need cleaning, especially in the magnet gap. I would keep the meter movement but upgrade to modern electronics
My pleasure Scott. this will be a great service for guys with stock tachometers that either don't work or are not accurate enough.
If it's off by 500-600 RPM like Larry described above with his then that's a problem. But if it's off by 100 RPM or so I can live with that. I wouldn't expect a 500 RPM error though with a brand new tach.
Easy enough to check. Your speedometer needs to be accurate first. Check that against a GPS, or use the GPS as your speedometer.
I would try the test procedure outlined by Larry above as a first step, if its already installed. If not installed, I can check it out and calibrate if needed
I spoke with Todd who I ordered this tach from and was informed it has "modern technology" in it. I was able to quickly wire it up and run the car at idle long enough to see it agree with the aftermarket MSD in dash tach it will replace, as well as the RPM displayed by the handheld programmer. I'm happy with it so far.
I realize this thread is over 6 months old but want to add another perspective to the errors exhibited by our factory tachs. While being accurate at low and high scale and inaccurate at mid scale is certainly true in many older tachs, it's not always this way. Many are inaccurate across the entire scale and that error is usually a percentage figure that's nearly the same at all readings. The inaccuracy at half scale only is a linearity issue. The other, and the one I've experienced with my last two factory tachs is a percentage across the board and can usually be corrected with component changes during repair/calibration. Before calibration my current tach readings are as follows: Read - Should Be - Error 2000 - 1750 +250 = 14% 3000 - 2600 +400 = 15% 4000 - 3500 +500 = 14% 5000 - 4350 +650 = 15% 6000* - 5200 +800 = 15% * 2nd gear The above readings are from my GS with Turbo 400, 3.42 gears and 295/50 x 15 in. tires. Readings were taken 1n 3rd gear except at 6000 RPM in 2nd gear. So, to get an accurate idea of the error across the full scale, we should take readings at known speeds (like GPS) and calculate what the RPM should be at those speeds using one of the online calculators like the ones on WallaceRacing.com.
