Tach accuracy

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Backally

Backally

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70 Duster, 340. I have the rallye dash tach. I haven't compared it to another tach yet but some symptoms point to it being inaccurate, if I am correct it is reading low. I have an Accel distributor, pn 59301, MSD blaster 2 with a Rev-N-Nator ignition box. I read on For B-Bodies only some people having issues with tach accuracy using electronic ignition. I see there are various tach adaptors listed at summit, are these needed to make the factory tic toc tach accurate, or do I have to go to a aftermarket tach? Would like to keep it stock looking if I can. This is a street car, but I would like it reading pretty close.
 
Another way is to calculate RPM by MPH. Plenty of online calculators based off rear gear and tire diameter.
 
Checking it shouldn't be hard, just need to get off my arse and do it. More wondering if there is a fix if it is at fault.
 
At highway speed it was coming up with rpm's consistant with a od 4spd using online calculators. But crawled under it today and looked and it's not. Can't believe I didn't know that. So it appears it's off by a bunch. But want to hook up another tach to make sure.
 
They are pretty far off and even if you upgrade to the solid state board they still react slow due to the heavy movements.
 
...............U may want to check ur speedo pinion in the trans if ur trying to calculate speed verses rpm.......cause tire size and gear ratio's affect it........kim.....
 
I have found the low cost $20 optical tachs work good. A relective target tape is put on balancer, aim and get a digital read out. I have verified with micro controller timing measurements used in my EFI and ignition systems.
http://www.amazon.com/CyberTech-Digital-Photo-Tachometer-Contact/dp/B001N4QY66/ref=pd_sim_263_1?ie=UTF8&refRID=046BGV1DR4C54R948QW8
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Put 3.73's in this past winter. Updated the speedo gear and verified it with a gps. In on the road for a couple days so will put the timing light on it wed. But I believe it's reading low, it would explain a couple things.
 
Put the timing light on it, from 1500-4000 it's a consistant 500 rpm's low. If anyone knows if a electronic tach adaptor will fix this I'll try it. Otherwise I'll wait till next winter and see if it can be calibrated.
 
i had a revnator,and my tach would be all over the place,put in chrome box and all is good.sent it back.6 mo later they said they got it lined out,and sent me another one.sent it back also.the car ran good with the revnator,but had tach issues
 
That's interesting. I got a orange box I'll stick on and see if the tach straightens out. It defiantly runs better with the Ren-N-Nator though.
 
Dr Diff. Gears, axles and 10' brakes. Yea, like them. Had 3.23's, most of my driving is around town. Maybe a real little bit of noise decelerating but if I wasn't listening for it I wouldn't notice.
 
Just a FYI, not the Rev-N-Nator, does the same with my orange box.
 
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