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66 Chevelle SS396 & 66 Chevelle 327 Convertible.
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Discussion Starter · #1 ·
Have a old Kneeknocker tack in my 66 chevelle. It reads about 3300 when it should read 2800 when traveling at 60 mph with a 3.55 rear end with a tire diameter of 26.1" Verified the speed with two other cars whoes speedometers are correct. Mine is wrong pretty sure the speedometer gear is for a 3:31 rear end, so my speedometer reads at about 67mph. Question is there any way to recalibrate the tack? Thanks, Jim
 

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How do you know it should be only 2800? Are you basing it only on your gear and tire dia? The only way to know if the tach is off is to try another tach and compare the readings at the same mph.
 

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Premium Member
66 Chevelle SS396 & 66 Chevelle 327 Convertible.
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7,045 Posts
Discussion Starter · #3 ·
Kind of, First I checked the Speedometer against two of my other cars, at 60 mph and recorded the distance traveled. The figured what my speedometer should read at 60 mph, wrong speedometer gear for the rear I have and the tires diameter is not factory specks. The followed my other car doing 60 mph and my speedomenter checked out at 67 what my calculations determined it should read at 60 mph. Noted my RPM at 60 mph and noted what they should read at 60.
 

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You didn't mention and I didn't ask was what kind of trans you have. If you have a stick shift the calculation can be pretty close, but if you have an auto it can be off. All autos, even bone stock ones have a certain percentage of slip in them and it can vary from one trans to another. If a performance converter is installed it can be even more and a whole lot more if the RPMs are below the converters stall speed. This is the reason why just knowing the tire dia and rear gear doesn't work well in knowing if the tach is off. If the other tach is a newer modern tach, then your original tach is off. I wouldn't know how to recalibrate one at home but you could have it done at one of the specially places that repair gauges and such. Here is one that someone else used at TC.

http://www.gaugeguys.com/index.htm

They can also help you with speedometer correction

http://www.gaugeguys.com/ratiotest.htm

See my post in the perfomance section, my car isn't anywhere near orginial and I an going to try them in getting my speedometer corrected.
 
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