Hey... maybe you guys might be able to help me with this. I took my 73 454 4 spd out for the first time this year. After sitting inside over this long Montana winter... it really wanted to go fast... which it did... right up to the point that the car comin at me turned on the blue lights. Now heres how it went... I'm in a 70 zone, I saw him ahead, looked at the speedo, saw 72, tach 32 (both parallel like they're 'sposed to be), kept on going. Imagine my surprise when his lights came on. Seems as though I was going 83. So after a few minutes of chatting... great guy btw... and looking for a 71-73 he can call his own, he let me off with a warning. He also paced me so I could see actual 70. 62 on the speedo is actual 70. The rear tires are Comp TA 275/60-15 (28" tall). I do not know what the rear end ratio is. The car has had the same size rears on it for 25 years and I have never been stopped before. I guess I always thought it was right on because the speedo always tracked with the tach in 4th...wrong... and thought if the speedo was off, the tach wouldn't read parallel. Is this a speedo drive gear issue, and if so, anyone maybe know the part # I would need to correct this for this size tire?
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If the rear tires are the stock diameter the speedo should be pretty close, apparently the rear end has been swapped with a lower ratio gearset. Explain the meaning of parrallel tach and speedo.

 

Parallel meaning they are on the same spot in 4th gear on both the tach and speedo. 70 mph =3k rpm and the needles are both in the same position on each dial. I guess Chevy did this on the vettes with dual round face dials so that if you lost one, you could tell where you were in 4th gear by looking at the other dial. All of the others I have had (all early 70's 4 spd's) have been the same. I know the ratio has never been changed since it left the factory (2nd owner) but I don't know which ratio I have. Any way to tell without taking the pumpkin apart?
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The stock tire for '73 was a GR70-15 which is close to a 255/60-15. Your 275s are about an inch taller which leads to your speedo being off. Two ways to go- change back to something close to stock tires or change speedo gear in tranny. Stock rear gears for '73 w/ a wide ratio-3.08 & 3.36 optional, w/ a close ratio-3.36 & 3.55 optional.

 

You are right, I need to change to a different speedo drive gear to work with the taller rear tires. I kinda thought that was the problem after thinking about it for a while. Aren't there different colored speedo gears for different rear end ratios? I replaced about 4 years ago and seem to remember using a tan one, whereas the one I replaced was white. In case your wondering... the 275/60's look incredible from the rear... gonna keep them! Tranny btw is the close ratio M-22.
Thanks,
Wedge
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Well I think logically then you'll need to get a gear with fewer teeth so it will turn faster. Here's old (circa 1976) P/Ns and teeth: 3987919/19 teeth, 3987918/18 teeth, 3987920/20 teeth, 3987922/22 teeth. Parts guy should be able to cross those to current numbers if they're still available.

Hope this helps. [sm=gears.gif]

 

Excellent... I will be at my local Chevy shop tomorrow with those numbers.
Thanks for your help,
Wedge
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GM also sells a ratio'd speedo adapter that your cable goes in one end and the other screws in the transmission. It is available in several ratios. Yours should use one that is more than a 1:1 ratio. Maybe like a 1.2 : 1 ratio. If they don't have it, do a search on Google for speedometer ratio adapters and see what happens. Good Luck.

 

Here's a few shots of my Vette. Do those rear tires make its *** look big? Thanks again for the advice...
Wedge
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Looks good to me. My '73 has 275/60/15's on the back and looks the same...nice aggressive posture....like " I dare ya!"


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