how fast was I going?

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Nov 13, 2005 | 10:01 PM

djlangford

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Joined: Apr 2001

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From: Las Vegas NV

how fast was I going?

Ok, I'm sure that this has been asked at least 1,000 times, if not more, but I have a 74 BB, all stock, not sure of rear end gearing, but sure that it is stock, had an indicated 100MPH, and 4,500 RPM Does that jive given an auto? I have MSD ignition, and distributor, but otherwise stock intake, tranny, rear-end, and tire sizes.

I'm thinking my spedo is off by about 10MPH, but have no evidence to support that.

Thanks,
David

Nov 13, 2005 | 10:03 PM

80TexasC3

&amp;quot;Rodstoration &am p;quot; In Progres

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From: Frisco TX

You really need to take into account the rear diff ratio, tire size and if the original speedo gear is in place... any of these three can change the accuracy of your reading.

Nov 13, 2005 | 10:13 PM

djlangford

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From: Las Vegas NV

That is the idea, I don't believe my speedo, I had some work done on the cable and gear, and I've been thinking the spedo gear ratio is off since then but don't have any real evidence (1/4 mile, radar shot, etc.).

the VIN is 1z37z4s4{xxxxx}, though I dont' know that you can tell much about the rear ratio from VIN, other than it is a BB coupe, auto built in '74 in the 400k range.

Rear tires are 225/70R15.

Nov 13, 2005 | 10:13 PM

big_G

Le Mans Master

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From: Austin Texas

You can have your speedo. checked on a chassis dyno. Kill 2 birds with 1 stone....

Nov 13, 2005 | 10:33 PM

PRNDL

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Joined: Jul 1999

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From: Huntersville NC

It is impossible to tell. At 100 mph you are going to get wind resistance so with an automatic I doubt you would get the exact calculated mph given rpm anyway. BUT, in 1974 the standard rear was 3.36, but 3.55 was optional. So with 3.36 and 27.4" tires at 4500 rpm with no slippage you would be going 109.2 mph... BUT, if you have 3.55s you would be going 103.4 mph, but I think you would get some slippage, just not sure how much.

If I wanted to test my speedo I would just get on the interstate and hold the speedo at 60 mph and time myself between mile markers.

Or... you could get on the interstate and hold the speedo at 100 mph and wait until you get pulled over and they will tell you how fast you were going!!!

Last edited by PRNDL; Nov 13, 2005 at 10:35 PM.

Nov 13, 2005 | 10:39 PM

WhichVette?

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Borrow a handheld GPS if you don't have one.

Nov 13, 2005 | 11:16 PM

supervett77

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From: Apple Valley MN

you could always get autometer guages

Nov 13, 2005 | 11:29 PM

djlangford

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From: Las Vegas NV

Funny, PRNDL. ;-) Of course, I would only do 100MPH in a secure environment, never on the open road, I would never support such a thing.

But, your calculations are actually quite close. I was figuring my speedo was about 10MPH off.

I tested this after I posted. I had my [reluctant, but understanding] wife drive in front of me in our '05 Murano, and she was reporting 60 MPH when my spedo read 50. I then had her repeat the test in my '01 Carerra, with the results duplicated.

So, it as I thought, my speedo is off. They must have put the wrong gear in when I had the cable replaced. I'll have to research that.

Thanks,
David