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Discussion Starter #1
A buddy of mine recently purchased a used 96 cobra. We're not sure what kinda gears he has. So the question is, what RPM are you @ 80 mph and what gears do you have? THANKS!
 

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I have 4:10's and at 80 I am at just under 3 grand
 

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80mph is right at 3000 maybe a tad over
 

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if i remember right...

4.10's, 80mph=2800rpm
4.30's, 80mph-3100rpm
 

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4.30s here and at 80mph Iam right at 3k.:D
 

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With stock tires (245/45/17), 80 mph equals...

2295 rpm w/3.27
2490 rpm w/3.55
2615 rpm w/3.73
2735 rpm w/3.90
2875 rpm w/4.10
3015 rpm w/4.30
3200 rpm w/4.56
3425 rpm w/4.88
 

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Bob Cosby said:
With stock tires (245/45/17), 80 mph equals...
2295 rpm w/3.27
2490 rpm w/3.55
2615 rpm w/3.73
2735 rpm w/3.90
2875 rpm w/4.10
3015 rpm w/4.30
3200 rpm w/4.56
3425 rpm w/4.88
That looks correct to me.
Remember a stock speedo usually reads 2-5 MPH higher than actual speed.
Also remember if the owner didn't compensate the speedo for the diff ratio, it will be way off.
To avoid these problems, it's best to use a GPS on a flat, straight section of freeway to measure actual speed.
My car with a 3.73 and 295x35x18 rear tires is 2650 @ 80 MPH.
 

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Or you could do it the old fashion way and jack the rear end up. Mark the tire and driveshaft. Rotate the wheel one turn and count the revolutions the driveshaft makes and that's the ratio.

John
 

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SSSSSS said:
Or you could do it the old fashion way and jack the rear end up. Mark the tire and driveshaft. Rotate the wheel one turn and count the revolutions the driveshaft makes and that's the ratio.
John
That works too... the only difficulty being trying to see the difference between 4.1 and 4.3, or 3.73 and 3.9. But it gives you a good round number.

For anal accuracy, you do all the math (RPM to tranny ratio, to diff ratio, to wheel circumferance). Just remember most tires in reality are about 3% smaller than the numbers indicate. For example, a 245x45x17 wheel has a circumferance of 80.7 inches. But that is just a mathematical calculation based on the tire size. The wheel's actual size is usually about 3% smaller.

To get the exact circumferance in typical installed applications, some manufacturers give a number which is total revolutions per mile. For example, take the BFG G-Force T/A KD. Its 245x45x17 size is rated at 811 revs per mile. That is 78" circumferance, about 3.2% smaller than the theoretical size of 80.7" circumferance.
 

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Just an FYI...not all speedo's are 2-5 mph high. Mine was very, very close before I changed gears (as it is now - as I have control via the Speedcal).

Another way to check your speedo is to drive down the interstate at a constant speed for a timed 10 miles, using mile markers. I would use 10 vice just one or two in order to get a more accurate distance. Here's one way to do it...

Set your cruise at whatever speed you wish - but it must stay constant.
Start timing at a convenient mile marker.
Stop timing after 10 miles.
Divide the amount of seconds it took to go those 10 miles by 10 (gives you average seconds per mile)
Divide 3600 by that average - that is your speed in mph.

For example...

Speedo reads 70 mph
Time to go 10 miles was 8 minutes and 50 seconds.
8 min & 50 seconds = 530 seconds.
530 / 10 = 53 seconds per mile
3600/53 = 67.9 mph

:)
 

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Discussion Starter #13
:curses: , stupid me... I left out one important part. He is on 285/35/18. How much would this throw it off? How can I tell if he has a SpeedCal, where is it located?
 

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I wouldn't do it that way. I would just get under there and figure out what 's there. See what speedo gear is in there also.

Because when you say 80 MPH, you're assuming it's really 80 MPH, that it's been corrected, or corrected accurately. 285X35X18 is a tad over 1% more diameter than stock, so would make speedo read a little less than 1 MPH slower.

You could more likely figure it out looking at the RPM's show in the above posts, and see if that matches any of your buddy's speed. Go by RPM, not speed.
 
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