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Discussion Starter #1
Here's one for the math buffs. I need to find out how far off my speedometer is. I have 4.11's in the rear with 27" tall tires. I just recently replaced both the drive and the driven gear in my M20 to match the 4.11's. My tach reads 3000rpm @57 mph or 3200rpm at 60mph. Is this very far off from what it should be and with the 27" tall tire what is my actual rear gear ratio? If other info is needed....


Thanks,
Gary


1969 Cortez Silver Z28
 

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62.5 mph @3,200 rpm
with:
4.11
27" tire diameter
1.0 trans ratio

If you were running:
60 mph, 27" tires, 1.0 trans ratio, 3,200 rpm
then you would have a 4.28 rear, but you do have the 4.11 in it, right?

[This message has been edited by Silver69 (edited 10-21-2001).]
 

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Discussion Starter #5
Thanks for the responses. Yes, I am using 4.11's. According to the the calculations from the website Dennis turned me on to, either my speedometer is off just a bit or my tach is 200 rpm off. At 3000rpm I should be going 59 mph with 4.11 and 27" tall tires. Right at 3000rpm on my tach mph is around 57 and at 60mph my tach indicates 3200rpm. This at least will keep me from getting a speeding ticket.


Thanks again,
Gary
 

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If it were me, I'd time and measure about 5 mile posts on an open stretch of interstate. Keep it exactly on 60, and it should take exactly 5 minutes. Ratio it from the actual time. If the actual time is 330 seconds then you would be off by 30/330= 1/11 or 9%. ie you were going 54.6mph. The odometer sould be off by the same percentage. Ricks has a table in their catalog, to tell you which gear to change to, based on your results.
 
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