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Old 02-01-2009, 11:59 AM
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gear ratio

OK, problem is driving me nuts...

Car is a '78 Datsun 280Z with a Chevy 350 engine, Turbo 350 tranny, Datsun R200 differential. The rear has a after market device that makes the rear gears lockup much like a posi-traction. Problem is it runs at a high rpm when doing 50 MPH. OK, I turned the drive shaft a little over 3.5 turns to get the wheel to rotate one turn.I would guess than that it has a 3.50:1~ or so rear gear. Why then is it taching high at speed? The tach is not running right now so I would have to guess that at 50mph it miight be running around 3000 rpm.

I've seen plenty of cars with 3.58:1 gear ratios be quite road worthy.

All ideas are welcome.

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Old 02-01-2009, 12:34 PM
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what size tires?
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Old 02-01-2009, 12:37 PM
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how sure are you that it is a 3.50 gear.sounds lower to me.and tire size does make a differance.
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Old 02-01-2009, 01:15 PM
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tire size

tirte size is 235 60 R14 and the official gear ratio is 3.545:1
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Old 02-01-2009, 02:31 PM
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thats only a 25" tall tire i would try and get somthing a little taller and see if that helps (maybee borrow one from a buddy)
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Old 02-03-2009, 12:40 AM
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When you turned the driveshaft, did BOTH tires turn the SAME amount at the SAME time? If so, your calculations would be correct. If no, then your calculations are wrong.
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Old 02-03-2009, 01:21 AM
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By my math, with a TH350 in high gear (1:1), a rear end ratio of 3.545:1, and 235/60-14 tires:

Speed vs. Engine RPM
50 mph @ 2375
60 mph @ 2850
70 mph @ 3325
80 mph @ 3800

Maybe your ear tach is calibrated for a 6 cyl.. (that's a joke)

Is it possible the transmission is not getting into high gear?

Like Matt said, taller tires will help. If you can determine how big of a tire you can fit on the car, I can recalculate and tell you how much it would affect the engine speed.

Hope this helps...

Joe G.
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