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Old 08-26-2010, 01:54 AM
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RPM's

How is it that with a m-21 4 speed, 3:42 gears,and a 26 inch tall tire I'm revving 3100rpm @ 60 mph . Ive checked with my gps had my cop buddy clock me, and put in the right hardware when the tranny was built ,which by the way did not change the rpm before or after the rebuild,could my 4th gear be numerically higher than 1 to 1. I've counted the ring and pinion gears and come up with the same number that is stamped on the gears 3:42.1.
I'm stumped any one else with this problem,or any logical explanation.Any help will let me sleep better.Thanks everyone

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Old 08-26-2010, 02:34 AM
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First question, are you absolutely sure the tire diameter is really 26". Don't go by published specs, published tire diameters are rarely ever correct(pet peeve of mine about tire companies, how hard can it be to publish the true #'s?)? Measure the tire yourself to find the true diameter.

Question two, is this rpm reading off the factory Z28 in-dash tach?? They are known to not be that accurate when they were new, and they get worse with age. Have you confirmed the rpm with a seperate tach??

There are no Muncie or Borg Warner T-10's with anything other than 1-1 in 4th gear, no overdrive versions were made(along with no underdrive). All 4th gear does is lock the input shaft to the output shaft via the syncronizer slider, 4th gear is not an actual seperate set of gears in the trans like 1,2,3 and Reverse are.
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Old 08-26-2010, 06:24 AM
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rpm's

The rear tires 255/60/15 I measured,with a straight edge on the surface of the tire "26" tall. I disconnected the factory tach because it was bouncing everywhere at all rpm levels and installed an autometer autoguage tachometer,and I have a msd timing light which shows rpm as well as advance and retard and it is dead on with the tach I istalled,? still stumped,But thanks for taking the time to get back to me
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Old 08-26-2010, 07:29 AM
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Either your measurements are wrong, you have a slipping clutch, or you have a 4.10 rear gear.
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Old 08-26-2010, 07:37 AM
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BTW, the 3.42 is a 41T ring gear x 12T pinion. The 4.10 is 41T x 10T. Would be a fairly easy miscount to make, especially on the pinion.
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Old 08-26-2010, 07:42 AM
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3.73's with a slightly under 26" (25.8) tire will produce the rpms you are looking at.
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Old 08-26-2010, 07:48 AM
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I thought about that, too- the GM 10-bolt corporate w/3.73 uses 41T x 11T gears. Even easier to miscount.

But it will take a 25" tire to be 3008 RPM.
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Old 08-26-2010, 07:53 AM
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I believe a better way to determine the diameter of the tire is to do a roll out measurement and compute it from that.
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Old 08-26-2010, 08:32 AM
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Shot in the dark? Is your tach one of those that has the selector for 6 & 8 cyl. dual purpose?
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Old 08-26-2010, 11:35 PM
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RPM's

The tach is set for for 8cylinder , at the track I shift into 4th at the 11ffot mark ,with 4zz'10's would I not have shifed sooner into 4th,as well when I had the diff cover off to clesn it up and put on a summit aluminum girdle the pinion gear was stamped 3:42,as well the ring had 41gears and the pinion had 12 =3:41.666rounded of 3:42.I also did a top end test with my gps and a friends radar laser gun that he was working and qualified to use and iterpret.
I hit 136mph at 6200rpm I dont think that would be possible with 4:10's at that rpm.???????????????????????????????? I cant figure it out.
In the end The car itself runs super smooth great road manners and has the ponies to back it up an gets 14mpg when I'm doing 60mph on the nose,so I'll drive it and enjoy it .
I didnt spend 4 years and 18 Grand to watch it maybe one day I'll go 4:10s and a 5 speed. Thanks everyone.
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Old 08-27-2010, 12:14 AM
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A 77' trans am with 26" tires 235/60/15 , a TH400, and 3.08's is at 3000rpms at 70 mph.

The Olds 403 is built for torque and has the Performer olds cam, and intake. Trans shifts out around 4800rpm. Stock had 2.40 gears.
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Old 08-27-2010, 08:53 AM
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Something has to be slipping, cause after doing the math, you should be doing 70.2 moh at 3100 rpm. i would say look at the clutch, and make sure its not.
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Old 08-27-2010, 09:25 AM
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Muncie

There is a overdrive gearset for muncie trans.I would hook up a second tach and compare it to the first one.
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Old 08-27-2010, 12:51 PM
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rpm's

Thanks Big Roy, but its a brand new clutch from LUK it's an 11 inch unit for big blocks 1965-1974 I have a bigger bell housing for my sbc and it grabs like crazy,I've gone through that test as well. 13.1@101.36mph not bad for 2tons.
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Old 08-27-2010, 01:11 PM
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I still this your rear gear is not what you think it is. It is the only thing that makes sense.
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