Rear gear ratio ? - Chevelle Tech
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post #1 of 18 (permalink) Old Mar 9th, 20, 6:10 PM Thread Starter
 
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Rear gear ratio ?

Hi Guys! Just a quick question. In a 70 CHEVELLE SS 454 Built with all the goodies. Driving in 3rd. Gear going 45mph. The tach is at 3000 rpm’s
What is the gear ratio?
And does anyone have the formula to calculate rear end gear ratio’s?

Thanks in advance!
DAVE
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post #2 of 18 (permalink) Old Mar 9th, 20, 6:40 PM
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Jim
 
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Re: Rear gear ratio ?

Dave -

What transmission do you have? Assuming it's a TH350 or TH400 automatic - i.e. 3rd gear is your final drive?

If you're turning [email protected] 45MPH, you've got a fairly low (high numerical) gear ratio. The best way to calculate would be to lift the rear-end of the car (in neutral with front wheels blocked) and spin the tire one full revolution and count how many times the pinion (driveshaft) turns. This may not get you an exact number, but it will be fairly close. BTW, this assumes you have posi, if not, *I think* you have to keep one rear wheel on the ground so it doesn't spin.

jim

Edit - Well it looks like from your other posting in the New Member Introductions forum that you have a four speed. What RPM's are you turning in 4th at highway speeds?
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post #3 of 18 (permalink) Old Mar 9th, 20, 6:57 PM
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Jim
 
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Re: Rear gear ratio ?

Aftermarket tach of factory. I assume it was in 3rd gear.

Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
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post #4 of 18 (permalink) Old Mar 9th, 20, 7:07 PM
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Steve
 
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Re: Rear gear ratio ?

Are you sure youíre going 45 mph? The speedo gears in the transmission may be off as well and you could be going faster than you think.

1972 Chevelle, 454, 200-4r non-lockup, 12 bolt 3.73
1981 Camaro Z28, 355, TH350, 3.73
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post #5 of 18 (permalink) Old Mar 9th, 20, 8:14 PM
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post #6 of 18 (permalink) Old Mar 9th, 20, 9:10 PM
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Re: Rear gear ratio ?

Sounds like 4.88s if everything stated is correct.

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post #7 of 18 (permalink) Old Mar 9th, 20, 10:22 PM
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Jim
 
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Re: Rear gear ratio ?

Quote:
Originally Posted by Alan F View Post
Sounds like 4.88s if everything stated is correct.
I was gonna say 5.13

Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
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post #8 of 18 (permalink) Old Mar 10th, 20, 6:55 AM
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Gene
 
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Re: Rear gear ratio ?

Dave, some really good reference tools here, scroll down the page to "useful gearing formulas" and run some math, don't guess!

https://www.chevelles.com/techref/

Gene
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post #9 of 18 (permalink) Old Mar 10th, 20, 9:44 AM
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Re: Rear gear ratio ?

Is this a factory tach, or aftermarket? Not sure about Chevy, but the Olds tach after years can be way off. Make sure your tach reads correct. Mine was almost 2K off. The Olds has adjustment on the back, not sure if the Chevy one does, some other Chevy guy probably knows? Also on the Olds if Chevy is the same, the adjustment is very touchy.
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post #10 of 18 (permalink) Old Mar 10th, 20, 12:02 PM
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Jim
 
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Re: Rear gear ratio ?

Reason i asked about the tach, if aftermarket it could be set to 6 cyl. that would drop rpm to 2250 that would be about a 3.73 gear, if mph is correct.

Jim

77 Monte
421 Profilers ported by Eric Weingartner, Straub HR 3200 conv. from Jake 200-4r w/ 3.73's
Machine work by Torvinen's machine
11.93 @ 112.7 & tuning
67 Imp.SS for the wife 275 hp 327-TH350
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post #11 of 18 (permalink) Old Mar 10th, 20, 12:16 PM
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Re: Rear gear ratio ?

HI
Rear tire size ????
RPM at 55 or 60mph in top gear
transmission type auto/man and brand M21 Th400 etc etc

Need all info to calculate rear gear
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post #12 of 18 (permalink) Old Mar 10th, 20, 6:25 PM
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Jack
 
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Re: Rear gear ratio ?

3.31 is the rear axle ratio on my 1971 SS 454 , 12 bolt ...

https://www.chevellestuff.net/1971/rear_axle.htm


https://www.chevellestuff.net/index.htm

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post #13 of 18 (permalink) Old Today, 2:09 AM Thread Starter
 
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Thank You All for your Great Information!!!

MY REAR GEAR RATIO QUESTION.

I am Truly Sorry, I didn’t fully explain all the Details.

454 built big, Factory Tach, 4 Speed manual trans, 3500 rpm at 65 mph. In 4th. Gear, L 60 rear tires on factory SS rims.

Thanks for all the awesome reply’s and the tach might be erratic. I never considered that. Maybe 3.73 ?
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post #14 of 18 (permalink) Old Today, 4:51 AM
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Re: Rear gear ratio ?

Hi
Monster info is a bit short U running 14 or 15 rims??? . Assuming 15 275/60/15 equiv to L60 /15
Tyre diameter is 711mm or 28 inch
3500rpm at 65mph = approx. 4.10 diff ratio
thank me later
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post #15 of 18 (permalink) Old Today, 9:02 AM
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Re: Rear gear ratio ?

Quote:
Originally Posted by MonsterSS396 View Post
Thank You All for your Great Information!!!

MY REAR GEAR RATIO QUESTION.

I am Truly Sorry, I didnít fully explain all the Details.

454 built big, Factory Tach, 4 Speed manual trans, 3500 rpm at 65 mph. In 4th. Gear, L 60 rear tires on factory SS rims.

Thanks for all the awesome replyís and the tach might be erratic. I never considered that. Maybe 3.73 ?
I think that if the speedo gear is the correct one and the tach is good, then the gear ratio is definitely more than 3:73... more like 4:30 or even 4:56.



Claude.

1971 Chevelle SS
1987 Buick Grand National 9.48 @ 142 mph
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