rear end ratio

Corvette Forums Corvette Classifieds Gallery Corvette News Member List Search Sponsors
Forum FAQ Calendars Contact Us Corvette Recalls TSBs Corvette Timeslips Register Login

rear end ratio

Printable Version

Corvette Forums >> Corvette Models >> Corvette C3 Forum >> rear end ratio

Page: [1]

Message

<< Older Topic Newer Topic >>

rear end ratio - 12/12/2005 8:02:46 PM

79vette

Posts: 4
Joined: 12/2/2005
Status: offline

I have a 79 vette with a 3-speed automatic. 235/60 x 15 tires. at 65 mph@2,800 rpm, 70mph@3,100 rpms. Can anyone tell me whats my rear ratio.
Thanks

Post #: 1

RE: rear end ratio - 12/13/2005 5:52:45 AM

NY Man

Posts: 25
Joined: 11/29/2005
Status: offline

First, you can't be running exactly 2800 at 65 and exactly 3100 at 70, the resulting ratios of RPM to speed are close but not exact (if you're running exactly 2800 at 65 you're doing 3015.4 RPM at 70). Its hard to read a tach properly and they aren't perfectly accurate anyway.

Regardless, I expect you have a 3.23 axle ratio. Using a 25 inch diamter tire (about right for that size), 3100 RPM at 70 computes to a 3.294 ratio, and 2800 at 65 computes to 3.203, neither of which Chevy offers, but which bracket the ratio 3.23:1, which was a popular ratio for the automatic in the Corvette.

_____________________________

1970 Chevelle SS 350
Suburu Forester (snow car)
1996 Corvette M6, CAI, headers, cat back

(in reply to 79vette)

Post #: 2

RE: rear end ratio - 12/13/2005 2:24:50 PM

Lee Willis

Posts: 1952
Joined: 7/27/2005
Status: offline

Yeah, I did the math and get the same #s, most likely a 3.23.

_____________________________

2007 Aston Martin Vantage Prodrive
Lincoln MKZ "Grandfather car"
''98 Z28 with iron LS 408, NOS, tricks, street driven mid 9s (sold to my youngest son for $1, Aug 28, ''07)
''02 SCd 427 cid ZO6, 704 RWHP, sold Aug. 28, ''07

(in reply to NY Man)

Post #: 3

RE: rear end ratio - 12/13/2005 6:09:12 PM

xrcrx

Posts: 74
Joined: 12/7/2005
From: houston texas
Status: offline

I checked one of my books, the 79 w/auto was standard w/355's, 336 and 370 were the only optional gears.

_____________________________

Eric
84 C4 Z-51 auto
05 CBR1000RR Repsol Rep
00 TLR

(in reply to Lee Willis)

Post #: 4

RE: rear end ratio - 12/14/2005 7:19:36 AM

Lee Willis

Posts: 1952
Joined: 7/27/2005
Status: offline

Then, he has a 3.36.

60 mph = 88 ft/sec.

Thus 65 = 65/.60x88 =95.33 ft/sec
x60 = 5720 ft/minute

A 235/60x15 tire is 26.1 inchs in diameter (I got this buy going to Tirerack website and checking "specs" on tires of that size. The first five I checked in this size, Pirelli, BFG, Goodyear, Firestone and Bridgestone, were all this diamter).

26.1 inchs = 2.175 feet

which means circumference around the tire is 3.14159x2.175 ft ==6.833 ft.
So, at 65 mph, the tires and rear axles are turning at 5720ft/6.833ft=837.15 RPM

He says the tach reads 2800 RPM ==> 2800RPM/837.15RPM=>3.34

_____________________________

2007 Aston Martin Vantage Prodrive
Lincoln MKZ "Grandfather car"
''98 Z28 with iron LS 408, NOS, tricks, street driven mid 9s (sold to my youngest son for $1, Aug 28, ''07)
''02 SCd 427 cid ZO6, 704 RWHP, sold Aug. 28, ''07

(in reply to xrcrx)

Post #: 5

RE: rear end ratio - 12/16/2005 9:14:07 AM

xrcrx

Posts: 74
Joined: 12/7/2005
From: houston texas
Status: offline

Based on your math I'd say he's got the optional 3:36 rear.

_____________________________

Eric
84 C4 Z-51 auto
05 CBR1000RR Repsol Rep
00 TLR

(in reply to Lee Willis)

Post #: 6

RE: rear end ratio - 12/16/2005 4:46:43 PM

Lee Willis

Posts: 1952
Joined: 7/27/2005
Status: offline

You can't be sure, until you know if the tach and speedo are really right, but its hard to believe they are so far off that that isn't it.

_____________________________

2007 Aston Martin Vantage Prodrive
Lincoln MKZ "Grandfather car"
''98 Z28 with iron LS 408, NOS, tricks, street driven mid 9s (sold to my youngest son for $1, Aug 28, ''07)
''02 SCd 427 cid ZO6, 704 RWHP, sold Aug. 28, ''07

(in reply to xrcrx)

Post #: 7

RE: rear end ratio - 12/17/2005 8:54:19 AM

xrcrx

Posts: 74
Joined: 12/7/2005
From: houston texas
Status: offline

There's also the possibility the gearing has been changed sometime in the past. I'd still have to say it's better than even oddsthat it's a 3:36-good math by the way-not to mention tire circumference varies by brand and wear.

_____________________________

Eric
84 C4 Z-51 auto
05 CBR1000RR Repsol Rep
00 TLR

(in reply to Lee Willis)

Post #: 8