TPI, T5, 3.42 swapped

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79 Goon

Not-quite-so-new-guy
Aug 28, 2018
25
19
3
MN
Hello. I just bought a ‘79 Malibu wagon project. A previous owner swapped in a 305 TPI & T5 manual trans from a ‘91 Trans Am. They also swapped in an ‘87 GN rear axle with 3.42 posi. It also came with 16” Camaro wheels shod with 245/50R16 (25.6” dia) tires. The gearing feels really tall! I’m debating running a 255/60R15 (27” dia) tire, but it’s going to make the gearing even taller yet! Would it be THAT noticeable? I’m also debating going 3.73s or 4.10s. It’s a daily driver - mixed hwy/city - 35 miles/day. I’m fine with 17+ hwy mpg. It gets 20+ now (running rich). Any input would be appreciated. Thanks!
 
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TURNA

Rocket Powered Basset Hound
Jul 24, 2009
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Socialist NY
Welcome to the site. Sounds like a sweet ride

BUT...............................................



:worthless:
 
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Oct 14, 2008
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79 Goon

Not-quite-so-new-guy
Aug 28, 2018
25
19
3
MN
I don’t have any interior pics at the moment. I’ve taken out a lot of the interior.
The shifter is in a decent spot. The car also has bucket seats so it’s good. Since the motor & trans is from a ‘91 TA, it’s a World Class T5. I THINK it’s the taller .63 5th gear if I remember correctly. I’d have to research it again.
 

79 Goon

Not-quite-so-new-guy
Aug 28, 2018
25
19
3
MN
79
 

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Texas82GP

Just-a-worm
Apr 3, 2015
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Spring, Texas
Welcome to the site from Houston. Cool car! I'm surprised you find the gearing to be tall. Are you sure its a 3.42? I really like this rpm calculator....

http://www.csgnetwork.com/multirpmcalc.html

Use the bottom section so you can plug in your overdrive ratio.

According to it, a 25.6" tall tire with a 3.42 rear gear and a .63 overdrive should turn 1838 RPM at 65 and around 2404 RPM at 85. That doesn't seem that tall to me which is why I ask if you are sure it is a 3.42. I don't doubt its an 8.5 but who knows if the gearset has been changed. Maybe a 3.08?

Is the identification tag still on the trans? If you can identify the transmission for certain (thereby identifying the gear ratios) and confirm the rear axle ratio (jack up rear end and see how many turns of the driveshaft = one turn of the rear wheel - will be close enough to tell if it is 3.42 or something else) that will help in identifying what change to make to get the car more to your liking.
 
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pontiacgp

blank
Mar 31, 2006
29,270
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Kitchener, Ontario
If you have the stock shifter you shoudl upgrade to a brand that has stops. I have a Hurst shifter with stops so I can't overshift like you can with a stock floppy shifter. I drive my GP daily and I like the 3.08 rear end so I don't have to shift it every 30 feet in the city.
 
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79 Goon

Not-quite-so-new-guy
Aug 28, 2018
25
19
3
MN
It
If you have the stock shifter you shoudl upgrade to a brand that has stops. I have a Hurst shifter with stops so I can't overshift like you can with a stock floppy shifter. I drive my GP daily and I like the 3.08 rear end so I don't have to shift it every 30 feet in the city.

It has a Pro 5.0 shifter in it. It’s ok. It’s too notchy for my liking but it’s fine for now.
 
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79 Goon

Not-quite-so-new-guy
Aug 28, 2018
25
19
3
MN
Welcome to the site from Houston. Cool car! I'm surprised you find the gearing to be tall. Are you sure its a 3.42? I really like this rpm calculator....

http://www.csgnetwork.com/multirpmcalc.html

Use the bottom section so you can plug in your overdrive ratio.

According to it, a 25.6" tall tire with a 3.42 rear gear and a .63 overdrive should turn 1838 RPM at 65 and around 2404 RPM at 85. That doesn't seem that tall to me which is why I ask if you are sure it is a 3.42. I don't doubt its an 8.5 but who knows if the gearset has been changed. Maybe a 3.08?

Is the identification tag still on the trans? If you can identify the transmission for certain (thereby identifying the gear ratios) and confirm the rear axle ratio (jack up rear end and see how many turns of the driveshaft = one turn of the rear wheel - will be close enough to tell if it is 3.42 or something else) that will help in identifying what change to make to get the car more to your liking.

I haven’t done all that yet. I’m taking the previous owner’s word for it. I’m pretty sure it’s just the T5 that makes it feel tall to me. It doesn’t have a tachometer either unfortunately, but it’s in my plans.
 
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