I have 3.75 gears...is this too much???

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65shlbycln

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Aug 24, 2002

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#1

• Jan 5, 2004

• #1

I took my fastback to the shop, and they got it running. Turns out it was the wrong starter and it was chewing up the teeth in the flywheel or something.

But anyways, he also took the rear end apart, cause i told him that there was some loud whining coming from the rear end. He said it needs to be rebuilt, and also said that i have 3.75 gears.

Now i had a feeling that they were higher than normal cause under full acceleration i spun like crazy through 3rd gear with my 4spd.

I have a 4spd close ratio toploader. With the 3.75 gears, is this car gonna top out way too soon? Its going to be a daily driverish type car. It has an open diff.

My 289 is stock, with just intake and exhaust components changed out.

I havnt driven it yet since he got it running, since its still at the shop. ANd i never previously drove it above 40 or 50 mph, and that was without a speedo or tach. What would you do?

 

bud4660

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Apr 14, 2002

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Jan 5, 2004

#2

• Jan 5, 2004

• #2

I put 3:73's in the 98GT I just sold. Everybody was like put 4:10's in it. It was an automatic so I felt 3:73's were enought. And they were. I have 4:10's in my 96 F250 Crewcab truck. Its a 460 with a 5 speed and other then then it gets 10mpg its ok on the highway. The 67GT I have now has 4:56 gears in it now. I also have a set of 5:14 and 6:20's from another toy I raced. So you should be fine with the gears. My .02 anyway.

 

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65shlbycln

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#3

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Bud4660,
Did the 98GT you had with the 3:73's, come out to a similar final drive ratio as my toploader with 3.75? I think its called the final drive ratio...

Was it ok on the freeway?

 

6

68DARKHORSE

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Oct 9, 2003

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Jan 5, 2004

#4

• Jan 5, 2004

• #4

65shlbycln, the '98 has overdrive. It's final drive would not be the same.
Your car sould run 62mph at 3000rpm. 72mph at 3500rpm. This is with a 26" tire.
Here are some nice calculators: http://www.speedworldmotorplex.com/calc.htm

Do you have a larger pic of your avatar?<SCRIPT type=text/javascript> vbmenu_register("postmenu_3684709", true); </SCRIPT>

 

12sec67

Active Member

Oct 6, 2003

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San Diego, Ca

Jan 5, 2004

#5

• Jan 5, 2004

• #5

final drive gear is 1:1 - everyday driver with a four speed top loader at a good cruise speed approx 70mph you should stick to 3.55.

 

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65shlbycln

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Aug 24, 2002

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#6

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68DARKHORSE said:

Do you have a larger pic of your avatar?<SCRIPT type=text/javascript> vbmenu_register("postmenu_3684709", true); </SCRIPT>

Click to expand...

yeah.

 

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68DARKHORSE

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Jan 5, 2004

#7

• Jan 5, 2004

• #7

Thanx for the pic.

As mentioned above, 3.55s are good for a daily driver or car that sees alot of highway time. I ran 3.89s with a C4 auto (also 1:1 top gear) before a very bad friend of mine talked me into installing a 5 speed.

 

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65shlbycln

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#8

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thanks for the info guys

 

pabear89

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High in the Hills of So Ca with the Voices in My H

Jan 5, 2004

#9

• Jan 5, 2004

• #9

I'm running a 3.50 posi unit with close toploader,
Great around town gear and not to bad on the highway.
Only feels like it could go faster with a overdrive gear.

She Tach's up Quickly.

 

G

gp001

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Jun 30, 2001

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Jan 5, 2004

#10

• Jan 5, 2004

• #10

68DARKHORSE said:

Thanx for the pic.

As mentioned above, 3.55s are good for a daily driver or car that sees alot of highway time. I ran 3.89s with a C4 auto (also 1:1 top gear) before a very bad friend of mine talked me into installing a 5 speed.

Click to expand...

My bad friend talked (well, taunted) me into installing a 6 speed

Shlbycln - I would go with 3.55 range if I were you. You'd be much happier

 

bud4660

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Jan 6, 2004

#11

• Jan 6, 2004

• #11

65shlbycln said:

Bud4660,
Did the 98GT you had with the 3:73's, come out to a similar final drive ratio as my toploader with 3.75? I think its called the final drive ratio...

Was it ok on the freeway?

Click to expand...

68DARKHORSE is correct on the overdrive. My old 67GT 428 toploader in the 70's I had I ran 3:89 gears in if I remember right. It depends on what you want for top speed. I dont drive over 70mph much on the roads. Now on the drag strip is another story....

 

allcarfan

The Answer Man

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Apr 8, 2001

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Jan 6, 2004

#12

• Jan 6, 2004

• #12

Clone:

Do you have an 8 or 9" rearend? I dont think I have ever seen a 3.75 gear ratio offered. That might just be me though.

You need to check out our site too. Its GA based. We get deals on all kinds of parts and are working on some sponsorships. For details just message me or go to street-machines.org.

 

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65shlbycln

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Jan 6, 2004

#13

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• #13

I have a 9" rear.

See, here's the thing. I didnt choose the 3.75 gears. I converted my 6cyl fastback to an 8 cyl one by finding a donor car, and changed out the suspension, tranny, rear end, etc from it. But the donor car was a drag car. Hence the 3.75 gears that im just now finding out about. I dont really have any money right now either so im just gonna have to stick with the 3.75s too. I just hope that i can cruise on the freeway relatively easily without being revved way too high and sucking insane amounts of fuel.

As far as the car 3.75 gears not being offered...i wouldnt be surprised if the donor car's rear end hasnt been used since the early 90's/ late 80's. At least it looked like that. The body was horrible and was all rusted out. We were just fortunate to take the toploader/rearend/spindles/driveshaft etc for $400. Only now is the rear and tranny having to be rebuilt. So as far as 3.75 gears not being offered now, i dont know. maybe they had them 10-20 years ago?

 

BullittStangV8

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Oct 5, 2002

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Jan 21, 2004

#14

• Jan 21, 2004

• #14

what rpm are 3.55's in a top loader 4 speed at 60 and 70 mph? im looking into getting some gears, seems that it really will make a difference in the acceleration department.

 

J

jesserose17

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Aug 21, 2003

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Jan 21, 2004

#15

• Jan 21, 2004

• #15

BullittStangV8 said:

what rpm are 3.55's in a top loader 4 speed at 60 and 70 mph? im looking into getting some gears, seems that it really will make a difference in the acceleration department.

Click to expand...

Assuming 26" diameter tire (235/60R15), it will be 2750 rpm at 60 and 3250 at 70 mph with 3.55 & top loader. It will have good take-off punch.

 

BullittStangV8

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Jan 21, 2004

#16

• Jan 21, 2004

• #16

how do you correct the speedometer after the gear change? i know rim/tire combo is a factor as well. thanks!
Mike

 

Edbert

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Jul 13, 2002

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Jan 21, 2004

#17

• Jan 21, 2004

• #17

http://www.edbert.net/rpm_chart.htm

For a C4/C6/toploader use the bottom chart and ignore where it says "3rd gear". That only means 1:1 drive ratio. The top chart shows what an AOD gives you.