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 3.42 or 3.73?

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Posts : 3087
Join date : 2013-02-20
Age : 49
Location : Marion,NC

PostSubject: Re: 3.42 or 3.73?   Thu Jan 19, 2017 10:23 am

If gas mileage isnt an issue,why not go for more smiles per gallon and run the 3.73's? Cool
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Posts : 9516
Join date : 2010-06-23
Age : 44
Location : Round Rock Texas

PostSubject: Re: 3.42 or 3.73?   Thu Jan 19, 2017 1:58 pm

Just my 2-cents here folks. That T-top '88 Cutlass that I yanked the 307 from and swapped in the Olds 350 had a 2.55 rear axle from the factory. Crazy right? Well I found a 3.73 from a Monte Carlo SS and put it in the Cutlass....and I don't like it. It's fine around town, but even with the overdrive transmission, the engine is working WAY TOO HARD at Texas highway speeds. Keeping in mind we have a legal 85mph speed limit here now, the Cutlass feels like I'm hurting it pushing it at that speed...and you can watch the fuel gauge drop. At 55-60, it's ok, but even at 65 it's runner faster than I like.

Now that said, I swapped a 3.23 rear into my '82 Camaro Z28 when I swapped in the Chevy 350 and 700R4, and I think it's about the perfect gearing for good power off the line while still being able to run down the highway too.

So bottom line, think about how your car will be used.

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Posts : 919
Join date : 2009-01-05
Age : 66
Location : Manotick, Ontario

PostSubject: Re: 3.42 or 3.73?   Thu Jan 19, 2017 2:14 pm

I agree with Mike. Its all about how the car is used.

I swapped out the 2:93 in the Caddy for the 3:42 for towing purposes and lost about 2mpg on the highway but did gain some around town. If your not towing, the 3:23 is the best compromise for both. My RMS gasser (L03) would top 18mpg(big imperial gallons) in town and close to 28mpg on the highway.

In reality, you won't feel much difference between the 3:42 and the 3:73.

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Join date : 2011-01-24
Age : 34
Location : CFB Bagotville, QC.

PostSubject: Re: 3.42 or 3.73?   Thu Jan 19, 2017 4:27 pm

IMO, go with the gears that will optimize your cam (doesn't say what you have).  If you bought your engine from Karl, seek his advice.  With the description you gave, if you're driving at 45 most of the time and have a 3000 stall TC, I'd go with the 3.73.  That being said, I find your tire diameter way small.  If you find the 3.73 is a bit much then go up to a 27 or 28 tire.  You can't really do the reverse (i.e. drop tire size if you find the 3.42 not to be enough gear)

Obviously retune.  With either gear you should be locking up in 4th well below 67.  With the 3.73 you should be in 4th with lockup around 40.  Make sure you optimize DFCO (some people don't like the feel though).

I'm in a similar situation.  55 is the max speed limit in my area.  Rarely drive 65.  Spend most of my time in town, 40-45 range.  3.73s all the way.  That being said, if I knew I had to drive on the highway, I'd do 3.42.  I had 3.42s on the DCM and PPM and loved them.

If you're driving a stroker with a 3000 rpm stall, I really don't think the difference in mpg between the two should be your deciding factor, given your commute characteristics.

also, yes, definitely get a truetrac.
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Fred Kiehl


Posts : 4731
Join date : 2009-11-13
Age : 69
Location : Largo, FL 33774

PostSubject: Re: 3.42 or 3.73?   Thu Jan 19, 2017 9:43 pm

I just bought a TrueTrac 913A481 for my 454 wagon transplant, and there is a $50 mail in rebate on until March 31. You have to access the rebate form through JEGS. The price just went up, but JEGS will price match. I saw someone selling them for $425 on Ebay. Buy it through JEGS or directly, not on ebay, or the rebate is no good (http://www.jegs.com/rebates/eaton/rebate.pdf). I do not know if Summit will price match, but I think they will.

I have 3.23s, and may stay with them, because I like to hear myself think when I am on the highway. The 3.23s are 200MPH gears (I've been informed that it is 208 at 6K RPM with 27 inch tires), and I will probably never see that, in fact the car probably will not go that fast, but at 75, the engine is not thrashing itself to death. I am running 255/50/17s on the rear (Impala size). That is 27 inch tires. My cam, intake and converter all come on at 2500RPM, so everything is matched. Exhausts are shorty headers with Flowmaster 40s.
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Join date : 2017-01-18
Age : 36
Location : Mount auburn, il. 62547

PostSubject: Re: 3.42 or 3.73?   Fri Jan 20, 2017 12:59 am

Most 3.42s aren't compatible with your abs, you need to make sure to find a set that is machined to allow the reluctor gear (I think that's what it's called) on the pinion for the abs to read. It's easy to find 3.73 gears that are made for it.
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Fred Kiehl


Posts : 4731
Join date : 2009-11-13
Age : 69
Location : Largo, FL 33774

PostSubject: Re: 3.42 or 3.73?   Fri Jan 20, 2017 8:02 am

You have to get the correct reluctor for the gearset as well.
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phantom 309


Posts : 5758
Join date : 2008-12-28
Age : 108

PostSubject: Re: 3.42 or 3.73?   Fri Jan 20, 2017 11:36 pm

i don't think it matters what the wagon people think,. you will put in the gear set that you think is right anyway,.

with what you have described with your driving trips,. with the big motor and the 3g stall,.i'd go 4:10's car would seem very light,.

i ran a basically stock motor with 4:88's and auto for a while,. car still topped out at 120+ mph,.car was a blast to drive,.

3:73's work good on the street,.and highway,. we cruise pretty quick around here,. nothing wrong with buzzing a sbc or a sbo,..back in the 70's there were no overdrives,. everything buzzed 3000 or better on the highway,.big or small blocks,. but the newer iron has everybody used to low low rpm,.

3;73's,.. program your lock up as soon as you can,.40mph,.with a 3g conv you'll be heating up the trans if it doesn't lock till 67??
even a stock motor can pull easily and never unlock the conv,.and don't unlock till more than 60% tps,.
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