Rear end gearing

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Rear end gearing

Post by yagermichael » Thu Dec 22, 2011 2:05 pm

I have a 1971 El Camino and am looking for some advice for rear end gearing. I drive this pretty much all year with some highway miles it has a 350 ci short block, less than 2,000 miles since complete rebuild. Bored .030 over w/ flat-top pistons, Vortec heads, Edelbrock Vortec intake manifold, Hedman headers, TH350 3-speed transmission rebuilt with shift-kit & 2600 stall converter. 10 Bolt NON-POSI original rear end/gears. FlowMaster 2 1/2" exhaust system, 15 x 8 rear rally wheels 15 x 7 front rally wheels. 275R6015 rears, 235R6015 fronts. I am thinking that 373 gears will be a bit much? Thanks for any advice.

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Re: Rear end gearing

Post by 67drake » Fri Dec 23, 2011 10:19 am

I put 3.90's in my 71' GTO this year. Kind of "buzzing" above 55 MPH. More of a street/strip type car so I knew this going in,and am fine with it. BUT, I can't imagine driving this thing above 60 MPH,and only for a short time period.
You do at least have a relatively tall tire. If you do highway driving you might want to consider 3.42's though.

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Re: Rear end gearing

Post by monzaz » Tue Dec 27, 2011 10:29 pm

Sounds good so far...

Gear ratios are one of those things that each person chooses to enhance what they want their own car to do for them...

IE- 3.90 may be good for the drake but may be totally worthless to you if you want to travel with the car....

Picking the ratio that makes the car it's absolute quickest for all the parts you have in the engine and trans MAY not be always the way to go. You may need to compromise to maker the car do what you really need it to do.

SO the million dollar question- WHAT do you want from your car? What do you do with this car? or what are you GOING to do with it?
burn outs light to light action hang around town like drake or are you going to do hot rod touring across the USA ??? Really think it over.

What I see right now is - lack of a starting ratio- WE need that ratio that is in your 10 bolt rear first then we can go from there because right now the ratio could be 2.56 2.73 3.08 or 3.36 these are the MOST COMMON gears they put into the A-body cars.

You have a 3 speed auto tranny - so you have a good low first gear 2.52 - This will multiple the ratio 3.08 making the rear feel like 6-7 to one on the launch. But in the end you will need to drive the 3.08 ratio once in third gears also know as 1 to 1 when your in 3rd you are driving 3.08 to 1 ratio.

Next thing you have a 2600 stall meaning the convertor will not hit full efficiency till 2400 ish this is where the car RPM should be keep for best mileage and least amount of slippage in the tranny.

Your tires are about 28" tall so as stated in the post before you have plenty of tire to support more steep gears like 3.55 or 3.73.

IF your rear has 2.73 in it and you think it is doggie then I would bump it up to 3.36 area
If you have 3.08 ratio maybe bump up to 3.55
if you have 3.36 and you think it feel doggie then 3.73 might help you out.

REMEMBER - there is NO substitute for a well tuned carb and ignition on the engine to give you snappy torque and gas mileage and power. This is why electronic fuel injection works Sooooo
well. :) Jim

Please keep the questions coming if you need more help.

The extra help by other members is ALWAYS appreciated, Thanks to you all-
J D Race

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Re: Rear end gearing

Post by uncleg » Thu Jan 05, 2012 11:19 am

What camshaft are you using?
If your cam is too big or to small, you can you can completely negate any change in gear ratio
If you use a cam that does'nt start making any power until 3000 RPM and your gear ratio is 3.08,
with that tall tire you will be cruising at 55 and the RPM's will never come close to 3000.
You definitely would not be happy, and you would be throwing money out the window.
Out of the hole it would be a dog.
But if you plan on driving 90+ miles per hour you might be happy.
Give some specs on the cam, or a brand and part number.
Matching motor trans and gear are very important, as Jim asked earlier what are you using this car for?

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