How does this compare to stock? 2.75:1 with 8" axle

[AgentDaleCooper 10]

Can someone explain how a 69 with '2.75:1 gears with 8" axle' compares to stock setup?

 

I know what it refers to in mechanical terms but don't really understand whether it's a good thing or not! Have just seen a car advertised with this and was slightly confused.

 

Many thanks!

Dan

[R4K 10]

Well, the only reason I can think they'd advertise that is to impress on either the top achievable speed, or fuel efficiency. Thems' highway gears. So if it's a "cruiser" rather than a musclecar/hotrod, it'd be a whole lot quieter and better on gas on the highway.

 

The only other upside to it that I can see is that you'd be able to achieve a higher max speed before your RPMs hit redline. These types of gears really aren't desireable for most folks that try to muscle up their cars, because those highway gears really eat into a quick launch at the local dragstrip.

[Spoofty 10]

Can someone explain how a 69 with '2.75:1 gears with 8" axle' compares to stock setup?

 

Dan

 

That sounds like a stock setup to me.

[ForceFed70 14]

While the 8" isn't bad, it's not very desirable. Neither is the 2.75:1 gear ratio.

[bnickel 10,004]

most 69/70's came with a 3.00:1 or higher rear gear ratio stock. very few came with the 2.79's and those were mostly 6 banger (69/70 250ci six cyinder cars had the 8" rearend stock) cars or 302-2v cars.

[ForceFed70 14]

Mine had the 2.75:1 ratio...it was a 9" tho.

[Protowrxs 111]

Mine had the 2.75:1 ratio...it was a 9" tho.

 

Same here.. 351W, 2bbl, 2.75:1 9" originally - would fly on the highway though.

 

-Stephen

[rwcstang 201]

yeah mine are 2:79's in my 8" soooooooo believe me im upgrading to 3:55 or 3:73's since my engine is going to give more horses..

[bnickel 10,004]

wow you guys had some oddballs then. most everything i've ever looked at, stock anyway, has generally had the 3.00 gears. that's what my car had originally, it now has a 3.25 from a 390 mach however.

[AgentDaleCooper 10]

Thanks for all the replies guys. The car is a 302 2v so sounds like it has its original rear setup still. Not planning to drag in it so would probably suit me fine.

 

Cheers,

Dan

[70vert 17]

and I just took a 200-mile round trip in it consisting of highway miles, 40+ miles of twisties, some non-twisty country roads. Combined mileage? Somewhere over 20mpg, figuring with the Mapquest directions and my fuel gauge, and that's a conservative estimate, could have been close to 25mpg maybe. Original carburetor, and with the top down half the time. This is with a TKO600 tranny, 5-speed, .64 final drive ratio.

[351carlo 10]

Wow, I didn't know any other ratio than the 3.00 existed for our cars *laugh*

 

I was a 3.00 ratio, 9 inch stock. Of course you know me... 4.11's with a trac lok now! Depending on what power this motor actually makes (Trap speed is important for me) I'll have 3.89's or the 4.11's with at least 31 splines and a spool. Call me dumb, but I want that all the time power without the cost of a locker. It'll be a nice lightened full spool, probably from moser.

 

2.75 with an 8'' is definitely not desirable. Unless you have a 5 or 6 speed with a nice low 1st gear to get you moving, a ratio like that is just a highway gear. Forget about performance.

[buening 63]

My F code 70 fastback (302 2V) came stock with a 2.79 8" rear. My mach came with 3.00 9". There was a plethora of rear gear combinations for our cars.

[bnickel 10,004]

oh yeah, there was definitely a bunch, thats' for sure, my point was that the 3.00:1 was THE most common ratio and i have never actually seen an early mustang equipped with the 2.79:1 ratio from the factory. around here almost everything had the 3.00:1 or the 3.25:1 ratios

[70vert 17]

I bet some of the wealth of knowledge on the VMF could benefit us if we're interested in figuring out how common different rears were.

 

Mine was bought in PA originally and came with the C-4 3-speed and is a convertible. My understanding is that 2.79 is the equivalent of an "overdrive" for the 3-speed. Still revved on the highway. I like my 5-speed with low 1st and high OD gearing. I'm still not sure I would go up to a 3.25, 3.55 rear when I get my stroker. I might stay 3.00 or (blasphemy) maybe even the 2.79. I would like to drag it at least once but would rather have road racing style gearing if I can ever get it on a real track. :yes: If anything, I'm looking for a "Trans-Am Boss Convertible" style of restomod. :thumbup1:

[ForceFed70 14]

There's nothing racing about a gear ratio in the 2's.

 

Well, unless you're racing at the boneville salt flats and have miles to wind up for a land speed record.

[Pakrat 1,043]

Well chalk up another one here. My car was originally a 302 2V (woken up nicely now though) with an 8", 2:79:1 gears and 3spd manual. Not everyone want's to own a muscle car and not everyone wants to get 8-10mpg especially these days. You should base your decision on how you intend to enjoy the car, not the amount of testosterone you need to portray. I intend to step up to something in the 3:50 range +/- when I go to a 5spd but I can tell you that for the 3spd these gears where a good choice and it probably could not have handled much more. It was like driving a semi automatic, I could cruise from 15 to 60 in 2nd gear and reach some pretty high speeds on the open highway with alot of RPM to spare. I don't think convertibles are ideal for off the line cars though anyway and I wouldn't have wanted to trade my cruisability for that considering the rest of the setup.

[70vert 17]

I should clarify the point I was trying to make. Certainly, there's nothing racing about a tired old 302-2v mated to a 2.79 rear, although the 5-speed TKO with the low 1st gear lets me do fine off the line. 2nd is great, 3rd and 4th not so exciting, 5th strictly highway cruising or straight backroad loping. But that 5th did get me over 20mpg.:yes:

 

When I upgrade to a 408w stroker sporting 500hp and tq, my thinking is that that much torque can get me off the line easily with a high rear end gear, so I can have both the power through all the gears and a little mileage to boot up top, with EFI. That stroker won't be a revver - 6500 redline and peaks usually around 5600, 5800, so I think it might actually work out.

 

So, does this theory hold? Can the massive torque of a stroker overcome high gearing?

 

There's nothing racing about a gear ratio in the 2's.

 

Well, unless you're racing at the boneville salt flats and have miles to wind up for a land speed record.

[Mufflerbearing 0]

I had a 69 fastback in high school with a 302 2brl with 2:79rear. I bought it from the original owner. My car went 15.8 in the quarter mile. Plus I always had more gas then my buddies who were running 4:10's Only draw backs is it's a one legger. You'll be king of the one tire fryers.:001_smile:

[70vert 17]

heh heh, good one - "king of the one tire fryers." No, I'd be replacing this one with a torque sensing diff, probably the Black Gold one, before the 408 goes in. But I am investigating ratios for that setup.

 

15.8? Nice! That's kind of surprising . . . 4 speed or 3-speed auto?

[bnickel 10,004]

I should clarify the point I was trying to make. Certainly, there's nothing racing about a tired old 302-2v mated to a 2.79 rear, although the 5-speed TKO with the low 1st gear lets me do fine off the line. 2nd is great, 3rd and 4th not so exciting, 5th strictly highway cruising or straight backroad loping. But that 5th did get me over 20mpg.:yes:

 

When I upgrade to a 408w stroker sporting 500hp and tq, my thinking is that that much torque can get me off the line easily with a high rear end gear, so I can have both the power through all the gears and a little mileage to boot up top, with EFI. That stroker won't be a revver - 6500 redline and peaks usually around 5600, 5800, so I think it might actually work out.

 

So, does this theory hold? Can the massive torque of a stroker overcome high gearing?

 

 

you really need to match the power band of the engine with the desired cruise rpm to get the best gear selection for your car. the 2.79's may lug the 408 stroker down into the very lowest part of the power band and running an engine at too low an RPM is just as bad as running it at too high of an RPM. true the torque off the line probably won't be an issue but if the engines best cruise rpm is 2800 and you lug it down to 2200 or so you'll be fouling plugs and other bad stuff like ignition knock etc. i'd suggest something in the range of 3.25-3.89 or so definitely no lower than the 3.25. that is really something you need to discuss with either your cam company or your engine builder or both, really

[Mufflerbearing 0]

Fresh C4 3 speed. I would go with the 3.89 ratio. This way your cruzzing is still there. With 4.10's your stuck at 50-55mph Max cruzzing speeds. Good luck

[rwcstang 201]

yeah i hate my rear end gears thought they are gr8 on the freeway i just tend not to go on them....also since im making my car to auto to manual conversion im going up the gears to 3:55-3:73s

 

not to hijack this thread, is there anyway to get an 8" posi diff? which few cars carried them?

[bnickel 10,004]

there was a traction-lok diff available in the 70's. mostly seen in v8 and v6 mustang II's, pinto's and mavericks and the mercury equivalents. when i had my 78 II i got a 3.50 trac lok from a 76 mercury bobcat wagon with the 2.8 v6.

[R4K 10]

Yeah, i'd never heard of such a thing...but I saw an ad in the paper here last week or so for an 8" Detroit Locker, for around $350 I think. I meant to ask here on the forums if anybody had heard of such a thing. I think I missed it though, as it wasn't in the classifieds this last weekend.

 

EDIT: I just noticed yesterday that Dark Horse Performance has a Locker conversion kit for both 8" and 9" for $225, if that might be of interest to you.