2.73 to 3.42 swap, Fuel milage? Please help

I currently have 2.73 gears in my 1971 nova, very light car, about 2700lbs. it has a 327 and a glide, the 327 is set up for more midrange and top end torque... while its not too bad off the line, i figure swapping to 3.42s would put me in the power band quicker and help out alot. However, i dont want to get terrible gas milage. Right now i can get about 19-20 if i keep my foot out of the carb, im wondering how this gear swap would affect my milage if any? My tires are 26.5in tall. with the 2.73s im running about 2100 at 63mph.. now if i go to 3.42s, itll jump to about 2600rpm. At that rpm the engine will be making more torque to carry the car, so will it be about the same, or how much less fuel milage? I do alot of highway driving, its all highway and back roads around here. I appreciate any input that can be given as always, thanks in advance. -Dan


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just to give you some imput. any time you change gears it will affect your mileage especially when you go to a lower gear ratio. you will definetly like the differance on take off but at highway speed it will affect you even if you keep your foot out of it.

i expect a slight drop, im just hoping nothing drastic. if it dropped 2.5 or so i would be okay with that, I appreciate your input! maybe someone else has a similar setup? a nova or first gen camaro with 3.42s and a 1:1 final drive in the trans?

You owuld be better off installing a T350 or 700r4 trans. You'll gain quicker acceleration and the 700r4 would increase your mpg.The T350 might allow you to gain some mpg if you can go easy on the pedal, but since it still has the 1:1 final drive, you wont gain as much as if you installed the OD trans.

Around town, my car will get the same mileage. I went from 2.80 to 3.5's on a 9 inch ford. The difficult part is keeping your foot out of the pedal. On the highway, since I'm not running a transmission with overdrive, I'm taching at about 2800 rpm going 60 mph. You can watch the gas gauge move with the 3.5's in the rear. Would I change back to 2.80? Probably not!

if you want mpg with better off the line power, then keep the gears and get a th350 trans.

That old story about having your cake and eating it too.

A three speed plus OD along with the gears will allow you to eat the cake. Of course the pay back period for the costs of installation will pay for lots of gas

a cheap way to see what what it will be like is,
put some smaller tires on the rear
what size rear tires are you running now?

With current rear gears a OD transmission is a bit excessive and can lead to other problems. A swap to a good rebuilt or lightly used TH350 can probably be done as cheaply as a gear swap and will give you better acceleration and mileage.

2.73 to 3.42 = 25% increase in power to the rear wheels. i have a nice little chart here that lists all the possible combinations so if anyone has any proposals i can answer them.


As far as mileage i guess the only way to find out is to try it. i don't think in all cases mileage may suffer and in some cases it might improve because it may put the engine in a more favorable power range during crusing, but probably not in this case.

i feel riding a 10-speed bike ALOT can really be educational in terms of gear selections/gearing.

Like Matt says. With your 26.5" tires @ 63 mph you should be at 2181 rpm with 2.73's and 2732 with 3.42's. Find or borrow some 21" tall tires, that would put you at 2752 @ 63. Drive it the way you would normally and check your mileage. Pretty much way back yard, but...

__________________
''Life's tough, Pilgrim, and it's even tougher if you're stupid.''---John Wayne

Midnight Sun Street Rod Association

Your mileage will not be the same as with 26.5" tires and 3.42 gears, your car will be about 3 inches lower and your tires weight will be much less, not to mention the tread width and pattern would probably be different. Even at the same engine and vehicle speed those factors can alter mileage a LOT.

The difference between 2.73 to 3.50 ratio is a big change.

Your question of mileage has 2 situations.

If you are in stop and go traffic the mileage change will be very minimal.

Highway or driving above 45 mph the mileage loss will be great. Kiss your 19-20 goodbye.

The best change you could make to your car is the transmission. With your current gear of 2.73 a TH350 would be a great improvement.

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Myself being a multiple Nova owner I can attest that we are limited to tire sizes. As with most other Chevy Makes allow tire size changes, Nova has very limited space for larger tires. 26inch tire is a short tire, but normal for Chevy II/Nova.

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The best and I have done several vehicles with this change.
A 3.23 to 3.55 gear with a 700R4 is the cake and eat it too.

I just completed a 63 Convertible with 3.08 - 700R4 with a GM 350/290hp crate Holley 600cfm and 23mpg is a easy task and the car is a tire burner. 1750 rpm at 65mpg is a easy task. Very impressive cruiser.


IMHO
AL

Could you send me a copy /link of the ""Possible combination chart" you spoke of to "Chevyman223"--I have a 67 2 door skylark with a '70 Buick 350-4bbl Turbo 350 trans--And I am leaning towards a 8.5 posi unit with 3.23/3.42
3.55 geared rear end. I currently has a "whimpy" 2.56 one tire fire. Thanks JIM

Im okay with losing some milage, just not a huge amount. do you think 16 would be attainable? thats a 3-4 mpg loss. i dont want to change trans, thats just not an option right now, one thing at a time. Someday id love to have an overdrive, but cant yet. so please answer as to how much of a drop and if you think 16 is about right? or even less? does anyone know where i could get some 3.23's Instead? or would i be better off with a set of 3.08s if i cant get 3.23's for what im looking for? I appreciae everyones input and as always thank you! Dan. p.s. Jim, it seems as though his "chart" may work something like this, if your going from a 2.56 to a 3.23, subtract 2.56 from 3.23, you get .67, which is about 26 percent greater than your original gear ratio. I dont know for sure if thats how his chart works though, just seems that way to me, hope it helps, im sure hell message you with it though