On my fury '74 project was on the list a rear gear change and a swap from TF727 to A518 with lockup just to have a better "take off" response and a bit better highway and town fuel economy. But honestly, I don't have idea which rear gear ratio should I choose and why.

I've read some forums and some people agree that a gear ratio like 4.10:1 will help town fuel economy, will boost the car "take off" and the highway fuel economy will be the same only if I the car has an overdrive transmission. But I've read too that this kind of gears will have worse fuel economy in all cases, just will provide a better take off.


In most of the forums thread that I've read, the ratio 3.53:1 seems to be the more reasonable option but I want to have more opinions

 

j -

There's no way in hell a 4.10 is going to help fuel economy anywhere, don't care who tells you otherwise. Use one of the on-line rpm calculators and you'll see why.

3.23 or 3.55 is really about as high as most people should go, unless the car is considered "special purpose", it mostly a strip car.

I'm running 3.23s and was considering 3.91s for a while. After looking at the calculators, it really showed that realistic highway speeds would be impractical.
I may still go to 3.55s, but the jury is still out.

Archer

 

The argument about why a high gear like 4.10s will help the town and hill mpg was that the driver will have to use less throttle to get the car moving, sounds a bit rational but 4.10s stills being a high performance gear.

According to this calculator, with my tires (235/70R15), 3.55 gear and A518 overdrive ratio (0.69) I'll be cruising at 60mph @1800 ~ 1850 RPM and on the town at 45mph at 1350 RPM

The car will be driven most of the time on the city.

What do you think? Is it a good set up or i will just waste money doing that change and I'll being better with my stock rear?

 

My 2 cent's...... You have to choose one side of the street or the other....
The only way to achieve what your trying to accomplish is to have all the paper numbers and do quit a bit of R&D which most of us don't have or funding for...
Fuel Misers and Performance just don't fit in the same sentence.
If your goal is 18 MPG in Town and 23 Highway. Strive for that point...

here you go
318... EFI... with mile or more camshaft.. Engine V.E. Range Idle to 4000RPM
727 T.F. with low stall converter
Gear Vendor Over Drive unit....
245/75/17" tire
3.23 Rear gear ratio.....
Note : 4000lbs is my guess....
Great cruiser but not much in the way of performance....
Like I said just my 2 cents....

Keep us posted....

 

j -

Didn't know about the over drive, and that changes things. In that case, yes you could get away with 4.10s on the highway.

Even with the over drive, which isn't enployed in typical city driving, you'll still be loosing gas mileage, since the engine is still running higher rpms in the lower gears. Fuel consumption is primarily based on rpm, not load.

Could you modify your driving habits to make it work? Anything is possible, but why bother?

I'd guess you would get good highway mileage (due the lower rpm) and loose it in city driving (since the lower gears would be the same).
In my case, without the over drive, I'd be loosing mileage in either scenario. That's why the idea about 3.91s were ditched.

Archer

 

My take is... You have to build the combo to work their. For that. Everything else Starts moving out of the triangle with change's....

 

As for the 3.91 gear... I loved them in my 67 Coronet 440 W/440 A.T.
Got out of the gate well... Just didnt like the fuel mileage on the freeway, Never any problem after that.... It would just Siiiiiiing along and purrrr like a kitten.... But always had a date with a fueling station....

 

18 MPG in town sounds great, but honestly I can be happy with 12 ~ 13 in town and 15 ~ 16 at highway. I prefer to have a bit more performance than such great MPG. I Just want to have an "efficient" drivetrain.

Forget the gear vendors OD, for the price of that unit I can buy an A518 with lockup and I think will be a better and reliable option. I don't like EFI systems and the price is so high that will be very difficult to have the investment back in fuel saving costs.

So... will be a better option a 3.23 gear ratio than 3.55? take in mind that I live on a hilly city

 

My Choice..... 3.55 For all around
3.91 For a little better Performance.
3.91... I believe will pull the hill better with less effort... The lockup will help on the other side of all this...

 

Seems to be that 3.55s will be the way to go

I have another question related to rear drive train... When is recommendable to have a positraction differential?

 

Any time you run into traction related problems.....

 

If you have a small block go 3.55
If you have a big block go with a 3.21

 

Our 5.2 truck had a 4 speed auto overdrive, 3.91 and felt right.

Changed to 318 carb 904 3 speed 1 to 1 final, 3.91 and felt like it needed another gear.

Changed to a 3.55 same engine trans and felt right again.

Recently put in a 4 speed manual trans with overdrive, with a heavy load up hill in 3rd is good, normal cruising in 4th O.D. is great and gets respectable fuel mileage.

All with 32 inch tires.

 

Thanks for all the help I'll try with the 3.55s, I'll be posting once I the chance is done

I just have one last question, Can be a good idea to buy this kit?

 

Don't forget a good trans cooler !!!!

 

i found this external trans filter check it out.
use ONLY steel brake lines. copper WILL break off. the factory uses steel for a reason!!!! mopar most use 3/8 dia tube, but some might be 5/16. i think most have the fluid in at the rear left side. here is an EXTERANL filter, i found. have one on my 01 dak. my idea is when you put a new filter in side this one will help from having to do the inside one as often.*
the old auto zone 2210 magenfine filter is no longer available at auto zone. BUT i did find the same thing as JX-160 brand-- ATP. at a real parts house uses 3/8" tube. a JX-150 for 5/16 tube. ONLY use high pressure hose. If you must. SAE J1019 217 psi working. NOT fuel hose for a carb engine.

 

If you drive on ice or snow NO suregrip

 

318 with 518 and 391_90 gear is a great combo
in my 86 5th ave
performance and economy.
With a theorical top speed of 160mph
best of both worlds for old school car.