Technical Piss-poor mileage from a 700r4 ??

Hey guys,

looking for some input: I recently switched from a TH350 to a 700R4 in my 53 chevy before Lonestar Roundup. I was hoping for big things on the mileage, but am actually disgusted now. I live in the sticks, and it is about an hour drive to any real city, that's why I decided to upgrade to a 700R4.

My buddy rebuilt the trans. I have faith in his abilities, and the parts he used. I have a 305 that has about 50K miles on it, EDL carb 1405 (600cfm), and 3.08 rear gears. I was consistenly getting around 14 MPG out of the TH350, and on the way to and from Austin, the 700R4 was averaging around 11-12. Why the hell is this happening? I was really hoping to bump my MPG up to around 18 or so.

One thing I did notice was a tremendous amount of down shifts, and then kicking back into OD. To the point that even unlevel surface of a bridge or slight incline would cause it to shift repeatedly. I don't have a tach on it, so I cant really tell what its turning.

I have done some searching on here, and keep reading people mention RPM and powerband. Seems a lot of people seem to start their rear gearing at 3.55 ish with 700's. Will this keep my trans from the constant shifting back and forth, and improve MPG ?

Thanks in advance.

 

That 700R is not going to work well with those 3:08 gears.

 

gearing is too stiff and the motor is out of the torque band for the cam ,for optimum milage you want the engine to run near to the torque band as possible if not in it ( torque is the measure of efficency ) , a 3:08 gear with the .78 od is a 2.40:1 ratio and you are probably spinning 500 or more rpm below the cam for a O/d trans to work right with a GAs engine the rule is about 3:55 is the lowest you want to go . ( diesels can pull lower as there powerband is basically off idle ) most of the cars that used the 700r4 had 3:73 and 4:10 gears , some had 3:55 ( squad cars) and they didn't see much of a improvement in milage due to the ammount of weight they carried , ISp did experiment and found 3:73's worked better with the heavier cars at cruise

 

The 3.08 gears are wrong for the overdrive in a 700 R4. The gearing puts the RPM so low at cruise speeds that the engine is lugging way below it's power band. A stock 305 would be happy cruising on the hiway at 2200-=2800 RPMs at about 65-70 MPH. Anything lower than that is working the engine at an RPM that the engine isn't efficient.

 

So will switching to a 3:55 put me more where I want to be, and improve fuel mileage? Do I need a cam swap as well over stock?

 

I would go to a 3.73.

 

all the good stuff has been said

 

Running a 3.08 with a 700R4 is the same as running a 2.16 with a TH350, when each are in top gear, only the 700R4 is even worse, when the torque converter locks, and torque multiplication is gone.

Change your rear gears.

 

If you use a set of 3.55s, it may get you right back to where you were with the 3.08s and a 350 turbo. Some simple math will tell you. If so, maybe some tuning and jetting changes would net you better gas mileage with less expense and work.
Still have the 350 turbo?
Will the 700R4 lock up the converter in 3rd gear? If it will, maybe cruising the hiways in 3rd gear will get you in the 2200-2800 rpm range
If you're going for mileage, a performance cam will not help unless you build some compression in it and then you'll need fuel with more octane...premuim gas and that's expensive enough to cancel what you save in gas mileage.
Put a super tune on that dog! My old malibu wagon with low compression 350/350 turbo and a Q-jet got 19 MPG on the hiway....was in tune and ran headers, 3.08 gears and 28 inch tall tires......3500 lbs.

 

So it has been written

Thanks guys, couldn't figure out why I was going backwards on MPG and power. Will look into this upgrade as soon as I can scrape together the change!

 

The hunting between 3rd and 4th is indicative of not enough torque to pull the rearend gear, as others said. The 3.08 is too high of gear with your engine and car weight. So the logical fix is 3.55 or 3.73 gears. Alternatively if you had an engine with more torque it could pull the 3.08 gear without having to kick down to 3rd. BTW, 3rd is 1.00 ratio, same as the 3rd (high) gear in your old T350. So if you leave your 700R4 in 3rd, it should get similar mileage as the old T350 trans.

 

I HAVE 373 IN MY STUDEBAKER WITH THE 700R AND LOVE IT. I HAVE BEEN GETTING IN THE HIGH 20 MPG RANGE FOR THE PAST 10 YEARS.

 

I run a 305, a 700R4, 4.10's (in a WAY lighter car), with a 30" tire, and can get 28MPG on the freeway.

Tuning is everything.

Wideband O2, Edelbrock 500.

 

What rear end gear suedestude?

 

Holy crap Gimpy!

 

It's a 2450lb. car.

 

<------That thar 55 Chevy wagon has a 305 with RV cam and Edelbrock 600. Has Camaro 3.08 gears thru a 200R4 and gets around 20-22 MPG. Been in there for years. It never up or downshifts on the road unless on a steep hill.
Well Florida doesn't have many steep hills. Lock up works properly.Torque converter is a red maroon if that helps.
Only reason I swapped was the guy I sold the wagon to had a sick ill-adjusted powerglide put in it and I lucked out on the rebuilt 200R4 at a swap meet. I'd never use a 700R4 after all the stories I read.

 

Your gears are too tall drop to a 3.73 gear or deeper and you will see a big difference.

 

All the Chevy's around here seemed to use either 3.43's or 3.73's with the overdrive trans.
3.43's being the most common to find.

 

BTW, to the original poster, what is your rear tire size?

 

Firestone 6.70s, so I think that is a 28.5 ish tall tire

 

A 3:73 gear is the way to go. That gear will not stop the trans from hunting for a lower gear completely if you are running the back roads with twists, turns , ups and downs but it will help. Best thing to do is put it in 3rd gear in those conditions. When you get out on the flat road and can run a consistent speed use the over drive. That is the way I run the the 700r4 in my 6000 pound suburbans. I got as much as 17 mpg on a long high way run probably a low of 13-14 on the slow back roads. If I am towing I will get about 12 and keep it in 3rd gear. Never tow in overdrive. I am looking for 18 mpg plus in my coupe with the 350/700r4 combo in my coupe If and when I ever get it on the road.

 

You bunch of pussys. I can't believe nobody told him to try cruising at 95 to get those gears up in the proper rpm range. Yeesh!
(try it, you may be surprised)

 

Are you sure the convertor is locking up, or are you not sure? If you are not sure, get someone to drive it, that knows what it should feel like as it locks.


You need to set these transmissions up correctly, especially the cable adjustment.

Also, there are different ways to do the lock up; by electrics, or by non-electric mods to the lock up solenoid.


You can't just do a simple "swap in" like a TH 350/or 400.

 

It's hard to do that here in the boonies with cops trying to make quotas everyday. About 80 is all I can get by with, especially in the city park.
What amazes me is we're talkin modern overdrives on a traditional thread. Traditionally this is the way it is now.

 

Gears and tuning, like everybody said.
I have a similar setup in my `63 C-10 pickup. 305 roller cam, Eddy 500, but it's a T5 stick and the rear is the OG 3.73. No MPG data yet without an operational speedo, but it seems decent.
Do you know what cam you have----stock?

 

^^^ as everyone has indicated 3.5:1 or higher for OD transmission
For best results, your engine RPM whilst cruising needs to be between 1800rpm to 2100rpm.
Make sure you lockup function is working correctly and that the TV cable is adjusted otherwise you will turn your 700R into toast in a very short time frame.

 

I ran 3.73 gears/700r4 on my last coupe and enjoyed it! Going that same route on my current coupe build.

 

Go to this website it will tell you all you need to know . A 305 likes 2200 to 2500 rpm. http://hotrodworks.net/hotrodmath/quarter.html

 

My '29 A Sedan with 472 Caddy gets 13-14 mpg @ 60-65 mph with TH400 trans and 3.23 Dodge rear. Tires are 30" tall. It running about 2500 rpms