Technical Piss-poor mileage from a 700r4 ??

My thoughts exactly. I swapped a 700r4 in and had to drop the pan and install a lock up kit from TCI, as well as a switch on the brake pedal. Did you do anything like this?
http://m.summitracing.com/parts/tci-376600

 

Bingo

 

OD's are highly over-rated for mileage increases. I have two Ram pickups, one a standard, one automatic, and tried running each in Direct for a tank of gas to see what diff it made. End result? Zip, nada, nothin'! 18 both ways. Had a 235 OD with 4.11 gears years back, it got way better mileage in Direct than OD. Engine wear is reduced in OD, that's the big deal.

 

Definitely a 3.73 rear. I don't know what my '84 Chevy wagon had in it, but it got decent mileage if I kept my foot out of the Q-Jet. The up & downshifting is a symptom of the transmission being freaked out.

 

I can't believe nobody (in 30+ posts) has told him to "DROP A 4 SPEED IN IT!". Man is the HAMB getting soft, lol....

 

That would be a given. I just assumed he must have a lame leg since he didn't have a 4 speed...

But- My famous Uncle gets much better mileage cruising closer to 80 than 60. Lugging along at 1700 rpm doesn't mean you're getting good mileage.

 

Drop a six-speed in it.

 

Good thing you posted. I was going to drop a 700r4 in my 51 with 3.08. Now I know I will probably need at least a 4.11 gear to keep it where I need it. Good post.

 

LOL the Raven got a ticket out in KS for going a little faster then OH WOW in his little roadster, he was running a 2.89 gear and a 200R and GAWD knows how fast he was going to keep his small block in cruising range. His ticket was for speeds in excess of 100

 

I think those saying that you should run a lower (numerically higher) gear in the rear with the 700R4 are correct, but ultimately I don't think that's going to help your fuel economy, and the issue is elsewhere.

With the original configuration, with a 1.00/1 final transmission drive ratio and 3.08 rear gear and a 28.5" tall tire, your engine RPM would be 2360 RPM. That's not a lot of RPM at all for highway speed, and frankly, right in the wheelhouse for low end torque from a stock/RV cammed 305. This is a setup that should, in theory, yield pretty good fuel economy on the highway. With the current set up of .78/1 OD, 3.08 rear and 28.5" tall tire, your RPM at 65 mph is 1841, which is too low and below the torque curve of the 305. As stated, this would be an explanation for the decrease in milage. However, if you swap the rear gear to the more OD-friendly 3.73, your RPM at 65 mph will be 2230, only 130 RPMs lower than your original configuration. With that essentially bringing you full-circle with cruising RPM, it would be safe to conclude that you would see comparable milage to what you saw with 3.08s and no OD, and not significantly better milage as you hoped for. Now the swap wouldn't be for nothing of course. The 700R4 has a much lower first gear, which will get your car off the line much easier than before with the 3.08s and the TH350. The 700R4 essentially lets you have the benefit of a tall gear for acceleration without sacrificing top end speed. This is all good news, but doesn't address your underlying fuel consumption issues.

If you're getting milage that poor out of a 305 with a cruising highway speed of ~2300 rpm, I'd start looking elsewhere in your setup. The TH350 and 700R4 are similar in size internally, so it's not as if you're running a bigger trans like a TH400 that would see parasitic horsepower loss. Is your carb properly adjusted? How many miles are on the engine? What is your compression like? Are your wheel bearings good? Are your brakes properly adjusted? What are you using for an ignition system? Are you driving with a parachute attached to the back bumper? These are all questions that need answers to get to the underlying issues.

 

or "gas mileage?"

 

Gas ain't free.

Not everyone here is a retired multi-millionaire.

 

Isn't the 700R4 OD 0.70:1?

 

Besides, its a leadsled.....don't need to prove how many tickets I can rack up. I leave that to the open-wheel guys!

 

Yeah it is, that's my incorrect memory. The GearVendors unit is .78/1, and some RV 700R4s are .75/1.

The point of my post remains the same though. 3.08 is a highway gear. If milage is bad with the TH350/3.08 setup, it's probably going to be just as bad with a 700R4/3.73 setup. All he's saving is a couple hundred RPM.

 

I haven't taken the time to read the response's you have received but I suspect the real culprit is your rear gear. HRP

 

Actually you make a valid point. If the engine is going to operate in its power band and you gear to run the same revs as before at highway speed you should encounter the same mileage.

Here is the deal with an OD most of our engines will not run below a certain RPM efficiently, the reason a late model can get better mileage from an OD is that with the onboard computer controlling spark. fuel feed and even cam timing it can run revved up or just off idle. Ideally what one wants is to be running just on the bottom end of the engines power band for fuel mileage, so if you own an engine that starts climbing up on the cam @ say 3000 then at cruising speed you want to be running right at 3000.

The upside to an OD is that you can claim better versatility, you can gear low for around town and still be able to cruise at highway speed without being revved to the max. What would be nice would be to have double OD, say 5th @ .35 and 6th @.7 od. Then you could gear in such a fashion that you could run normal highway speed in 5th and do felony speeding in 6th.

 

That's my plan. Two overdrives.

 

Could just use a 3 speed turned around backward behind the drive transmission. LOL

 

Math may be what it is but before doing anything else make sure the TV cable is adjusted correctly

 

T56.

 

Better get it fixed, you may be burning up the 700r. Establish what road speed you want to cruise. Then go to the computer and find the peak torque of you 305. Set your cruise speed half way between that curve on the lower side. You can get the needed formulas on the web for you gearing combination. Now go to Eldebrock and find their economy carburetor, research the jets, springs, and rods for that carburetor. Buy an Eldebrock tune up kit and set your carburetor up for maximum MPG. This is what I did and my 36 Chevy Tudor gets 19 MPG with a cruise control at 75 MPH. Oh yea the plugs are brown.

By the way pull that transmission detent cable tight and pull it 3/8" more and lock it down. You don't want that transmission shifting between 3rd and 4th as it will burn the clutches up.

 

Do you have the 700R4 TV cable ratio corrector on that Edelbrock?

 

It needs adjustment. There no problems using a 305/700r/3.08 that's a common factory package. And should net good mileage . check the TV cable. It should not shift the way you discribe.
Godspeed
MrC.

 

Unless you have the aforementioned geometry corrector, you will not have the correct pull on the TV cable.

 

The geometry corrector is certainly needed, I'm having to work one up now for my friends tri-power 348 '61 BelAir.

 

305 just doesnt make enough torque for that rear gear. I run a 3:08 with a LT1 350/4L60E (same as 700r4 but electronic) and average 20 mpg city/highway with my 4000+ pound cadillac. 350 makes enough low end torque to pull fine in 4th. I run a 305/700r4/4:10 gear in my 37 buick with 28" tall rear tires and also average 20+ mpg but that car is lighter. With the 4:10 gear the 305 has plenty of pull and only downshifts out of OD on steep hills. I have my converter wired to lock up in 4th gear only and I am running a 750CFM qjet, too big for the 305 but runs flawlessly. I shoot for 2000-2200 rpm at 60-65 mph in my builds, I do a lot of long distance car shows.

 

Put the th350 back in, I doubt you will get more than 3-4 mpg better changing gears with the 700r4.

 

But 4 mpg is a 30% increase--

 

I would never go back to a TH350, OD is the way to go.