westside807

Hello,
I'm trying to make sure I understand how the rear end gearing and transmission work so I can make the right decision for my application. Since the 3rd gear ratio is 1:1 in my TH350, and my rear end gear ratio is 2.56, this means that 2.56:1 is my final drive ratio (excluding tire size, for now) correct?
If so, if I switched to a bigger numerical rear end gear ratio like 3.42 or 3.73 and a 200r4 tranny, I would need a lower final drive ratio number in my new setup than the current 2.56 if I was looking to lower RPM's and improve fuel economy, correct?

Specifically, I have a 1971 cutlass with the original 350 that's now a 355(bored .030 over). Stock cam. Headers. According to the online calculators, my tires are 29.3 inches tall. The rear end gear ratio is 2.56:1. I don't race. This is simply a cruise around town and hit the highway car. I would like more get-up-and-go or take-off from stop. I'd also like to improve my fuel economy if possible. So, reading this forum I know that the overdrive gear helps when you increase(numerically) the rear end gear ratio.

Main questions
So, if I want better take-off and fuel economy, not only do I install an overdrive tranny and increase the rear end gear ratio and go higher numerically, but I also need to pick the rear gear ratio that when multiplied by the final transmission gear ratio(.67) and taking the tire size in account, the final gear ratio will equal less than 2.56, Correct? If so, Which is better for my application 3.42, 3.73, or another ratio? I don't currently have the 200r4 but I will. Right now I have the original TH350
I've done some playing with online gear calculators, but I'd like to hear from you guys before I decide which ratio to buy.
I hope this make sense,
Thank you

matt69olds

With a overdrive, you could replace those 2.56 gears with 3.08 or maybe 3.42, and have basically lower cruising engine speeds than you have now, with acceleration that feels like you installed a engine with 100hp more than you currently have. I’m too lazy to do the math, but 3.08-3.73 with a overdrive and lockup converter is about perfect in my opinion

Fun71

I have done this as a thought experiment for many years and with 3.90:1 rear gears and a TH200-4R you would end up with the same effective top gear ratio (3.90 x 0.67 = 2.61) but have substantially increased first gear acceleration due to the 2.78:1 ratio vs the TH350 ratio of 2.52:1, so essentially the best of both worlds regarding acceleration and mileage.

69CSHC

Is 29.3 inch tall tires a misprint ? If accurate that is extremely tall and devastating to off the line performance. Factory for your car is likely 26.3 so that additional 3 inches is reducing your 2.56 rear gears effectiveness to 2.30

If this was done on purpose to maximize miles per gallon understood. But if your car has always had such a tall tire and you reduce it to factory height. Off the line will be much improved.

If you insist on keeping such a tall tire, go with 3.73 and the TH200-4R. Effectively it will be a 3.35 gear which is reasonable with overdrive.

If your current or future tires are factory height spec 26.3 you will have effective gearing very much like a G-Body 442. MPGs for Car and Driver 1985 442 are 14 with its 180 HP 245 LB-FT 307

For comparison Car Craft tested a then new 1974 Z28 with a 350 CID 350 THM 3.73 gear and saw 10 MPG

joe_padavano

The ratio that matters is the EFFECTIVE final drive ratio. This is the rear axle ratio multiplied by the gear ratio in the trans. Yes, tire diameter matters also, but I'm assuming for the purposes of this thread that you are not changing the tire size.

For a trans that has a 1:1 top gear, it's just 1 times the rear axle ratio, so in your case, effective final drive ratio is 2.56:1 in third. Note that with a TH400 2.48 first gear, your effective ratio off the line is 2.48 x 2.56 = 6.35:1 in first. This makes for a VERY lazy car off the line.

Rule of thumb is that you want about 8:1 to 10:1 effective ratio off the line. With a TH400, that's about a 3.42:1 rear axle (2.48 x 3.42 = 8.48)

Now let's use an OD trans. The 200-4R has a 0.67:1 fourth gear and a 2.74:1 first gear. That means that with your 2.56 gears, changing to a 200-4R gives you 0.67 x 2.56 = 1.72:1 effective final drive in fourth, and a slightly better 2.74 x 2.56 = 7.01:1 effective final drive off the line. Best of both worlds.

If you use the 200.4R with those 3.42 gears, you get 0.67 x 3.42 = 2.29:1 in fourth (for better mileage on the freeway than even your 2.56 gears) and 2.74 x 3.42 = 11.70:1 off the line for much better acceleration.

And you don't need online calculators to figure this you, you just need to know how to multiply.

RetroRanger

A 2004r plus a rear end rebuild could run 3-5 k $. Thats alot of gas even w premium at $3 gallon. Idk how much you drive but the roi is 30-50000 miles, and even then idk how much improved gas mileage will be. My guess is gas mileage will go down w the increased off the line power.

then again if your selling it to the general as better mileage but the troops want the better acceleration then carry on sir!

Gpc1

Well I had the 2.78 gears with a stock 200r4 , 350 4bbl engine and it was sluggish from start. Pulling out onto busy streets it takes a lot more throttle to get up to speed and hard on the clutches too. I was using 2nd gear around town, and this resulted in tranny overheating and losing fluid out the dipstick tube. I didn’t realize until it was too late and burned up the clutches. Tranny is presently getting a stage 1 rebuild , $1400 . While tranny is out I swapped out the 2.78 for a 3.90 posi with boxed control arms and sway bar, another $1400. Should be much better plus final drive .67x3.90 = 2.61. Not as good as the 2.78 @1.86 but liveable. 1st gear was 7.61 new ratio is 10.6 which will be great on starting out. Should get it back this week! Yay!

Tuffnut

I did the th350 swap with 2:56 gears to a 2004R OD with 3:42 gears with posi. Totally different car off the line now. Great for highway cruising and lots of grunt off the line with rubber if you desire.

JohnnyBs68S

My car had a 2.78 open rear and Jetaway and got really decent highway mileage w/ my 2bbl 350. It was pretty sluggish stepping off from a stop though. A 3.90 posi helped that tremendously, but it ran alot of RPMs on the highway. Swapping in a 200-4R regained a final drive a little taller than I had originally (2.61 vs. 2.78..........not including the positive effects of a lock-up TC) and now it launches REALLY well.

westside807

Thanks for the responses! In my case, based on what I put in my original post, would you go to 3.42 or 3.73. These are really the only 2 rear end gear sizes I'm looking at. With my 29.3 tall tires and adding a 200r4 is 3.73 just right or over kill or would 3.42 be better. Just right=perfect balance between acceleration and economy, in my setup

joe_padavano

The difference in RPM will be less than 10% between those two. With 29" tall tires, I'd err on the side of the steeper gears (3.73), but it depends on your priorities - acceleration or low RPM cruising.

westside807

Are you running factory size tires? I ask because I was leaning towards the 3.42 but I'm not sure how much the tire size will effect performance good or bad. Mr. Joe says go with the 3.73's which were my original selection.

westside807

[QUOTE=69CSHC;1277311]Factory for your car is likely 26.3 so that additional 3 inches is reducing your 2.56 rear gears effectiveness to 2.30

If you insist on keeping such a tall tire, go with 3.73 and the TH200-4R. Effectively it will be a 3.35 gear which is reasonable with overdrive.

How are you calculating the effective gear based on my tire size?

joe_padavano

Taller tires result in a lower numerical effective final drive ratio. As an example, 29.3" tall tires with 3.73 gears is nearly identical to using OEM-sized 26.5" tires with 3.42 gears.

Olds64

I merged your threads.

westside807

thank you!!!

monzaz

I noticed that many of you are thinking that because you lower your RPM you think your getting better economy.
We have seen many pick a freway gear with a over duration cam thinking it would help the economy...NOT so. It got worse...
You have to remember that the camshaft in your motor and how it meters fuel is where your going to get your fuel economy.

If you have a Big Powered or even what most guys use loosely MILD BUILT motor...it is usually way more than any stock engine ever used so fuel economy really makes not difference here with out paying attention to the carb air to fuel ratio and how much performance your trying to get with the camshaft.

Instead of using the term economy...use drive- ability in all ranges of driving.

Traditional Muscle cars built today (455 cube) are rarely getting fuel mileage... Big cubes are always going to eat fuel. Unless your using fuel injection LS motor do not expect more than the motor can give in economy. Just enjoy it for what it is. Overdrive will definitely help tons to making the old technology more usable and much more fun with new tech cars out there with out mucking up the whole cars originality.

Camshaft will determine what ratio will be best at the RPM you want to drive at on the freeway.

Jim
JD

joe_padavano

Excellent points, Jim. Obviously the cost to make all these changes will never be recouped in fuel savings anyway. I think more people are interested in the lower freeway RPMs than anything else, while not giving up performance.

westside807

The truth for me is when I purchased the car, I liked the idea of the original engine. The guy I bought it from had it sitting for months and couldn't get it started. I bought it and after some time, we got it started. He then got in and smoked the tires instantaneously, needless to say I kicked him out because it wasn't his car anymore. Once he did that, I was sold on keeping the engine. After the rebuild of the engine back to stock(except the for the .030 bore and matching pistons etc.) and some time had passed I began to think that performance and fuel economy(or drive-ability) mattered more for me. I considered going the LS route, but I already had a low mile rebuilt engine. So I decided that I'll switch to an overdrive and upgrade my rear end, and eventually get Holley fuel injection. Those changes will help in performance and economy is what I've gathered over time and reading this forum and others. If I'm able to get 18 MPG or better and great acceleration I'll be happy. If not, and I get better acceleration and similar fuel economy. I'll be ok with that, but I won't like these changes and it result in worse fuel economy.
Thanks for the responses. I believe I'll be going with the 3.73's with the 200r4.

69CSHC

Yes, calculated with your current tire height of 29.3

I prefer a combination of calculators from Wallace Racing, but if you want to cut to the chase.

https://www.intercotire.com/tire_siz...tio_calculator ( you must clear all info with reset in order to go forward)

THM 350 with 2.56 gear and 29.3 inch tire makes for an effective 2.30 ratio

THM 200-4R with 3.73 gear and 29.3 inch tire makes for an effective 3.35 ratio

( the transmissions are irrelevant in 1 to 1, only put in by me to help make point clear)

Something else to keep in mind, a full 1 point move up numerically in gear. Reduces ETs by about 1/2 a second for cars in our performance ballpark. So if your car is running a 15 flat and you complete this mod. She could potentially be running a 14.5 all else the same. Which is a tremendous improvement. This is the reason gear swaps are the best bang for the buck in cars like ours. You need to add an enormous amount of horsepower to achieve this otherwise all else the same.

Thank you Jesse, I was thinking our earlier comments were going to be lost in the abyss....

69CSHC

Absolutely !

I have fiddled with the 69s carb for years here and there while gradually reducing tire height. Steadily improving the MPGs to my current peak of 15 MPG highway.

Initial tire diameter was 27.4 when I first got her, over time swapped down to 26.6 and now currently 26.1


Gotta love those 300 LB FT of torque !

Performance and fuel economy are tied together. Absolutely nothing is more important than a close to ideal tune.

Thank you for keeping it Olds V8, for many of us "going the LS route" is like nails on a chalkboard.

I think you may see 18 MPG highway with the overdrive. Acceleration will be better no matter what. I would say its virtually impossible for your fuel economy to get worse. Unless you are going to beat on your ride morning, noon, and night. Of course everything must be in good working order for the improvements to materialize.
I believe you will be very pleased in the long run. The only thing I would consider is a high performance 200-4R instead of run of the mill. That 300 LB FT mentioned earlier hits hard...

westside807

Thanks for the information 69CSHC!!! And everyone else!!!! I know this may sound very ignorant, probably because it is, but I thought my v8 was the low horsepower and lower torque engine. Is 300 lbs of torque correct for my close to stock 1971 engine? If so what does that equate to in horsepower? I will say I heard that oldsmobile rockets were high torque engines at one time, but I assumed it was the 60's engines not my 71.

Fun71

There are different ratings depending upon carburetor and exhaust setups.

Below is the info I have for the 1971-1972 350 engines (these are SAE ratings, not the earlier GROSS ratings).

Carb...Exhaust...HP @ RPM.......FT-LBS @ RPM
----- ------- ----------- ---------------------------
2-bbl...single.......160 @ 4000...275 @ 2400
4-bbl...single.......180 @ 4000...275 @ 2800

2-bbl...dual..........175 @ 4000....295 @ 2600
4-bbl...dual..........200 @ 4400....300 @ 3200