Gear ratio question

My car currently has a 350 turbo tranny, 3.08 posi and 225/60 14 wheels. I am changing the tranny to a 200r4 with the following gear ratios: 2.74 first, 1.57 second, 1.00 third, .67 fourth. I am also changing my wheels to 235 60 r15s. Would there a major benefit (seat of the pants feel) if I changed my rear gear ratio to something higher?

 

Not only a major benefit going with a higher numeric ratio but very necessary. Your current 3.08 will cause the engine to lug way lower than it should when you are in OD with the 200r4. (Don't have the calculations handy right now.

Most 200R4 users would agree that the rear gear should be at least 3.42 and many would go with the 3.73 or 3.91. If I went to a 200 4R it would be the 3.73 with a 245/60 x 15 tire which is the equivalent diameter as the original G60 x 15 Poliglas that came on our GSs with the 15 inch Rallye wheels from the factory. The 235s you mention are actually about 1/2 inch shorter.

Good luck with your project. I've thought about doing the same with my GS.

 

You are getting an extra gear so let's think what you want. My pulling truck had a 400 in it with a 4.10 gear. It was topped out at 60 MPH. I put a 7004r in it and it tops out at 85 MPH at about 3500 RPMs or more. So yes A gear change to a higher number will always make the launch better.

 

Go here: http://www.ringpinion.com/Calculators/Calc_RPM.aspx
You can test any number of tire size & ratios.

With a set of 3.73's you'd be spinning 2,091 rpm at 65mph.
With a set of 3.90's you'd be spinning 2,187 rpm at 65mph.

 

I've got 4.10s behind my 200-4R with 275/60-15 tires and it runs 2,400rpm at 70mph and the same rpm at 45mph in 3rd gear.

Just for the sake of comparison, the 200-4R and a 4.10 gear gives a final ratio in OD of 2.75. A 3.73 would give you a 2.50. I tried a 3.73 and didn't like the way it lugged at 55mph. You didn't mention your engine combo but in my case the 3.73 gear was putting the rpm too low for the power band.

 

Your current tire is only 24.63" in diamater. This would result in an engine speed of 2522 RPM at 60 MPH. With the extra (reduction) gear of the 200R4 the engine speed would drop to 1690 RPM at 60 MPH. This as Ray states, would be considerably lower than where your engine makes peak torque.

235/60-15s with a 26.10" diameter would result in engine RPMs of 2380, and 1595 respectively. Even lower than your current combination.

Lets assume that your engine makes peak torque between 2400 and 2800 RPM. That would be close to where you want to be running at speed in overdrive for top performance. For comfortable highway driving, I would recommend the lower end of this scale. So if you set your car up with the 235/60-15 tires, and a 4.10:1 ratio, you would be able to cruise at 60 MPH with an engine speed of 2123 RPM in overdrive, or just under peak torque. Your engine is going to respond with less throttle input if it is operating at, or slightly under peak torque. So adequate acceleration can be achieved without the need for a downshift.

Some here would argue that 4.10 is too low, and that is a valid argument. The bulk of Buick's top performing cars from the period were set up with 3.42:1 final drive ratios, and tire diameters of about 26.5" diameter (G60-15, or G70-14) tires. This combination resulted in engine speeds of about 2600 RPM at 60 MPH. 2600 is uncomfortable to some people at highway speeds. And the 2123 RPM from the above combination may be uncomfortable for some. If this is the case for you, a 3.90:1 final drive ratio would be a good compromise. This would drop the engine speed in OD about 100 RPM at 60 MPH, and performance would suffer little.

This is where an overdrive really shines. It allows you to take advantage of the performance benefits (mainly torque multiplication) of a low final drive ratio, without the discomfort of high engine RPM at highway speeds. Going through the gears from a stand-still, you will find the car to be very responsive to the lower final drive ratio.

 

Thanks for the information. Engine is a mild build 350 with a 284 cam, headers, HEI ignition and a custom jet Rochester 4bbl. I'm always looking for off the line performance but never wanted to scream on the highway with the turbo 350 which was why I originally went with 3.08's. Highway driving however is limited to short trips at the moment. Tranny change is due to my 350 turbo needing a rebuild so I decided to go with a new set up. Was thinking about changing the gears while the cars in the shop. Currently have an Auburn unit and have no idea what the gear change would run me. As a side note, I was planning on running around town in third gear and using the electronic lock up in fourth gear on the highway.

 

Well, the key comment here is that you are looking for "off the line performance". If this is your priority, than definitely go with either the 3.91 or 4.10. Your little 350 will scream around town and the highway speeds will take care of themselves when you put it in overdrive. And you really should consider the 245/60 x 15 tires as they more accurately replicate the original size and will look better in your wheel wells IMO. Have fun. :grin:

 

Thanks for the info Ray,

When I was ordering the rims and tires from the tire shop 235 was recommended to go with my wheels based on their back space and off set plus I got as great deal. From what I see the 245's would have been .47 higher, maybe next time! Just had the 235's installed and they fill the wheel wells much, much better than my 225/60 14's did. Tranny is next as its already ordered and at the shop, then maybe the gears. Now it’s time to sell my tires and Cragars.

 

People always say you "need" to run low gears with the 200-4r. From the factory the majority of cars with that trans ran a 2.78 or 2.93 gear ratio (I've had them). They also used 27" tire diameters so that has to be taken into account. Based on the calculator that trans with a the 3.08 gear and your current tire height puts you in at around 1,800 rpms at 65mph. That'll give you the best fuel economy from the options you're considering, but it won't be a screamer off the line compared with the other options.

 

Thats exactly my concern.

 

If you want an overdrive and off the line performance than consider the 700r4. It has a 3.06 first gear which will help you jump off the line with a higher rear gear ratio. It may end up being cheaper and easier than doing both a 2004r and a gear change.

 

Although it sounds good on paper, the trouble with a 200-4R or a 700R4 and a 2.73 or similar is they simply don't work. GM experimented with OD and gears back in the `80s in the attempt to get better mpg. Of course, they also experimented with a lot of other stuff that were miserable failures as well. I had an `83 GMC pickup back then with a 2.73 gear/700R4 trans and it was useless. The only way it would stay in OD was if I was going 80mph on a straight flat highway with no headwind. If not, it would constantly shift in and out. I finally gave up and never used the OD again.

 

+2 on that. I also had an early 80s car ('82 Cadillac) with a 200 4R and, I believe, 2.73 gears and it would lug terribly in overdrive at anything lower than 70 MPH. I currently have an '87 Turbo Regal WE4 (like the Grand National) and it has the 200 4R ahead of 3.42 gears and a 26 inch tire like your 235/60 x 15s. It will shift into overdrive at about 45 MPH in town at about 1700 RPM and still have plenty of power to move around in traffic without shifting out of OD. The engine in this car has about 500 HP.

So, all said, I'd strongly recommend (as have several others) that you go with at least a 3.42 with your 200 4R and since you said you like performance driving, I'd consider the 3.73 thru 4.10 range. :grin:

 

You guys are both off. Just about every fullsize GM made after around 1983 came with either the 2004r or the 700r4 and a 2.93 gear. The issues you guys had is in no way related to the gearing and is based entirerly around the fact that the early overdrives had design issues. My '84 Lesabre had the 2004r and 2.93 rear gears and had no trouble on the highway or on the back roads. Hit the gas and the weight transfer would make the ash tray slide open. On the highway I was doing 70 at around 2,000 rpms which was getting me a hair over 20 mpg from a carb'd fullsize with a trunk full of crap and the aerodynamics of a brick.

If you ever want to see how the gears work with an overdrive than get a B-body. Every one had 2.93 gears with the 4l60e except for 2 packages. The wagons/9c1/SS got 3.08 gears, and the L99 9c1 got 3.23 gears. The L99 9c1 is slightly faster to 30 mph and then the LT1 car out powers it and pulls away. The pre-94 (tbi) cars had the 700r4 in place of the 4l60e. Very few outside of the drag race crowd go above 3.42 gears and most go after the 3.23 gears since the car is fun but still useable on a daily basis.

 

So that said, going to something higher than my current 3.08 wouldn't be worth the time and money performance wise?

 

Quite the contrary. If you want a quicker car through the first three gears, you will benefit from a lower final drive ratio. How much lower will depend largely on the intended use of the car. If it is for daily use and extended highway driving, I would not recommend anything lower than 3.42:1. If the car will just be used on sunny weekends with limited highway driving and the occasional trip to the drag strip, the better torque multiplication of a ratio between 3.73 and 4.10:1 will give you the "seat of the pants" improvement you mentioned in the original post. You will need to decide if it is worth the expense.

I would say if the car is for performance use, and you intend to use the 235/60-15 tires, go with a 3.90 gear.

 

Try the 3.08 gears with the 200R4,and see if you like it. If you could spin the tires with the T350 in first gear,they'll spin even easier with the 200R4 in first gear.

For myself,I like to use the 9.5 first gear multiplication rule for a street/strip car,where you multiply your first gear ratio with your rearend gear ratio,you want to get as close as you can to 9.5 as you can with available rear gear sets for your axle.

The best gear with a 200R4 would be a 3.42, 2.74X3.42=9.37:1 first gear multiplication ratio. Anything more on a street car with a stock suspension,is going to roast the snot out of your brand new tires with a first gear launch.

If you went with a 700R4,and the gears you have now,you'll end up with a 9.4248:1 first gear mulitplication ratio,3.06X3.08=9.4248. This would get you closer to 9.5 without having to change your gear.But you would need to shorten your driveshaft,and use an adapter plate to bolt the trans to your buick,but may still be less expensive than a gear change.
Seeing how you have the 200R4 trans already,you may want to upgrade your rear suspension before you shoot for a 9.5 first gear multiplication ratio with a gear change,or you'll be buying new tires sooner than you want.

I have a 65 Impala with a mild sbc 383 that I built,with a 700R4,and a 3.07:1 rear gear and posi. Quarter mile 60' time with this heavy monster is 1.80s with 28" tall drag radials(traction limited),and low 13s at 101mph. The camsshaft in it has 202 degrees @ .050 on the intake,and 212 degrees @ .050 on the exaust,with .550in. and .546ex. lift hyd.roller. This cam makes power right off of idle to 5,000rpm(I shift at 6,000rpm),and I get about 24 MPG,cruising on the freeway at 70 about 1,900 rpm plus or minus a couple hundred with NO engine lug. No lug at 45 mph in OD. My point is if you have a cam with the power range in the lower rpm band,lugging won't be a concern,if you have a full on race cam,or a cam that doesn't start making poweruntil 2,500 rpm,then thats another story. You will need to match your gear to your cam,and where the power range its in.

 

Not so. What is worth the money time and performance wise is up to you to decide. Look at it this way - If you go to a higher numeric ration than your 3.08 you will notice improved acceleration from a dead stop. I believe your OP said you were interested in performance so, you will definitely notice seat of the pants improvement going from a 3.08 to a 3.42. And, even more with a 3.73.

I can give you an example using the first GS I owned which we bought for my Son back in the mid '80s. It was a '72 GS 350 with a 3.08 rear. It went mid 15s at about 86 to 87 MPH in the quarter bone stock. We built an engine for it with 1970 compression, a mild KB cam, aluminum intake and KB headers. It then, went mid 14s at about 98 MPH. We then changed out the rear end for a 3.42 posi and the car improved to low 14s (with a best of 14.00 flat) at 100 MPH.

So, yes, going up numerically in gear ratio will improve your performance and your highway mileage will improve substantially with the 2004R. So, again, you have to decide how you want to play the performance vs. fuel economy game.