Technical Dilemma: Install higher gear ratio or get an overdrive transmission?

I have a 41 Buick coupe that I built with a stock 455 buick motor and 400 transmission. The rear end is a late 70's item with a 3.08 gears. I am running a 215 75R 15 inch tire.
I drive this car locally as well as on the freeway. At present, it seems like I'm turning too high of RPM's at 65 mph plus. So I'm faced with changing the tranny to a properly built 200R4 or 700R4 or leave the tranny and go with a 2.56 or 2.72 ten bolt GM rear end.

I'm not sure which direction to take. It seems like the gear change might be the easier of the two options. Probably less expensive as well. But will the gear change give me the proper RPM range at the higher speeds. I look forward to your responses. Thank you.

 

The car will have another gear, and because the rear ratio will be lower numerically, the gears effectively will be closer together. It's a much better deal unless you're racing.

The OD has lots of rotating mass to accelerate. That ring gear with the planetaries is like another mini-flywheel you gotta wind up & that costs ya time. Anybody who's shaved a flywheel will understand this. Light flywheel = faster 1/4 mile.

Whenever I changed a 3-speed to an overdrive trans, it was slower in a drag race.

 

If you go to an OD transmission you DO REQUIRE a taller rear end ratio, 3.08:1 isn't high enough. 3.5:1 or higher are better as you need to keep the engine in the torque band and RPM between 1800rpm and 2100rpm for cruising around. Any lower RPM in OD and your engine will labour and your ride NO fun to drive. If you go OD upgrading the rear end is mandatory. 2.56:1 or 2.71:1 is NOT a good ratio for an OD transmission.

 

with the Nailhead in my 50 sedan delivery I'm limited on trans choice. I installed a gear vendors OD behind it and absolutely love it, all but the price that is. I'm running 3.08 gears also.

 

I run a 700R4 with 3:55 rear end gears. It seems to be the perfect combination for back road and I-State driving.

 

In the early Ninety's I had an OT GM Commodore 5.0 with a T700, 3.08 rear, and 205/65x15.
It worked well, first gear is low in a T700, and OD was great on the highway at the (70mph) speed limit or faster.
That said, it would have been a dog if it had a big cam.

 

My vote is the OD trans , and the 700 R4 built 1985 or later and not the 2004r as you are putting it behind a big motor.
You get two bonuses with the 700R4 obviously over drive , and its first gear ratio is 3.06 to 1 versus the 2.48 to 1 you have in your TH400 .
You get geared lower and higher with the same trans.

 

Hi. Sometimes keeping it simple is an easy solution. What is your rear tire size & tire diameter? Sometimes a couple sizes larger tire will give you enough to lower the rpm for highway driving assuming you are not going to drag race.
Use the following formula to figure your current tire size to rpm & thdn try with a larger tire diameter & see what you get. 366 times MPH times gear ratio divided by tire diameter = RPM

I currently have 600/16 tires & 411 ratio.
For me going from a
600/16 with 411 ratio & 28.3" diameter at 65 MPH = 3171 RPM
650/16 with 411 ratio & 29.26" diameter at 65 MPH = 3067 RpM
700/16 with 411 ratio & 30.18" diameter at 65 MPH = 2974 RPM
750/16 with 411 ratio & 31.34" diameter at 65 MPH = 3882 RPM
OR
600/16 with 355 ratio & 28.3" diameter at 65 MPH =2746 RPM
650/16 with 355 ratio & 29.26" diameter at 65 MPH = 2649 RPM
700/16 with 355 ratio & 30.18" diameter at 65 MPH = 2569 RPM
750/16 with 355 ratio & 31.14" diameter at 65 MPH = 2489 RPM

As I live out in the country & there is heavy oil field traffic all the time I must be able to drive at least 65 MPH to not cause an accident. With this in mind I am going to do 2 changes - go to 750/16 tires & the 355 ratio rear end that will result in over 500 RPM less than current amount. The resulting RPM from these changes will not overwork the 235 CI 6 cylinder engine in my coupe at sustained speeds of 65 MPH. Will do a lot for fuel economy also. We have just finished the engine overhaul & will be installing the new engine & hooking up everything this & next week. After a new fuel tank it will then be the ring & pinion change & then back on the road.

Jimmie

 

Hold on here. You have 3.08's and a fairly big tire.
That is just the sort of combo that would have been on a big Buick in the '60's or '70's, cars meant to be cruised silently at 80 MPH.
With that gear, you don't need higher ratio, IMHO, and with a 455 you probably don't need a lower rear axle for more torque (the other big reason for going to OD) either.
I suspect you are cruising at a very low RPM already.
Using a 27" tire diameter, which is probably smaller than you have, your whole highway range from legal to flashing red lights in mirror is from low 2,000's to 3,000.
I suspect you have noisy tires or something like that convincing you engine is working too hard... it isn't.

 

I agree with Bruce, you just need a better radio in the car.

If you really can't deal with 2700 rpm on the highway, swap the rearend gears.

 

Overdrive, all the way. More gears is always better. Match it with the proper rear ratio, and you will have a far more drivable, flexible, and economical ride.

My '60 has 5, the '29 has 6, and we are about to finish a Cummins Dodge, with 10!

 

X2 on it being what it used to be. Today's computer-controlled six and eight speed overdrives have conditioned us to going down the highway at 1800 RPMs. The gears you have are really what Buick engineers though you needed. Rule of thumb was to gear to be at torque peak at highest cruise, i.e. 75-80 MPH. IIRC, your stock Buc had it's torque peak @ 2600 RPMs.

Having said that, I run overdrives in all my cars and trucks. It will require some tuning, as the ignition advance requirements change with more load and throttle opening at lower RPMs and lower cruise vacuum.

jack vines

 

27.7 tall tire, I would go with a taller tire before I changed a gear. All my junk and I seem happy at 2200-2500 at highway speed. (except Texas' 80mph limit)

 

do a search for ratio calculators like at: project33.com

 

Best do a lot of home work before you go changing anything as it can get spendy quickly. Bruce has it correct as well as squirrel. I spent a lot of $$$ putting a 4l60 in my roadster to get better mileage. Could have bought a lot of $4.00 gas for what I gained. I had a 350/350 combo with 31" rears & should have left it alone. I had to change rear gears,tires & rims & didn't gain all that much more MPG. I must say the 4L60 is a blast to drive tho. I also installed a Lokar shifter. Would I do it again? Probably not. It i

 

I run 4.11 gears in my roadster most of the time, it is under 3,000 rpm at 65mph with 31.5" tires. My 40 Pontiac with a 63 super duty 389 has 3.00 gears in the 9" and about 28"tires and runs about 2600rpm at 60mph. They both work great on the highway and are exciting drivers when needed. I think just taller tires will be your answer.

 

One thought maybe look at the torque converter you may need a tighter one (less stall)

 

I'm cheap. I'd go with a 235/75 set of tires and keep the gears ya got. (coming from a guy running 235/70/15's on an 8" Ford, with 2.78:1 gears, with a 350/350 turbo. The sweet spot, in my coupe, is 72mph, and will give me about 18mpg in light to moderate wind. That's even with a pretty lumpy cam!

 

I agree with Bruce and Squirrel. I think your engine probably sounds like its reving higher than it is. The size of your tires and gear ratio sounds about what would have been in a car with a 455/400 combo.

 

I have the same setup in my 49 except I have 235/75 tires and it loafs at 65 mph. You either are mistaken or have another problem.

 

Moose (Bruce) and Squirrel are good refs! I checked diameters, if you went to a 245/75R15
you'd gain about 6.5%, with a comparable drop in rpm.

 

I'd go 200-4R and plan on changing the rear gears to be happy with overall drivability. Overall = mindless "put it in OD every time you drive." Or just leave it in D until you want OD on the highway.

Why? Driving the same platform car with 2.29, 2.41, 2.73, 3.08, 3.42, and 3.73 and various TH350, 200-4R, 700-R4, and manuals will give a driver an opinion.

 

Excellant comments all the way around. Thank you very much.

 

When I ran my '60 Pontiac it had 3.08 gears and with 235/75R15 tires on it pulled around 2100 RPM at 65. And was very comfortable doing it. But the 455's torque can handle a 2.41 or perhaps higher. Then you can do 80 all day.

 

I actually think that big 455 torque can pull the 3.08 gear with an OD trans without any problem. So my vote, if you go to the expense and hassle to change the trans, is to get an OD trans. A 200R4 will bolt up and be easiest, but make sure you get one that is built up for the higher torque of your engine. A 700R4 requires an adapter to the BOP bellhousing pattern of your Olds. All 700R4 are Chevy bellhousing pattern. While a 700R4 is stronger to start, the 200R4 can be built to handle the power you have with ease. Run a lock-up converter for sure with either trans.

Or do the low-buck approach and put taller tires and just let the engine rev a little bit on the highway. My guess is with your 3.08 gears it is approx 2600 at 75 mph now. That is not too much for that 455. it only causes more noise and more gas to be used, but is not bad for sustained running at that rpm level.

 

The other option is to shift out of second gear. ;-)

 

Rockable... I have a 215 75 R 15 tire. If I had the 235, I would agree with you, The difference in size seems to equate to 400 - 600 higher rpm at 70 mph

 

The Gear Vendors can be moved to another car, and you can return to the prior set up. I love my Gear Vendors, they are a blast. I am working on putting one behind a C-6 406 C.I. FE.

 

Agreed. I run a 4L60E (same as a 700r4 as far as gear ratios) with a 3:08 rear gear and similar sized tires. Cruises the highway just fine with that gear, and I also agree to go 200r4 with a multi pattern bellhousing so it will bolt up to the 455. It will still need to be built to handle the torque but will work great.

 

I'm running a 350/350 EFI 2:78 rear don't get real good gas mileage can't hear the engine because of the wind noise so we just leave it alone and keep on going.