How Low a gear ratio can you go on a 350 and still drive on the HWY?

How low a gear ratio can I go on my 72 GS 350 and still drive it on the HWY? This is with the stock TH350 w/BM shift kit. So an overdrive must be considered in the equation. Mainly, the lack of one. Eventually, I will like to add an overdrive unit. But after the cost of ring/pinion and "posi" it will take a while to save up for that. So, how low can I go with a TH350 w/o an overdrive and not run the guts out of her on the HWY? It is not driven daily and occasional driven on the HWY. I have three sons and we use her for road trips.

 

3.42 is about as steep as I'd be willing to go, but I like to drive fast. If you want to limit yourself to 60'ish mph then 3.90's to 4.11's are decent. Also depends how tall of a tire you are willing to run too.

 

I forgot about tire height. I am running 31" tall tires and I like them. I too like to go fast, mainly from signal light to signal light. I like speed but not for sustained periods. Hope that helps.

 

It's all about how much you can stand - engine screaming or wallet draining....

In my book, if you drive 70-75 on the highway for any length of time, 3.55s are about it and even thats pushing it. I have 3.42's on my 73 and they're ok with a 29 inch tire.

My buddy has 30 inch tires with 3.55s on his 70 Trans Am, and he wont get on the highway if you put a gun to his head. But thats him.

 

That's a pretty tall tire probably can't go wrong with any gear at that point. Probably would drive nice with 3.73's. 3000rpm at 75mph. Even 4.11's would be ~3300rpm at 75mph. These engines will turn 3,000 rpm all day long.

Both my Skylark and Centurion turn around 2,700rpm for hours on end at 75-80mph. 300-600rpm more isn't going to hurt it.

 

As a disabled vet it is my wallet that is screaming. I know I am trying to eat my cake and have it too. I want to go fast but I want my engine to be happy too. At the cost of differential parts I want to get it right the first time.

I really appreciate the advice...

 

My favorite gear calculator. Been using it for years.

http://www.f-body.org/gears/

 

I have 255x60x15 tires on my '72 Skylark with a 350 and a 3.23 posi rear. It is a very nice combination and I can drive at 70-80 on the hwy and make around 3,000-3,300 rpms on the tach.

 

Chris it's about time they raised the speed limit on LI:grin:. Out here you can get run over at 80.

As for the gears I would also go with a 3:42- 3:55 enough gear without ringing out the wallet:TU:

 

OK I will blow you all out of the water. I ran my 72 Stage 1 455 NHRA legal stocker and record holder almost every day on the street. It had 4:78's with a 28" tall tire. I never blew up the engine and turned it often over 6000. At over 110 mph on the freeway, I would never approve of doing this today for safety and legal reasons. Since some Buick builders would say the 350 is more reliable than the 455 you just have to decide how low you want to go and how much fuel you want to burn, and o yeah, how much trouble you want to get into. Take your car to the strip and rod shows!

 

I wouldn't doubt they'd live just fine at 4,000rpm on the highway.

 

You will need a 3:73 to turn those tires :laugh: With those tires it will not be too bad unless you are Ken Betts Would need some gear in the 5's then. When you get the 200-4r you can go to 28 inch tire and perk the car up even more.

 

3000 rpm is a completely livable rpm for driving on the highway. I've done it for years. The '76 has 3.42's and 255 60 15's and with a tight converter it cruised at a shade over 3000 at 70 mph. If you have a loud car it may get annoying sooner than later but mine is pretty quiet. It would still get over 14 mpg and was running 11's with a 455. Up until last year when I started trailering I drove it everywhere to race including Bowling Green 700 miles one way every year since 2006.

In the early days of the GSCA I drove my '65 down many times with 4.10's in the rear. That put it into the 3500 range and proved no problem plus the speed limit was lower so that helped.

IMO the only real reason to run an OD is to prolong the life of the engine and make the car a little easier to drive. Gas mileage is a bonus. Most people never put enough miles on a hobby car to ever get a payback from better gas mileage because a good OD/trans is serious $$$$. If you plan on lots of highway miles(1000's and 1000's) then it could pay off. I drove my Buick 455 powered 1987 TA about 60,000 miles in the years I owned it all over the place so in that case I used the factory T700 with some HD mods but I did all my own work on it.

For a 350 powered car with those tires I wouldn't even think about a gear less than 3.73's. In fact what I would do is get some shorter 295 50 15's on some 10" rims, run 3.42 gears and when you are going to hit highway swap the tires out to your 31's.

For an exact measurement of the tire's diameter do a loaded measurement of the rollout and then calculate backwards for the diameter. You'll find your tire really isn't 31" tall. Not quite as good is loaded tire diameter which is closer to reality than the given tire diameter in the specs or calculating it by using the tire size numbers.

 

My vert has a 255/60/15 tire and 3.73's. The M21 is 1:1 in 4th gear and its brutal on tge highway. Im swapping to a 5 speed.

 

Reasonably priced used gears are all over the place even here:

http://www.v8buick.com/showthread.p...t-8-5-3-73-3-42-gear-sets-available-very-good

A nice set of factory gears will install easier than any new set and more than likely they'll be quieter because they were broken in correctly from the factory. If they have the factory pinion bearing still on you're 99.9% sure the shim will be very close and all you'll need to do is set the side to side play. That's the only way to go. I've done it many times including the 3.42's in my '76 right now. All you need is somebody who knows their way around a rearend.

Your '72 should have the corporate 10 bolt which uses these gears and carriers. Plenty of good gears and posi's all over E-Bay. Heck you could install the gears now if your rear has 2.73 gears or better. Probably 3.08's or 3.42's in there right now because it is a GS.

 

We are going to go with 3.73 ratio. We do drive it on the Hwy. But by far most of our driving is in town: Red light to red light. Once we get the rear end set up, then we can decide if we need an over drive or 4 speed automatic transmission. We are definitely going with Monzaz. His prices are fair. Plus he can set us up with everything we need. Additionally, we can be confident that all the components are compatible. He offers three different posi units: Yukon, Eaton, and True Trac. Any recommendations on which one is best? Especially has anyone had experience with True Trac?

 

All are quality brands you can't go wrong with IMO.

I've had Yukon gears and currently have a True Trac (in a 9" Ford). Eaton is well known and I wouldn't hesitate to use them.

 

I have the TrueTrac in my '76 and it works great. No complaints. I went with the TrueTrac because it distributes the power to the axle which has the best traction which helps when you are going around turns under power. I like that it doesn't need any special posi fluid. Hooks great at the drags.