I have a 1970 ss 454 with what I beleive is 408 rear gearing. The car is turning 3200 at 60 mph on 15 inch TA radials (raised white letters of course) This seems like really high revs for 60 mph and with a 454 there no shortage of torque off the line anyone have any suggestions on how low to gear to? 308s or 270 something. Also where to get the new gears from?

Thnak ruti

ruti@comcast.net

Hey Ruti,

My friend did the rear in my pickup recently. He is partial to Richmond Gears but I'm sure any reputable aftermarket company would do. As for gear ratio, 3:08 is probably as low as you want to go. You could go as low as 2:73 but even that torque monster you have under the hood will tend to bog if you put your foot into it off the line. I would probably look into the 3:30 - 3:40 ratio for your setup. I am guessing you have a Turbo 350 or 400 (with a final gear ratio of 1:1).

The tires you have also play onto the high RPM's on the highway. The width is not as important for RPM's, but the height plays a crucial part. The lower the tire height, the higher your RPM's are at top speed.

Have you considered swapping your trans for an overdrive unit like a THM 700R4 or a 4L80E? They are not cheap (especially to beef up to handle a 454) but they will solve your highway RPM issue instantly since the final drive ratio is lowered to roughly .85:1 rather than 1:1.

There is also a company (Gear Vendors - www.gearvendors.com) that makes an add-on overdrive gear for the THM 350 or THM 400 trans. They are not also cheap, but you only have to add the new tailcone with the gear onto your existing trans and shorten your driveshaft as opposed to reworking the underside of your car for a different trans setup.

Good luck and keep us posted as to what you decide on . . .

Dan

Dan thanks for the info. I would like to keep the car original so I will opt out for just the gear change your advice 3.08 is just what my mechaninc suggested with the same fear about over gearing for off the line performance. I will post my results when I finish.

As an odd aside, this car will spin the tires but only for a short distance as the speed of the car quickly catches up to the tire and engine speed with such tall gears , I am looking forward to smoking up the neigborhood

Ruti

Overdrive is the way to go! And with a big block a 2.73 gear is fine.

My 454/700R4/2.73 rear/2.75-50X15 tires turns about 1800 at 75mph, runs 8.60 in the 1/8th mi@80 mph, and gets 20mpg.

In a few weeks i'm going to a 3.31 for improved 1/8 mi times. If it doesn't hurt my fuel milage much I may go as low as a 3.55 gear, but no lower. The 3.31 would put me at around 2200 RPM@75, still reasonable.

Ruti.

I would take great pains to get a good stout overdrive if I were you and did go that route. That big block can tear a stock one up pretty quick.

Its pretty simple to figure
MPH X Gear Ratio X Final drive Ratio X 336 / Tire diameter = RPM
60 X 3.73 X 1 X 336 / 26 = 2892.1846
60 X 4.08 X 1 X 336 / 26 = 3163.5692

http://www.drivetrain.com/rearend.html

I found this handy chart of final drives that calculates RPM at 60 mph at the above site makes it real easy to determine

Thanks

Ruti

Here's another good calculator

http://www.chevellestuff.com/generic/rpm_calculator.htm