Rear End Gears

[Tom B.]

My 1959 Corvette has 411 Gears Im in 4th gear at 35 MPH I want to change
to gears that allow me to go 70 MPH or higher Now I get to about 60 MPH and tack
about 3000 Any suggestions on what I should change to?
Thanks
Tom Brennan 60456

[Joe L.]

Re: Rear End Gears

My 1959 Corvette has 411 Gears Im in 4th gear at 35 MPH I want to change
to gears that allow me to go 70 MPH or higher Now I get to about 60 MPH and tack
about 3000 Any suggestions on what I should change to?
Thanks
Tom Brennan 60456

Tom------

The reason that most C1 Corvettes had high numerical rear gear ratios was that the 283 engines did not have a lot of torque. The high numerical gear ratios helped to "compensate" for that. To give you a meaningful reduction that you're after you'd have to go to about 3.55:1. However, if you do that you might not like the acceleration you end up with.

[Jim L.]

Re: Rear End Gears

Tom, my FI '60 came to me with 4.11 gears and I drove it from SC to CA with them. Not the most pleasant experience.

I've since changed the gears to 3.55s and am quite happy with the change. Cruising RPM is about 14% less and overall cabin noise is noticeably less (my car has the off road mufflers). Some will tell you that you'll do a lot of clutch slipping to get moving if you have a close ratio transmission, but I don't find that to be the case. I think my car is extremely easy to drive.

Bottom line, I recommend the 3.55 ratio.

Jim

[Terry D.]

Re: Rear End Gears

Tom
Try this website, you put in the rpm, tire size, rear end ratio, etc and it will tell how fast you are going. Not 100% accurate, but close enough to do what you want. http://shiftsst.com/speed-analyzer

Terry

[Tom B.]

Re: Rear End Gears

A 3.55 gear is not bad I personally like a 3.36 I think it gives the best compromise in all situations.

[Duke W.]

Re: Rear End Gears

Tom------

The reason that most C1 Corvettes had high numerical rear gear ratios was that the 283 engines did not have a lot of torque. The high numerical gear ratios helped to "compensate" for that. To give you a meaningful reduction that you're after you'd have to go to about 3.55:1. However, if you do that you might not like the acceleration you end up with.

Back in the fifties there were no interstate highways and most US and state highways had speed limits of 55 MPH. Maybe a few were as high as 65.

So short gears were the norm, especially on performance and sports cars. With OE revs/mile (760) tires, revs at 60 are 760 X 4.11 = 3124 or 3644 at 70.

At 3000 revs on a 283 mean piston speed is only 1500 ft/min, which is actually quite low. HD truck diesels with a six inch strokes are usually governed at 1800 RPM, which is 1800 ft/min, but they cruise on the flat at about 1200 revs, which is 1200 ft/min. Large marine diesels with strokes up to 7 feet (yes, I mean feet) run at about 1100 ft/min in the 90-100 RPM range.

So stress and engine wear cruising at about 70/3650/1825 FPM is really not going to cause excessive wear.

A couple of years ago I accompanied a local chapter member from LA to San Diego for a chapter meet. His '65 FI coupe had 4.11s so we were cruising at about 70/3600 to stay with the flow of traffic. The engine was very smooth and was basically loafing. John wanted to do more road trips, so he installed a 3.36. It's a little quieter, but a bit tougher to get launched with the very high overlap 30-30 cam and has sluggish on acceleration compared to the 4.11s.

You didn't say what your engine configuration is, but a base cam engine (230, 245, 250 HP) will have more low end torque than a Duntov cam engine and will be more compatible with a taller gear.

A 3.55 represents a about a 15 percent reduction, so revs at 70 will drop from about 3650 to 3150, which is not a huge reduction.

A better combination would be to convert your CR four-speed to a WR and install a 3.36 or even 3.08 which would drop revs at 70 to about 3000 and 2750, which is still double what modern Corvette engines are turning at freeway cruise speed.

I think that's an issue with a lot of guys. They see their vintage Corvette engine is "screaming" at freeway speed compared to a modern big V-8 car and think the engine is going to wear out, but that's not really true.

So give it some thought. Gear changes are expensive and there are tradeoffs, such as poorer acceleration, so maybe you should live with what you have and the understanding that design philosophy and driving conditions have changed a lot in the last 50-60 years.

Duke

[Richard P.]

Re: Rear End Gears

Tom,
I faced a similar issue with my 1956. 4.11's and a close ratio 3 speed. After some research and soul searching I went to 3.55 and a WR Muncie (gasp!) 4 speed. The acceleration final drive gear ratios are similar to a 3.70 CR with the lower RPMs in 4th gear. I have been pleased with the result. With a Hurst competition plus shifter and a bolt on reproduction stick the change is not obvious. With a 3 speed shift pattern on the console it also is a theft deterrent (of minimal value).
Rick

[Tom B.]

Re: Rear End Gears

Im still contemplating changing my gears My 59 Vette has a 283 with 2 4 barrel carbs 245 Horse It has been bored to 302 and
has a roller cam I assume I have 411 gears now Does replacing them do anything to the value of the car does 356 gears enough to
make a difference? If I go to 336 wikk I be unhappy with the low end power?


Thanks
Tom Berennan 60456

[Duke W.]

Re: Rear End Gears

An OE 245 HP engine would probably work okay with the CR trans and 3.36 as long as you don't plan on racing V6 Camrys from stoplights of have to start on hills often. The OE cam is very torquey and could likely handle it.

I'm not so sure with a hot rod roller cam. Chances are it has a lot less low end torque due to excessive overlap. Measure manifold vacuum and stated idle speed and post it here. That will be a good indicator.

Duke

[Jimmy G.]

Re: Rear End Gears

I have a 1960 270 HP and a 3.36 Works like a champ with five NCRS National Road Tour stickers on the windshield including coast to coast in 1999 and Canada later on. Car can cruise at 100 with no issues and often does as long as kepp the oil plug in it i.e. "Blowout Award" winner in Hampton

[Tom B.]

Re: Rear End Gears

I decided to go with 336 gears The car runs good It appears the speedometer is not accurate Also the tach Is that
what happend when you switch gears What is the best method to correct Thanks Tom Brennan (60456)

[Joe L.]

Re: Rear End Gears

I decided to go with 336 gears The car runs good It appears the speedometer is not accurate Also the tach Is that
what happend when you switch gears What is the best method to correct Thanks Tom Brennan (60456)

Tom------


With a rear gear ratio change, the speedometer will not be accurate. You will need to also install new speedometer gears. In your case because of the change from 4.11:1 to 3.36:1, you will have to change BOTH the speedometer drive and driven gears. Changing the speedometer drive gear will require removal of the transmission from the car and partial diassembly. The only other alternative would be to install a speedometer adapter.

The rear gear change should have no effect, at all, on the tachometer, though.

[Richard P.]

Re: Rear End Gears

The speedometer error can be corrected by changing the driven speedometer gear in the transmission. Not a difficult task. The tach should not be affected by the rear end change.

[Joe L.]

Re: Rear End Gears

The speedometer error can be corrected by changing the driven speedometer gear in the transmission. Not a difficult task. The tach should not be affected by the rear end change.

Richard------


Changing the speedometer DRIVEN gear is quite easy. However, given that the original rear gear ratio was 4.11:1 and the current ratio is 3.36:1, BOTH the speedometer DRIVE and DRIVEN gears need to be changed. Changing the DRIVE gear is not so easy.

Put another way, there is no DRIVEN gear that is compatible with the current DRIVE gear that will give an accurate speedometer reading with the 3.36:1 rear gear ratio.

[Terry D.]

Re: Rear End Gears

Tom
I agree with Joe but, if you don't want to pull the tranny you can get close with changing the driven gear and changing diameter of rear tire. They are lots of calculators out there to figure this out. (just google it or go to the Corvette Forum). When you get close then I would use a GPS to see how far off and just compensate for the error by remembering what you tach at a few posted speeds.

Terry