Gear Ratio Change?

[JENSHAMMER@HOTMAIL.COM]

Thinking of changing out my 3.08's with some new gears, anywhere between 3.50 something an 4.11. This is my daily driver, and most of my driving is from light to light, with occasional long stretches on I-5. I'm running a 200-4R overdrive, obviously I want to have my cake and eat it too, but what would be the best ratio to help milage around town without ruining the highway cruise? Keep in mind I'm only pushing about 220HP now, but in less then a year hope to have 300+. Thanks!!

 

[81 Corvette]

here we go again. seems like its the "in" mod to do this year on this forum

anyways, i swapped from 2.87 to 3.54 in my 81 w/ th350. For me, the 3.54 was the perfect gear. I don't have od so i didn't want to get carried away and go w/ 3.73 since highway cruising rpm would be nuts. i turn 2600 rpm when going 60, so multiply 2600 times your OD ratio to come up with your rpm's at 60 mph

imo, a 3.54 gear is perfect for my long term application of getting the HP up to 400 and an OD tranny. i figure that if the c5 can do what it does with a 3.42 i should be able to match it in a few years with the 3.54.

i'm sure there be plenty of more input on this by some other members.

anyways, my suggestion is to put together a spreadsheet and compare the various gears and find the one that will work best for your driving style.

 

[Glensgages]

81 Corvette did it this year, and I had it done last year to my '82:
it'll really wake-up the performance of the typical 'wimpy' late-C3.

To calculate your cruise-RPM at any given MPH with any-other gear than 3.08, simply divide whatever gear you are considering (for example, 3.36:1) by your current 3.08s, and multiply that number by your current cruise RPM at that MPH.

Example:
if you are turning 1600 RPM at 60 MPH with your 3.08s (a rough guesstimate, based-on 27" tires, lock-up converter, and .7:1 OD-ratio in the 200-4r), your equation would look like this:

3.36 divided-by 3.08 = 1.090909 x 1600 = 1745 RPM at 60 MPH with 3.36 gears

with 3.55 gears, it'd be 1844 RPM at 60 MPH, with 3.73s, it'd be 1937, and with 4.11s, it'd be 2135.

 

[81 Corvette]

I just took a look at my excel spreadsheet and adjusted it for a .80 over drive. I don't know if that is your gear or not, but if it is, and it was my car, i'd probally opt for the 3.73. That would give you a cruise rpm of 2600 @ 70 mph, and should also give you a really nice off the line. Glen will tell you to factor in torque converter slippage which i havn't built into my spreadsheet since i have a lock up converter

the problem then becomes, if you bump up your HP, at what point do you start spinning your tires at the line when you mash the gas, which tells me 3.54 is a good gear to have. I guess i'm just biased since thats the gear i choose.

 

[81 Corvette]

see i told ya Glen would say that

btw, i can send you my excel spreadsheet if you want to compare various gear ratio's

its only set up for the final drive ratio, but if you give me you tranny gears, i can set it up to show your speed in each gear based on a given rpm input.

 

[Glensgages]

..... the problem then becomes, if you bump up your HP, at what point do you start spinning your tires at the line when you mash the gas, which tells me 3.54 is a good gear to have.
I guess i'm just biased since thats the gear i choose.

This is, by my estimate, what is wrong with the early OD transmissions:
1st gear is too low, OD is fairly-tall, and the gear-ratios are far apart.

Other than the 3.73s, my '82 is box-stock, and even with IT'S pathetic Cross-Fire, it can spin the tires with ease in Low Gear, which is great if yer trying to impress "High-school Harry" and the 17 year-old tenderonies.....
but on the 1-2 gear change, going from 3.06 to 1.63 ratios in the 700R4 is just too-much for the L-83 (imagine what is was like BEFORE the 3.73s, with the OEM 2.87s.....):
it literally 'falls' on it's own face.


If ya make enough 'smoke', ala a stroker 383, a 406, or a ZZ502, it can 'pull' that big gear ratio/RPM drop with ease, but with that kind of torque, you wouldn't need such low-1st gear/ tall OD ratios, resulting in the wide spacing between ratios.....
:crazy

The 'newer' Corvettes, with their computer-assisted high-tech fuel-injection systems, keep the engine making maximum power at all times, re-adjusting and re-calibrating several times per second:
that's almost impossible with an older Corvette, using a carb.....

I guess I'm just "used-to" driving cars with deep rear-gearing, and 'traditional'-type automatics (THM350/400), with closer gear-ratios (2.48, 1.48, 1:1) and a 'loose' converter.....

 

[JENSHAMMER@HOTMAIL.COM]

Well this has really helped me with the information I need. Sadly, I dont know my gear ratio's in the tranny, I would have to email bowtie overdrives for that. Since I fell off the money wagon (I.E., constant military paycheck) I will probably take the 3.73's since I dont know when I actually will be able to get the horsepower I deserve. 390HP with 400 lb/ft of torque... is that too much to ask? No. Is it too much to recieve? I dont think santa's gonna be smokin reindeer hooves to get me a crate motor this year. I appreciate the help everyone! Once again, CAC forums spreads the knowledge.

"Dimebag" Darrell Abbot RIP Dec 8th, 2004.