Help understanding Gear Ratios

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cgc68

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May 4, 2008
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Location
Southeast IL
I am looking for some help with my education. Gear ratios is something I know very little about. In my 68 Corvette I am running a mostly stock 327 with a TH350 and 3.70 rear end. Automatic Transmission Gear Ratios for the TH350 are 1st gear 2.52, 2nd gear 1.52, and 3rd gear 1.00. What can you tell me about these nunbers? Will I be running excessive RPM's at highway speeds? Is this why I can't do burn outs? How does different rear end ratios affect power/RPM's. I hope this makes some kind of sence. It's hard to ask an intellegent question when you know very little about the subject.

charlie
 
As far as I know all TH350 have the same gear ratios, no overdrive. The higher the number the "shorter" the gearing.
I used to run the TH 350 with 3.55 rear gears and 70mph on the highway was no fun, the engine was revving around 3000rpm at 70 mph as far as I remember.

With your 3.70 gears it will be even worse. Acceleration will be pretty good though :cool:

The number 3.70 for example means that it takes 3.7 revolutions of your driveshaft to make one revolution on the halfshafts.

The math is actually very simple: let's say you have a 27" diameter wheel, the circumference of that wheel is (27X3.1415) 84.82". Per revolution of your wheel you're traveling 84.82 ".

One mile is 63360", 70miles is 4435200", devided by 84.82" that's 52289 revolutions in one hour or 871.48 revolutions in a minute (at 70mph)

871.48X3.7 (gear ratio) = 3224.48 revolutions per minute on your driveshaft

your last gear in the TH350 is 1.00 so that's your engine RPM (3224.48 at 70mph)

With a 0.68 overdrive like my TKO600 the engine rpm drops to 2192.65 at 70 rpm
 
Thank you so much MYBAD79. Your answer was just what I was looking for. My car was a manual at one time but I had to change it to an Auto to get the ok from the wife to buy it. My tack doesn't work but I always thought the motor RPM's was high at highway speed and the car likes it's gas too. You are right about the acceleration. So is there a good compromise with this transmission??

charlie
 
cgc68 said:
So is there a good compromise with this transmission??
Click to expand...

I don't believe there is a good compromise between highway driving, acceleration and a TH350.

If you change the rear gears you will loose acceleration but the highway rpm are still higher than with a overdrive transmission.

871.48X3.23=2814.88

I think 3.23 gears are available, that would drop the rpm at 70mph from 3200 to 2800.

I'd look into 700R4 or 200R4 conversions if it must be a auto trans.

Not sure what the overdrive ratio on these transmissions is but even if it's 0.8 the rpm will drop from 3200 to 2600.
 
Methink Karsten must be a engineer, too much with the calculator there, I B lazy and go to this site....www.secondstrike.com /Technical/GearCalc

and just enter the information.....I have a print out for my car.....

the 700 & 200 4r have the final drive listed as .68 or .70 depending on your information source....

The 200 4r is about a direct bolt in swap....have to do the converter lockup function, and kickdown/throttle cable....

with my 336 gearset, I get 2300 rpm at 80 mph cruise....

I know the tach is accurate.....


:smash::eek:
 
Gears need to be selected with respect to the engine in your car. If your engine produces its best torque higher up in the rpm-band you need to select a higher numerical gear in order to get the engine in that rpm-band at the speeds in which you drive most.

The transmission provides you with gears to get the cars launched at start/lower speed.

To know the total gearratio, you need to multiply the gear ratio of the axle with the gear ratio in the trans in which you drive.

Overall, most agree that in first gear you should have a total gear ratio of minimum 9:1 to get the car going.

For example : most M21 and M22 transmission which had a 2.20 first used a 3.70 and even a 4.11 to get the car going in first. Of coarse the downside are higher rpm's at 60/70 mph.
The positive side of these transmissions was that the gearspread was more equal and closer spaced together which produced a stronger, racelike combo.
 
Belgian1979vette said:
Gears need to be selected with respect to the engine in your car. If your engine produces its best torque higher up in the rpm-band you need to select a higher numerical gear in order to get the engine in that rpm-band at the speeds in which you drive most.
Click to expand...

agree..... and that is exactly why a overdrive trans is a better choice than just swapping the diffy gears. The lower gears are pretty much the same (might be slightly different between TH350,200R and 700R, not sure), you just add another gear for highway driving :thumbs:

The current 3.70 diffy gears will tolerate a slightly longer (numerically lower) 1st or 2nd gear).
 
That is why i went with a 4.11. I hope one day I'm able to buy a tremec, in the mean time she will be a bitch to drive on the highway but fun to drive around town.
 
The TKO600 is worth every penny, even if the engine is a stock 350. The trans makes a bigger difference that anything you do to the engine..... it's the best modification I have done so far.... :D
 
"The 200 4r is about a direct bolt in swap....have to do the converter lockup function, and kickdown/throttle cable.... "

What is this converter lockup function and how hard and expensive is it?? What is the availability and source for a 200 4r?


"Gears need to be selected with respect to the engine in your car. If your engine produces its best torque higher up in the rpm-band you need to select a higher numerical gear in order to get the engine in that rpm-band at the speeds in which you drive most."

I have no idea where in the rpm-band the best torque is produced. My car will be driven mostly around town and short highway trips with few longer highway trips.

Thanks guys. Your posts are right on. I have learned so much. Still digesting your inputs. Hope to hear more.

charlie
 
rpm band is determined mostly by your cam and then by intake, exhaust and so on. Hard to say just by looking at it. Probably you notice when it starts to 'pull real strong' and where the 'pull' of the engine drops of.
If by example you have a cam that has a power band in between 1500 and 3500-4000 you can stay with a long axle (3.20 and lower)
 
cgc68 said:
What is this converter lockup function and how hard and expensive is it?? What is the availability and source for a 200 4r?
Click to expand...

T/C lock up is a clutch in the T/C itself that engages at higher speed/gearing to prevent slippage and improve milage. The clutch may be activated electrically, or set up to lock up hydraulically.
Keep in mind, both the 700-R4 and the 200-R4 BOTH have some good points and bad points. :huh:
The 700-R4 was not built well till after 1988. The 200-R4 has a plastic spacer between shafts that when it goes, the tranny is done.:clobbered:
 
cgc68 said:
"The 200 4r is about a direct bolt in swap....have to do the converter lockup function, and kickdown/throttle cable.... "

What is this converter lockup function and how hard and expensive is it?? What is the availability and source for a 200 4r?


"Gears need to be selected with respect to the engine in your car. If your engine produces its best torque higher up in the rpm-band you need to select a higher numerical gear in order to get the engine in that rpm-band at the speeds in which you drive most."

I have no idea where in the rpm-band the best torque is produced. My car will be driven mostly around town and short highway trips with few longer highway trips.

Thanks guys. Your posts are right on. I have learned so much. Still digesting your inputs. Hope to hear more.

charlie
Click to expand...



We sell ART Carr 200R4s here at CI. We sell about two a month.:D
 
Please be patient with me and forgive my ignorance but what are ART Carr and CI?
How much??

charlie
 
cgc68 said:
Please be patient with me and forgive my ignorance but what are ART Carr and CI?

charlie
Click to expand...

Unauthorized & nonsupporting vendors/shops who should be summarily thrown off of this forum for supplying quality products, information & work without paying us our percentage!!!!


Oooops--sorry, wrong forum.


Nevermind!:goodnight:
 
CI- Classic Industries. Art Carr is one of the most well known Auto Trans builders.



:blush::blush::blush:
 
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