Differential ratio
Thread Starter
Le Mans Master
Joined: May 1999
Posts: 6,692
Likes: 29
From: Texas
Differential ratio
What would be the best ratio to "upgrade" my stock 82 rear end to something better. 308, 355 seem to be the choices. Maybe 200 miles per month. Lets just say its a stock engine. Have done things, but, really, its an 82. Have not broken open motor, so anything changed is external.
Racer
Joined: Mar 2005
Posts: 393
Likes: 3
From: Dallas, TX
with a 4.10 or a 4.56 youll snap your neck if you mode the engine a little, if your not mechanically savvy you could buy a new diff from ecklers, or mid america or somewhere like that, one of them has a 4.30 equipped diff for around $1k. good luck
-alex
-alex
Race Director
Joined: Jan 2003
Posts: 15,172
Likes: 86
From: State of Confusion
Originally Posted by Larry82
What would be the best ratio to "upgrade" my stock 82 rear end to something better. 308, 355 seem to be the choices. Maybe 200 miles per month. Lets just say its a stock engine. Have done things, but, really, its an 82. Have not broken open motor, so anything changed is external.
I put 'em in my other-wise stock '82, and I like 'em a bunch.
Now..... everything IN-FRONT of the pinion-gear.....
Drifting
Joined: Aug 2003
Posts: 1,424
Likes: 0
From: Lakeland FL
Originally Posted by Larry82
What would be the best ratio to "upgrade" my stock 82 rear end to something better. 308, 355 seem to be the choices. Maybe 200 miles per month. Lets just say its a stock engine. Have done things, but, really, its an 82. Have not broken open motor, so anything changed is external.
Depends on what you are trying to accomplish. As previously said, something in the 4's will give you neck snapping take offs but could very well kill your gas mileage and you will probably be running high RPMs at any kind of highway speed. If you're only driving 200 miles per month, that may not matter to you.
Race Director
Joined: Jan 2003
Posts: 15,172
Likes: 86
From: State of Confusion
Originally Posted by ShinodaVette
Depends on what you are trying to accomplish. As previously said, something in the 4's will give you neck snapping take offs but could very well kill your gas mileage and you will probably be running high RPMs at any kind of highway speed. If you're only driving 200 miles per month, that may not matter to you.
I believe with 4.11s, it'd be 2200+ at 60, and if you'd run .56s, it'd be almost 2500 @ 60 MPH.
Racer
Joined: Mar 2005
Posts: 393
Likes: 3
From: Dallas, TX
if he only drives it 200miles a month and only hits 60 the few times he drives it well...why not make those 200 miles as much fun as possible right. 2500rpms on the highway isnt that bad, try it with side pipes with and then with out the inserts, loads of fun, in any event get the 4.11s or something like that, so what if your mpg blows, you dont drive the car enough for it to make a huge dent in your wallet.
-alex
-alex
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Racer
Joined: Oct 2006
Posts: 473
Likes: 6
From: New Port Richey FL
From what I calculate.... Based on a 255/60/15 tire and a 1:1 final gear in your trans...
It would look something like this:
RPM 2.73 3.08 3.55 3.73 4.11 4.56
1000 29.5 26.1 22.7 21.6 19.6 17.6
1500 44.2 39.2 34.0 32.3 29.4 26.5
2000 58.9 52.2 45.3 43.1 39.1 35.3
2500 73.6 65.3 56.6 53.9 48.9 44.1
3000 88.4 78.3 68.0 64.7 58.7 52.9
3500 103. 91.4 79.3 75.5 68.5 61.7
4000 117. 104. 90.6 86.2 78.3 70.5
4500 132. 117. 101. 97.0 88.1 79.4
5000 147. 130. 113. 107. 97.8 88.2
5500 162. 143. 124. 118. 107. 97.0
6000 176. 156. 135. 129. 117. 105.
Sorry its hard to read, the text editor limits spaces to two so I can't spread the columns out. Basically the chart says with 2.73s you'll turn about 2000 rpm to do 60mph (actually 58.9mph). 4.11 you'll be turning 3000 rpm (and go 58.7mph). It doesn't take into account drag. No you can't put in a 2.73 gear and make it to 180mph.
It would look something like this:
RPM 2.73 3.08 3.55 3.73 4.11 4.56
1000 29.5 26.1 22.7 21.6 19.6 17.6
1500 44.2 39.2 34.0 32.3 29.4 26.5
2000 58.9 52.2 45.3 43.1 39.1 35.3
2500 73.6 65.3 56.6 53.9 48.9 44.1
3000 88.4 78.3 68.0 64.7 58.7 52.9
3500 103. 91.4 79.3 75.5 68.5 61.7
4000 117. 104. 90.6 86.2 78.3 70.5
4500 132. 117. 101. 97.0 88.1 79.4
5000 147. 130. 113. 107. 97.8 88.2
5500 162. 143. 124. 118. 107. 97.0
6000 176. 156. 135. 129. 117. 105.
Sorry its hard to read, the text editor limits spaces to two so I can't spread the columns out. Basically the chart says with 2.73s you'll turn about 2000 rpm to do 60mph (actually 58.9mph). 4.11 you'll be turning 3000 rpm (and go 58.7mph). It doesn't take into account drag. No you can't put in a 2.73 gear and make it to 180mph.
Last edited by Green73; Dec 1, 2006 at 12:02 AM.
Racer
Joined: Oct 2006
Posts: 473
Likes: 6
From: New Port Richey FL
well then this would be for .7 overdrive
RPM 2.73 3.08 3.55 3.73 4.11 4.56
1000 40.6 36.0 31.2 29.7 27.0 24.3
1500 60.9 54.0 46.9 44.6 40.5 36.5
2000 81.2 72.0 62.5 59.5 54.0 48.6
2500 101. 90.0 78.1 74.3 67.4 60.8
3000 121. 108. 93.7 89.2 80.9 72.9
3500 142. 126. 109. 104. 94.4 85.1
4000 162. 144. 124. 118. 107. 97.3
4500 182. 162. 140. 133. 121. 109.
5000 203. 180. 156. 148. 134. 121.
5500 223. 198. 171. 163. 148. 133.
6000 243. 216. 187. 178. 161. 145.
RPM 2.73 3.08 3.55 3.73 4.11 4.56
1000 40.6 36.0 31.2 29.7 27.0 24.3
1500 60.9 54.0 46.9 44.6 40.5 36.5
2000 81.2 72.0 62.5 59.5 54.0 48.6
2500 101. 90.0 78.1 74.3 67.4 60.8
3000 121. 108. 93.7 89.2 80.9 72.9
3500 142. 126. 109. 104. 94.4 85.1
4000 162. 144. 124. 118. 107. 97.3
4500 182. 162. 140. 133. 121. 109.
5000 203. 180. 156. 148. 134. 121.
5500 223. 198. 171. 163. 148. 133.
6000 243. 216. 187. 178. 161. 145.
Thread Starter
Le Mans Master
Joined: May 1999
Posts: 6,692
Likes: 29
From: Texas
Looks like the 355 is it for me. Freeway driving is a good bit of the 200 miles. Probably 60%. But, 2500 rpm or so is not bad. Car seems to run better at the higher rpms. And, how do I describe this, you can't seem to hit the lower gear if you are at, say, 20 mph and want/need to take off. The 355 would help solve that.
355 is about 30% bigger than 273. Is it reasonable to expect a 30% +,- decrease in 0-60 times? Stock 82 is rated at 8.1 seconds (terrible), but mine, based on a G-Tech will hit about 7. 30% improvement would put me about mid-6's? Which I would be most happy with.
355 is about 30% bigger than 273. Is it reasonable to expect a 30% +,- decrease in 0-60 times? Stock 82 is rated at 8.1 seconds (terrible), but mine, based on a G-Tech will hit about 7. 30% improvement would put me about mid-6's? Which I would be most happy with.
Race Director
Joined: Jan 2003
Posts: 15,172
Likes: 86
From: State of Confusion
Originally Posted by Larry82
Car seems to run better at the higher rpms. And, how do I describe this, you can't seem to hit the lower gear if you are at, say, 20 mph and want/need to take off. The 355 would help solve that.
as 'weak' as a stock Cross-Fire is, if you don't keep it some-what wound-up, it is even WORSE:
my car still lacks, but it a MUCH-more enjoyable ride with the .73s.....
Originally Posted by Larry82
355 is about 30% bigger than 273. Is it reasonable to expect a 30% +,- decrease in 0-60 times?
1st Gear will be incredibly quick off the line, but it'll still 'bog' with the 1-2 gear-split being so-wide (3.06-to-1.63 ratios ).....
4th Gear
Joined: Nov 2006
Posts: 4
Likes: 0
I am new to this site and only recently aquired an 80 C3. The rear had been run wothout gear oil and I did a total rebuild. Based on experience with other cars, I went replaced the 307s with 307s. Having had 411s in another car, the only enjoyment was at the track. Nothing worse than driving down the freeway with all the metrics and old K cars zipping by you. I get decent gas mileage and don't hesitate to take my car out of state on long trips. I have gone past the 85 MPH spedo many times, keeping up with all the fast traffic with my stock (305 has been replaced with a 350) CA smog legal engine. The car has plenty of power and great handling for the country roads. Your car is, after all, stock. If you want to snap your neck, really snap it with the gears and engine to match. Otherwise, you will have something between a muscle car and stock one that does not do a good job for either and lousy gas mileage to boot.
Drifting
Joined: Sep 2006
Posts: 1,259
Likes: 2
From: MS
No overdrive in 1980 so really can't compare.
He has a factory installed 700R4 so you can run the lower gears and still cruise the highway.
I agree with you though, absolutely can't stand low gears on the highway.
He has a factory installed 700R4 so you can run the lower gears and still cruise the highway.
I agree with you though, absolutely can't stand low gears on the highway.
Safety Car
Joined: Nov 2004
Posts: 4,359
Likes: 57
From: Mundelein Illinois
I drive mine a lot on the highway. I have a 81 with 4 speed, rear end 2.72. At 70 MPH 2500 RPM. I changed the rear end to a 3.55. Car was great around town but on the highway, 70MPH 3400 RPM. The drive to Effingham would of been two tanks of gas and accesories on motor taken a beating. Put the original 2.72 back in it.
I just ordered a 5 speed and going back to the 3.55 rear end. Advantages are 1 gear will put you in the seat and fifth gear for highway 70MPH at 2000 RPM.
I would leave the car alone and just drive it. or maybe add a B&M shift kit.
I just ordered a 5 speed and going back to the 3.55 rear end. Advantages are 1 gear will put you in the seat and fifth gear for highway 70MPH at 2000 RPM.
I would leave the car alone and just drive it. or maybe add a B&M shift kit.
Thread Starter
Le Mans Master
Joined: May 1999
Posts: 6,692
Likes: 29
From: Texas
Decided on the 355. Took car to shop and should be ready next Saturday. Will be interesting to see and feel results. Did not want to go to a higher ratio so as to not kill freeway driving. Looks like about 2600+ rpm. Not too bad. But from the chart, will only be able to hit 187 mph rather than 243 mph with the current 273 
Thread Starter
Le Mans Master
Joined: May 1999
Posts: 6,692
Likes: 29
From: Texas
Found out that after 203,000 miles the pinion gears are worn badly, the bearings are worn badly and the pinion gear and seal also. Authorized a full rebuild, about the same price as an Ecklers/Mid America ship-it-to-me-pay-core-charge-hope-you-get-something-back-pay-shipping-both-ways price.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Originally Posted by 1fastC3
if he only drives it 200miles a month and only hits 60 the few times he drives it well...why not make those 200 miles as much fun as possible right. 2500rpms on the highway isnt that bad, try it with side pipes with and then with out the inserts, loads of fun, in any event get the 4.11s or something like that, so what if your mpg blows, you dont drive the car enough for it to make a huge dent in your wallet.
-alex
-alex
A 4.11 wont hurt gas mileage. If your mileage is bad it will not be from the gear. To best illustrate, my 61 with a 4.11 at freeway speed and over [mostly over] is a flat 20 MPG.
Safety Car
Joined: Aug 2000
Posts: 3,909
Likes: 26
From: Clymer PA
Originally Posted by Ironcross
A 4.11 wont hurt gas mileage. If your mileage is bad it will not be from the gear. To best illustrate, my 61 with a 4.11 at freeway speed and over [mostly over] is a flat 20 MPG.