Gears - do I have 4.11's?
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
Gears - do I have 4.11's?
I have a TH400, and 26.035 diameter tires (255/45/R17).
Attached is a pic of my speedo and tach.
According to an online gear calculator, I should have 4.11 gears, but that also means I should be able to roast my tires for days, right? If so, that's not the case in my situation. She's kind of a dog off the line.
Going about 68 mph at 3400 rpm.
Attached is a pic of my speedo and tach.
According to an online gear calculator, I should have 4.11 gears, but that also means I should be able to roast my tires for days, right? If so, that's not the case in my situation. She's kind of a dog off the line.
Going about 68 mph at 3400 rpm.
Last edited by Killingsworth 73; Jul 10, 2014 at 05:01 PM.
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
According to this:
http://www.angelfire.com/fl/procrastination/rear.html
with 3.70s, I would be going 71mph.
Maybe I should look at the difference at a higher MPH?
http://www.angelfire.com/fl/procrastination/rear.html
with 3.70s, I would be going 71mph.
Maybe I should look at the difference at a higher MPH?
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
But I am with you, it could go either way. I "gave it some gas" the other day, and sprinted up to 120 for about 5 seconds, and with 4.11s I would have to be at 6400 rpms to do that. Pretty sure I didn't go over 5800..
Also I'm not sure how accurate these old speedo's are at that speed......
Also I'm not sure how accurate these old speedo's are at that speed......
Racer
Joined: Aug 2009
Posts: 363
Likes: 72
From: Somers New York
Jack up the rear end of your car and mark the tire and driveshaft. then count how many times the driveshaft turns to make the tire turn one revolution. If the shaft turns 4 and a bit, then you got 4.11's. if it turns a bit less than four, then you cot 3.70's.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
This is the best way to tell what your diff gear is, short of removing the cover and counting the teeth. Trying to guess what your diff gear is using the stock speedometer and tach is an exercise in futility, as they can both be wildly inaccurate, never mind accounting for transmission slip.
Scott
Drifting
Joined: Dec 2004
Posts: 1,714
Likes: 42
From: Austin Texas
There is a code on the bottom of the rear end I believe. Here is a site that you can use to decode it.
http://corvettec3.ca/axle.htm
Also, I think sites like http://tri-5.chevyrides.com/rpm.php can get close for this purpose but I don't think they account for converter slippage. I would think you need to add a few hundred rpms to the calculator number.
I hope that helps.
Cheers
http://corvettec3.ca/axle.htm
Also, I think sites like http://tri-5.chevyrides.com/rpm.php can get close for this purpose but I don't think they account for converter slippage. I would think you need to add a few hundred rpms to the calculator number.
I hope that helps.
Cheers
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Burning Brakes
Joined: Mar 2014
Posts: 904
Likes: 46
From: Crestline CA
I doubt the accuracy of c3 speedos. I thought I had 4.11s since the speedo to tach relation showed it but then drove with a accurate GPS based speedo and found them to be 3.70s. If you have a accurate tach, a accurate hand held GPS with a speed read out and you know the tire diameter you can figure out the ratio pretty accurately. Automatic transmissions foggy it up a bit due to torque converter slip but under low load the slip isnt much.
The problem of doing the count the drive shaft revolutions method is that, depending on your shocks, the half shaft u-joints bind up with the suspension in full droop.
The problem of doing the count the drive shaft revolutions method is that, depending on your shocks, the half shaft u-joints bind up with the suspension in full droop.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
+1
3.70 and the short *** tire... Assuming your speedo is correct....Stock converter?
My dad has 3.70s with a 26.5" tire and 70 mph is 3500 rpms.... I've never GPS's his speedo tho...
We have 3.90s and a 28" tire on one of our other cars and 60 mph is 3000 rpms on the nose.
Mine with 3.36s, 28" tire is around 2900 rpms at 70 mph....however that is with some converter slip...
3.70 and the short *** tire... Assuming your speedo is correct....Stock converter?
My dad has 3.70s with a 26.5" tire and 70 mph is 3500 rpms.... I've never GPS's his speedo tho...
We have 3.90s and a 28" tire on one of our other cars and 60 mph is 3000 rpms on the nose.
Mine with 3.36s, 28" tire is around 2900 rpms at 70 mph....however that is with some converter slip...
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
+1
3.70 and the short *** tire... Assuming your speedo is correct....Stock converter?
My dad has 3.70s with a 26.5" tire and 70 mph is 3500 rpms.... I've never GPS's his speedo tho...
We have 3.90s and a 28" tire on one of our other cars and 60 mph is 3000 rpms on the nose.
Mine with 3.36s, 28" tire is around 2900 rpms at 70 mph....however that is with some converter slip...
3.70 and the short *** tire... Assuming your speedo is correct....Stock converter?
My dad has 3.70s with a 26.5" tire and 70 mph is 3500 rpms.... I've never GPS's his speedo tho...
We have 3.90s and a 28" tire on one of our other cars and 60 mph is 3000 rpms on the nose.
Mine with 3.36s, 28" tire is around 2900 rpms at 70 mph....however that is with some converter slip...
Yes stock converter. ..need a bigger one. I can feel the slippage when stomping the throttle. My speedo I believe to be decently accurate to at least 60. The tach is an original electric from a 75, that I hooked up to my HEI. it works MOST of the time, but every once in a while it does its own thing... 3500 in neutral..lol. I'll jack her up this weekend and see if I don't have a code on it.
Either way, if I have 3.70 or bigger, I should be able to light the tires up, right? I have some decent rubber but still.. The only time it will spin is if the tires are cold and from a 1-2 shift they chirp, or I power brake it.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Yes stock converter. ..need a bigger one. I can feel the slippage when stomping the throttle. My speedo I believe to be decently accurate to at least 60. The tach is an original electric from a 75, that I hooked up to my HEI. it works MOST of the time, but every once in a while it does its own thing... 3500 in neutral..lol. I'll jack her up this weekend and see if I don't have a code on it.
Either way, if I have 3.70 or bigger, I should be able to light the tires up, right? I have some decent rubber but still.. The only time it will spin is if the tires are cold and from a 1-2 shift they chirp, or I power brake it.
Either way, if I have 3.70 or bigger, I should be able to light the tires up, right? I have some decent rubber but still.. The only time it will spin is if the tires are cold and from a 1-2 shift they chirp, or I power brake it.
Low gears (high numerically) doesn't mean it will spin tires...The engine power/torque is really more important.... But yes low gears help.. If its still the stock smogger small block, it probably barely spins them in the rain..
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
Your tranny will need to have the correct speedo gear that matches the rear end ratio for the speedo to be remotely accurate...and then the tire height affects that as well..
Low gears (high numerically) doesn't mean it will spin tires...The engine power/torque is really more important.... But yes low gears help.. If its still the stock smogger small block, it probably barely spins them in the rain..
Low gears (high numerically) doesn't mean it will spin tires...The engine power/torque is really more important.... But yes low gears help.. If its still the stock smogger small block, it probably barely spins them in the rain..
It's not stock, but it IS a big mystery. It is an 87 block, from who knows what, bored .030, in theory a zz4 cam (what I was told), but no roller rockers.. World torquer s/r heads, edelbrock torquer 2 manifold (soon to be replaced with a rpm air gap, edelbrock 750, headers, shift kit.
Going to pull it all apart or swap it out this winter is the plan.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
So I crawl under my car to look for a rear end code and.....Attachment 47811359
So I figure I need to replace this bolt... 3 out of 4 holding the rear lead on can't be good right?
So I figure I need to replace this bolt... 3 out of 4 holding the rear lead on can't be good right?
Scott
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
Lol kinda scary. I wonder how long it's been like that! I usually only put 3 of my wheel lugs on too. . Guess 3 is my lucky number!
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Thread Starter
Racer
Joined: May 2010
Posts: 448
Likes: 3
From: Olathe KS
Side note - called VB&P (can I plug them here), they are sending me a bolt for free, since I only needed one! They are so great to work with! In the mean time, I am going to put her on jack stands so I can actually get under it, and also solve this gear mystery.