pg0099

Hello,
I am trying to figure out what my rear axle ratio is. The car is at exactly 60mph at 2500 rpms.

I am not sure if 3:36 was the first gear ratio available or 3:07 (I think) on the LT-1. I have an opportunity to put in a 3:55.

I would love more acceleration from this car. To me it feels very slow and weak. My 1990 L98 will kill it in acceleration.

If my ratio is 3:36, will 3:55 gears make much of a difference in acceleration? I am not concerned with MPG or highway driving.

Nowhere Man

if you want acceleration you need to go up like to a 3.70 or 4.11. if you really want to get there in a hurry 4.56

mstanton

Have you looked at the code on the bottom of the differential? That will tell. Most LT-1s came with 3.70 or 4.11 and M21. If you have M20 you could have 3.36. I don't think an L98 should kill a properly running LT-1.

Mike Ward

Standard ratio was 3:70 with an M21 or 3:55 with M20.

65 vette dude

What is your rear tire diameter? If it's 27", you have a 3:36 gear.

Mike Ward

Tire diameter makes no difference in speedo indication. Accuracy yes, indication no.

wmf62

BUT, it does make a difference in the initial selection of the speedo gear; which determines what the speedo reads
Bill

Mike Ward

Yes, but that's a different subject. The speedo has no idea how fast the car is actually travelling- it only knows how fast the cable is rotating and indicates accordingly. The correct driven gear must be selected according to the rear end ratio and tire radius so that the gauge represents an analogy of vehicle speed.

I 'presume' the OP is going off the speedo indication and not actual vehicle speed measured with a GPS or similar.

kenba

I have found that 4:11 runs 3000 at 60 MPH 3:70 2700 & 3:55 2500 give or take. I doubt that all tacks & speedos are always right on.
KEN

pg0099

Thank you all for your reply's. It sounds like a probably have 3:36 or 3:55 gears in it now.

I looked under the car to try and find the numbers on the differential, but couldn't find it. Can anyone tell me exactly where it is?

Also, are there any other ways to give this car some more power, but still keep it as original as possible?

Thank you in advance.

haccpguy

A quick way to check the ratio:

Jack up the the rear end, put it in neutral and rotate the passenger's rear wheel one complete revolution. While you do this count the number of revolutions on the drive shaft. If it goes about 3 1/3 times then you probably have about a 3.36, if it goes almost 3.5 times around then you have a 3.55 etc.

MikeM

I think you're trying to fix the wrong end of the car.

Mike Ward

You might want to post in the C3 section, lots of guys there with plenty of advice.

66BlkBB

Might work on a straight axle but it is a little more difficult on a IRS. U-joints tend to not like to turn when the axle shafts and tires are not level or close to level with the rear differential. He would have to support the frame and then raise both trailing arms enough to get the axle shafts to turn to be able to rotate to count the revolutions.

haccpguy

Good point.

Another way is to have someone drive the car slowly 1 tire rotation while counting the shaft rotations. More of a pain, but easier than pulling apart to count teeth.

JohnZ

The ratio code and assembly date are stamped on the bottom edge of the diff housing, between the spring retainer plate and the camber strut bracket; it'll take some cleaning to be able to read the numbers.

toms silver 60

It never hurts to check out your speedometer with a portable gps. It sure beats going down the interstate with a stop watch. Try it at various speeds, such as 30, 60, and 90 and see if bias is consistent %.

At the same time, take a portable tach with you and see if tach is correct. A passenger makes this a lot easier. Make a chart with metered and car rpm and mph.

You can now easily figure what you have as well as close or wide ratio.

I drove 30 years with a tach overstating rpm by 10% and speedo understating 10%. It took a generator pulley and rear end change to make it correct.

wmf62

whatta guy...

Bill

vt65

Hi John, what does those #'s denote on the rear, mine reads AS3 15 65, sorry for the hijack pg0099

Nowhere Man

AS=3.70.1 standard axle for a close ratio 4 speed and 327. 3 15 65 is march 15 1965