How to tell gear Ratio?

[Tepot]

Hi,

I would like to know what gear ratio I have in my rear end on my 74. I don't really want to open the rear cover from the rear end.

How else can I tell? The Vin on the block is not the original so no help there.

The only thing I know is that I've got a L-48 Engine with the 4 speed manual transmission.

RPM goes as follow:

2500 RPM @ 60 MPH
3000 RPM @ 80 MPH
3500 RPM @ 100 MPH
4000 RPM @ 120 MPH
4500 RPM @ 140 MPH

I was wondering if someone Had the same engine and transmission and would already know there gear ratio could compare the speed at different RPM and could tell me what can of gear ratio I got in my 74.

One thing to take in consideration, I've got L48 engine with Edelbrock performer intake , headers, True dual exaust, no cat, flowmasters 40's and of course the 4 speed manual transmission.

Can someone help me? Any suggestion on what might be the rear end gear ratio?

 

[craig32]

your engine doesn't have anything to do with rearend gear ratio. I t may have from the factory, but anything could have been changed in between then and now. The rearend gearing is driveshaft revolutions to one rear wheel revolution. Easy ballpark estimate - jack up the rear, mark a tire and turn the driveshaft til it makes a full revolution and count the turns. Hope this helps.

Craig

 

[bobs78]

Go here http://www.idavette.net/tech/ratioc.htm

 

[Tepot]

Go here http://www.idavette.net/tech/ratioc.htm

Ouf that calculator is complicated, first I don't know what Enter the Gear Ratio - 3.0 for 3.0:1 means. I don't know what the -3.0 for 3.0:1 means.

2nd is that when I've entered 60 MPH for 2500 RPM it's giving a different gear ratio than what i put for 100 MPH for 3500 RPM. It's should give the exact same results right? Cause that's what I can see on my gauges when I drive.

If you can help me with this. good. if not I'll have to try craig32's suggestion. I sure hope I don't have 2.73 gear ratio because damn they will be replaced with 3.55 very quick !!!

But I still need to know before I order 3.55's.

 

[bobs78]

Ouf that calculator is complicated, first I don't know what Enter the Gear Ratio - 3.0 for 3.0:1 means. I don't know what the -3.0 for 3.0:1 means.

2nd is that when I've entered 60 MPH for 2500 RPM it's giving a different gear ratio than what i put for 100 MPH for 3500 RPM. It's should give the exact same results right? Cause that's what I can see on my gauges when I drive.

If you can help me with this. good. if not I'll have to try craig32's suggestion. I sure hope I don't have 2.73 gear ratio because damn they will be replaced with 3.55 very quick !!!

But I still need to know before I order 3.55's.

Read the instructions at the top of the page... to calculate rear end ratio you need to enter: Engine RPM, Tire Diameter and Speed!

 

[67HEAVEN]

Read the instructions at the top of the page... to calculate rear end ratio you need to enter: Engine RPM, Tire Diameter and Speed!

Exactly right. Tepot, your tire diameter will be somewhere in the mid-20" range. Mine are 26.5" in diameter. After entering your numbers, click on Compute Ratio.

 

[Mac]

I'm far from expert but I believe the "stock" rear diff ratio was 3.36-to-1 but, like your engine, your rear diff may have been changed in the past thirty years.

-Mac

 

[GerryLP]

Tepot,

There is also the possibility that you are getting an error on the speedometer. You basically recorded the indicated speed on the speedometer; however, were you really going as fast as the indicated speed?

Get on a stretch of highway where the road is pretty straight and level and where there are mile markers. And along with a buddy (or significant other) and stopwatch time between mile markers. Keep an eye on the road and pick an indicated speed that you will follow. Let your companion do the timing.

Generally speaking, at 60 MPH, your Vette should be able to cover the one mile markers at or very near 60 seconds per mile marker. This will tell you how accurate or inacurate is your speedometer. For example, if your travelling at 65 MPH indicated, but you cover a mile marker in 60 seconds...guess what...you speedo has a 5 MPH error. If it was accurate, then you would have covered the mile marker in 55 seconds.

Remember to document the engine RPM and time it took to cover the mile markers. Then calculate what your actual speed really was irregardless of wheather you were going, coming, or turning (that would be calculating your velocity).

So once you know how fast you were really going, then we can perhaps use the calculators.

Here are a couple of tire calculators: http://www.csgnetwork.com/tiresizescalc.html

http://www.discounttire.com/dtcs/infoTireMath.dos

Automotive converters:
http://www.csgnetwork.com/automotiveconverters.html

And other science calculators:
http://www.onlineconversion.com/

GerryLP:cool

 

[Tepot]

Well. I guess before I start using that calculator I'll have to make sure my speedometer is acurate.

Maybe I'll just wait and this winter I'll open the rear end and I'll know for sure.

 

[Chas]

Well. I guess before I start using that calculator I'll have to make sure my speedometer is acurate.

Maybe I'll just wait and this winter I'll open the rear end and I'll know for sure.

I went from 3:02 to 3:55 what a diffrence.....it jumps out of the hole and still is good @ 70mph for hours on end.
3:02 in my vette was about 2500 rpm @ 70 mph, 3:55 is 3600 @ 70 mph.
If you go higher than 3:55 it is almost impossible to calibrate the speedo to match....mine is within 1 mph , so im very happy with it as it is now.
this may not be the same for yours but it is a good comparison i guess.
Good luck, Chas