Gear ratio and RPM's

**vannatte** · 08-15-2009, 08:21 PM

I just got 3.73's installed in my car and the RPM's seem to be much higher than they should. I have an auto and at 50 I'm at 2000 rpm's and at 70 about 2800 rpm's. Does this seem correct? I reprogrammed the computer with my Hypertech programmer, but maybe I did something wrong?

**Bandit1** · 08-16-2009, 01:01 AM

its normal. ur good

**SteveC** · 08-16-2009, 05:39 AM

Try this sight for the correct numbers:

http://www.f-body.org/gears/

@ 1500 RPM you SHOULD be doing 50 MPH

@ 2700 RPM you SHOULD be doing 80 MPH.

With your numbers you are up in the 4.40:1 rearend gear range.

I am assuming that you are in OD.

SteveC

**pajeff02** · 08-16-2009, 05:52 AM

Or this works also: http://www.richmondgear.com/101032.html

**vannatte** · 08-16-2009, 06:05 AM

I bought the rearend used and I didn't bother to count the teeth before having the rearend rebuilt, but I'm sure they were 3.73's. Is the programming wrong? What should the tire height be for a stock 16" tire?

**pajeff02** · 08-16-2009, 06:17 AM

Easy way to check your ratio:

Block your front wheels and jack up the back of your car. Support the rear on jackstands to allow you enough room to get under the car. Take a piece of chalk or a crayon and place a mark on your wheel where it is easy to see, preferably, staight up and down. Now place a mark on your driveshaft or the rear flange and a matching mark on your differential. Again, somewhere easy to see and where you can line up the marks.

Place your transmission in neutral and begin rotating your driveshaft, counting the number of shaft revolutions it takes to make your rear tire go around one complete revolution. If you have 3.73 gears, your driveshaft should make approximatelty 3-3/4 revolutions to make your tire rotate once.

It's easy to approximate it in this manner as the next lowest standard ratios are 3.42 and then 3.23. There should be sufficient margin for error that you will not confuse these as it is almost a 1/2 turn of the driveshaft difference.

**SteveC** · 08-16-2009, 07:41 AM

Originally Posted by pajeff02

If you have 3.73 gears, your driveshaft should make approximatelty 3-3/4 revolutions to make your tire rotate once.

It's easy to approximate it in this manner as the next lowest standard ratios are 3.42 and then 3.23. There should be sufficient margin for error that you will not confuse these as it is almost a 1/2 turn of the driveshaft difference.

The next LOWER ratio is 4.10:1, the next HIGHER standard ratio is 3.42:1, then 3.23:1. The HIGHEST standard ratio is 2.73:1 which I believe is the GU2 option.

SteveC

**dklowrider** · 08-16-2009, 08:06 AM

Originally Posted by vannatte

I bought the rearend used and I didn't bother to count the teeth before having the rearend rebuilt, but I'm sure they were 3.73's. Is the programming wrong? What should the tire height be for a stock 16" tire?

around 25.7"

**pajeff02** · 08-16-2009, 08:09 AM

Originally Posted by SteveC

The next LOWER ratio is 4.10:1, the next HIGHER standard ratio is 3.42:1, then 3.23:1. The HIGHEST standard ratio is 2.73:1 which I believe is the GU2 option.

SteveC

My bad. I have left and right down pat... still working on high and low. Thank you for the correction.

**35th-ANV-SS** · 08-16-2009, 08:13 AM

Originally Posted by pajeff02

My bad. I have left and right down pat... still working on high and low. Thank you for the correction.

It happens

I still use righty-tighty lefty-loosy.....until you run into those damn left-hand threads!!

**vannatte** · 08-16-2009, 03:19 PM

Originally Posted by dklowrider

around 25.7"

I programmed the Hypertech at 25" tires, would that be the error?

**dklowrider** · 08-16-2009, 03:48 PM

Originally Posted by vannatte

I programmed the Hypertech at 25" tires, would that be the error?

that could make some difference. the rpms will be correct, but the car thinks its traveling farther than it is with each tire revolution and shows a higher mph than you are actually doing. but that would mean your rpms are even higher compared to a given speed, if you correct the tire size issue. no, wait a minute. the car is using 25" so the mph will read lower than you are actually traveling. if you correct the tire size input it will read higher mph thus giving you a lower rpm reading at an indicated speed. sorry, i confused myself.

in other words, you show2k rpms at an indicated 50mph, but you are actually going faster than 50.

**vannatte** · 08-16-2009, 04:57 PM

What should it be exactly on a Hypertech? It lists 25.XX", so 2 decimal places.

**35th-ANV-SS** · 08-16-2009, 05:04 PM

Originally Posted by vannatte

What should it be exactly on a Hypertech? It lists 25.XX", so 2 decimal places.

What size tire is stock? Width, Aspect, and Size.

Or simply use this website to figure it out..

http://www.ringpinion.com/Calc_RPM.aspx

Bottom of the page is the tire height calculator.

**vannatte** · 08-16-2009, 07:34 PM

Originally Posted by 35th-ANV-SS

What size tire is stock? Width, Aspect, and Size.

Or simply use this website to figure it out..

http://www.ringpinion.com/Calc_RPM.aspx

Bottom of the page is the tire height calculator.

Great resource. Thanks. Is this going to be as accurate as making chalk lines on the pavement, measuring the distance for one revolution, and divide by Pi? The calculator gave me 25.65, but Hypertech only has 25.50 and 25.75. Which should I use?

**SteveC** · 08-17-2009, 04:08 AM

I would use the 25.75, that way you should only be off by 1/2 to 1 MPH on the slow side.

SteveC

**vannatte** · 08-17-2009, 08:24 PM

Changing the tire height to 25.75 seemed to make a big difference. Now at 50 mph, I'm at about 1800 rpms. I also adjusted the shift points and that seemed to help it shift better.