figure out my gear ratio

Marx · Nov 7, 2010

i've had my 05 4x4 for a year, starting to think it has different gears then on the sheet (3.92) how would i tell if the gears been changed??

i have not told the computer that i'm running 35's (stock 33's) and the truck still has great pick up

just wondering because i was doing 60km/h and drove passed a speed sign and it said i was doing 63km/h

maybe someone can tell me what rpm's they are running at different speeds and what gears they have

thanks guys

Redtruck-VA · Nov 7, 2010

Simon, that sounds about right. If your truck came with 33" tire and you have 35" on it now you would pickup a couple of actual MPH. The stock 20" rim has a 33" tire and the stock 17" is 31". The truck (PCM) determines speed by monitoring the tone ring located next to the ring gear. So changing gear ratio is automatically adjusted as it is still simply counting axle rpm. The variation comes from tire hieght which is programed into the PCM. This can be done at the dealer or with a DSP or SC tuner and maybe others. I had my PCM reflashed and they got the tire size wrong and ****** up the reflash. Of course they wouldn't accept responsibility, so all I call say about taken it to the dealer it is at your own risk. Hope this babble helped...

lilfroger · Nov 7, 2010

As for the RMP's at different speeds for different gears / tire sizes ... 4 Wheel Parts has a pretty good chart that will give you an idea...

Lift Kits, Tires & Wheels by 4 Wheel Parts

Ramunit · Nov 7, 2010

Here is your formula:
mph x gear ratio x 336
___________________ X Trans gear ratio = RPM
Tire diameter

Example: 70mph x 3.92 x 336 = 92198.4

92198.4 / 35(tire dia) = 2634.24

2634.24 x .67(trans ratio in 5th) = 1764.94 RPM

Pluging in these example numbers you are at 1765 RPM at 70 mph in 5th gear.
Now, use your numbers and you have your answer.

OlDirtyJohn · Nov 7, 2010

jack up your rear end, mark a spot on your tire, and mark a spot on your drive shaft. spin your tire one full rotation and count how many turns the drive shaft makes.

Marx · Nov 7, 2010

thanks, i'll give that all a try this week.

Marx · Nov 7, 2010

just found a really cool page
Gear Ratio Calculator

03MopaRamman · Nov 7, 2010

If your truck came with twentys dude it prolly ain't 3:55s. There is a Tag on the 3:55 diffs that match the sticker in the glove Box (2003 models anyhow).

Marx · Nov 7, 2010

03MopaRamman said:
If your truck came with twentys dude it prolly ain't 3:55s. There is a Tag on the 3:55 diffs that match the sticker in the glove Box (2003 models anyhow).

i know there at least 3.92's, but i got the truck used and don't know if the guy may have changed them out (to 4.11's or 4.56's), i'm just curious because the truck is fast for having 35's on it (butt dyno)

glass3222 · Nov 7, 2010

If you have 35s, here is approximately what your RPM at 70 mph will be:

gears....RPM
3.92....1770
4.10....1850
4.56....2055

Marx · Nov 7, 2010

glass3222 said:
If you have 35s, here is approximately what your RPM at 70 mph will be:

gears....RPM
3.92....1770
4.10....1850
4.56....2055

i'll have to test it out tomorrow thanks for the info

Marx · Nov 8, 2010

so i was running 1850rpm at 70mph so i think i have 4.10's in my truck
gonna spin the wheels tomorrow if its not to cold

glass3222 · Nov 8, 2010

If your build sheet said you have 3.92s, that is probably what you have. The 100 RPM difference could be tires slightly less than 35", torque converter slip, you could have been on a slight uphill grade, etc. If the previous owner did install 4.10s, then cool, but it was a waste of money to to from 3.92s to 4.10s.

AF_Hemi · Nov 8, 2010

agreed. i'll bet you have 3.92's. your driveshaft shouldnt quite make a 4th rotation

Off_Road_Teacher · Nov 8, 2010

Marx said:
so i was running 1850rpm at 70mph so i think i have 4.10's in my truck
gonna spin the wheels tomorrow if its not to cold

Let us know how it goes. If someone did install gears for you already that is a nice bonus.

Ram Man · Nov 8, 2010

well you said earlier when your speedo said 60 you were goin 63, which means when your speedo said 70 you were actually going faster...so if you were actually going 70 your rpms would be a little lower since before it was 1850 at around 73/74. Try using a gps to find out when your actually traveling 70..you will probably find you rpms around 1770 lol

Marx · Nov 9, 2010

Ram Man said:
well you said earlier when your speedo said 60 you were goin 63, which means when your speedo said 70 you were actually going faster...so if you were actually going 70 your rpms would be a little lower since before it was 1850 at around 73/74. Try using a gps to find out when your actually traveling 70..you will probably find you rpms around 1770 lol

ya i took that into account when i was at 70 put the needle at 66

Ram Man · Nov 9, 2010

damn, way to pop my little bubble

Off_Road_Teacher · Nov 9, 2010

Marx said:
ya i took that into account when i was at 70 put the needle at 66

So did you jack it up and count the rotations?

03MopaRamman · Nov 9, 2010

3.92's I Bet, cause 35's are prolly more like 34.6.

figure out my gear ratio

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