Actual Gear ratio VS Effective Gear Raio

[MuddiGGEr25]

Not sure where to put this, but just wondering about determining your axle ratios by turning 1 tire and counting driveshaft rotations....

I will link the chart im using for reference: http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.msg256328#msg256328


For example my "new to me" K5 blazer should have 3.42 axles under it (10 bolt) using a 31x10.50 tire from the factory according to the glovebox so i should get just under 1 3/4 turns with a 31" tire mounted

However with the current 225/75?r15 tires that are mounted on it, the seller said he gets almost exactly 2 rotations ( i will be verifying this after work) and claims it has 4.10 axles. Im thinking it has the 3.42 axles and because of the smaller tires the "effective gear ratio" has now increased (numerically)

I put in 31" OEM tires and the current 27" tires with a stock 3.42 ratio here: http://www.4lo.com/calc/geartire.php and it says the effective ratio is 3.93

And another example is my V10 suburban that came with the same 3.42 axles but the door sticker says stock tire size was 235/75r15 (27.1" diameter)which is quite a difference from the 31"  the blazer got stock. and by putting on 31" tires on the suburbab my effective ratio went down to basically 3:1.  by checking the driveshaft


SO are the axle ratios based off the tires the truck came with stock or is there a chart that says its this ratio with xx tire size? because GM putting various stock tire sizes on changed the effective ratio.

I know taking the cover off and counting is the easiest method, but this has had me intrigued for a while now, the same axles with the same gears but different "stock" tires end up with varying actual ratios.


I wonder the same about the 14FF i have... what size tire will actually give it a 4.10 ratio...

[dphipps]

The method of checking gears by spinning the driveshaft with the wheels off the ground will not be affected by tire size. Gears are gears and will always be the same ratio, rotations in: rotations out regardless of tire size.


According to that calculator 3.93 is your "new gear ratio" , your effective ratio is 2.98:1. That calculator is showing what you would have to re-gear your axle to to achieve the same effective ratio as 3.42 with your taller tires.

[Captkaos]

I know taking the cover off and counting is the easiest method, but this has had me intrigued for a while now, the same axles with the same gears but different "stock" tires end up with varying actual ratios.

Counting the teeth is NOT the easiest method.  Rolling the ring gear and finding the tooth count stamped on it and dividing is the guaranteed and easiest method.
Axle ratio has nothing to do with the tires size.  It is what it is.  Change to a bigger or smaller tire changes the effective ratio which impacts your rpm range.

[Irish_Alley]

like stated, tire size has nothing to do with the axle ratio. tires will only effect your rpms. that being said if hes almost getting 2 revs i would assume he has 3.73

[MuddiGGEr25]

so what your all saying is that ratio... 3.42 in my case is set in stone and only the effective ratio that affects rpm, and speedometer is what changes with tire size?

[Irish_Alley]

3.42 is you axle ratio, your trans will also play a role in your final ratio. im thinking the term "effective ratio" is the effect the tire size has on your final ratio.

lets say you have 3.73 rear ratio and 28" tires and for simplicity sake a 1:1 final manual transmission gear. at 65 mph your rpms will be 2900. now if you step your tires up to 37s and keep the rest the same your "effective ratio" around 2.75 if you still had the 28" tires

things to play with
a rpm calculator
http://www.csgnetwork.com/multirpmcalc.html

and a chart

 

[MuddiGGEr25]

Yes i am referring to what some people call "effective" or "final" or "useful" ratio, I fully understand that the physical gear ratio of 3.42 cannot be changed without replacing ring & pinion.

I have a 3pd spd auto with a 1:1 3rd gear, and lets ignore the TC slippage just for a moment.


If the blazer came with 31x10.50r15 tires and is advertised as having a 3.42 axle ratio, did it in fact have a 3.42 effective ratio with the 31" tires stock? because now there are 27" diameter tires on it (probably due to being on sale) and now the effective/final/useful ratio is actually ~3.93 bringing the engine RPM up by ~300-350 at highway speed and also increasing acceleration because the driveshaft is now turning nearly 4 times for the 1 rotation of the smaller tire, correct?

Just glancing at the chart the engine would be spinning the almost the same RPM at 65mph with 31" and 3.91AR = 2755RPM and also with 27" tires and a 3.42AR = 2766RPM.

again i know the physical gears never change, but by putting tires on the blazer that are 4" shorter than OEM the seller changed the effective ratio to about 3.93 but did not adjust the speedo gears to compensate so im sure the dash is wrong, but my phone GPS would be correct. We should have it running in a few days and my sunpro 5" tach should be here next week ill see where the RPM sit with the much shorter tires.

[hatzie]

If you know the axle ratio and tire size you can plug em into my spreadsheet and do all kinds of What-If stuff including optimal speedo drive and driven gears and speedo ratio adapter box calculations.
It works in Excel and the free Open Office / Libre Office branches of the old Star Office suite.

http://forum.73-87chevytrucks.com/smforum/index.php?topic=32456.msg274736#msg274736

[1967KaiserM715]

If the blazer came with 31x10.50r15 tires and is advertised as having a 3.42 axle ratio, did it in fact have a 3.42 effective ratio with the 31" tires stock?

No, gm didn't give effective/final ratio numbers. They give you actual gear ratio and tire size.



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[75gmck25]

I have never seen anyone refer to effective ratio for a rear axle.  The rear end ratio is fixed, and is unaffected by tire size or transmission.

A much more useful measurement for comparison is revolutions per mile, which was shown as a table in an earlier post. Revolutions per mile in high gear (1:1) will account for both tire size and rear end ratio, and you can also add in factor for  overdrive ratio if you have a 700r4.   

For example, my truck has 31" tires and 4.10 rear end ratio, so I have 2896 Revs per mile.    If I wanted to swap to 35" tires I could use 4.56 gears, and would still come up just a little short of my current ratio at 2845 revs per mile. 

Bruce

[MuddiGGEr25]

ok i understand, and i went outside and jacked up one wheel and used a marker to mark one of the wheel weights on the back of the rim to use as my reference, i got just under 1 3/4 full rotations, so that would be the 3.42 AR if i read the chart right from http://forum.73-87chevytrucks.com/smforum/index.php?topic=30546.msg274738#msg274738

Thanks for the help guys. Now just to decide 33's or 35's