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Thread: Effective Gear Ratio
11-28-2012, 01:22 PM #1
Effective Gear Ratio
Just wanted to give you guys a quick class on "effective gear ratio". Effective gear ratio is your gear ratio after you've changed your tire size. I'll give you a couple of examples of this and how to calculate your effective gear ratio.
Effective gear ratio can be calculated like so: (old tire diameter / new tire diameter) x gear ratio = effective gear ratio.
Ex 1) 3.92 gear ratio with stock 33" tires replaced by 35" tires. (33 / 35) x 3.92 = 3.70 (rounded to the nearest hundredth)
Ex 2) 3.55 gear ratio with stock 31" tires replaced by 33" tires. (31 / 33) x 3.55 = 3.33 (rounded to the nearest hundredth)
Ex 3) 3.73 gear ratio with stock 32" tires replaced by 37" tires. (32 / 37) x 3.73 = 3.23 (rounded to the nearest hundredth)
You can also calculate what you would need to re-gear in order to get back to your stock gear ratio (or close to) by swapping the old and new tire sizes around.
Ex 1) (35 / 33) x 3.92 = 4.16 (rounded to the nearest hundredth)
Ex 2) (33 / 31) x 3.55 = 3.78 (rounded to the nearest hundredth)
Ex 3) (37 / 32) x 3.73 = 4.31 (rounded to the nearest hundredth)
Hope this has been helpful and didn't confuse anyone too much.-2008 Chevrolet Avalanche LT 4x4 w/ Diablo inTune 87 Octane, K&N CAI
-2012 Kia Soul !
-2003 Kawasaki Ninja ZX-6R 636
11-28-2012, 02:25 PM #2
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Would that formula work for determining effective gear ratio after changing both tire size and gear ratio? I'd think it would need to be changed, but I'm not sure how. Here's an example of what happened to me when I changed my truck from 1/2 to 3/4 ton which involved changing gear ratio and tire size
old tire size= roughly 29.5"
new tire size= roughly 32"
old gear ratio= 3.08
new gear ratio= 4.10
Thanks for the knowledge either way.Christopher
1991 Chevy Suburban 1/2 ton 2WD w/ chevy SBC 350-3/4 ton drivetrain upgrade w/4.10 gears 199K miles
2005 Saturn ION-2 Stock 255K miles
1982 Bronco, 1993 Bronco (sold), 1971 M35A2 Deuce and a Half
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11-28-2012, 06:22 PM #3
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2004 Silverado 2500 Crew Cab 4x4, AMSOIL EA air filter, Granatelli MAF sensor, Throttle body spacer, Magnaflow exhaust (true dual to 2 in 1 out muffler), 6" ProComp lift (add a leaf and 5" superlift rear block), Bilstein shocks, 35's (Cooper Disoverer ST) and 4.10 gears, Rhino Liner, EGRUSA fender Flares and widow visors, extended stainless steel brake lines, firestone airbags w/onboard air compressor, Pioneer Avic X940BT navigation, Accel backup camera.
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11-29-2012, 02:11 AM #4
Honestly, if I were calculating it I would leave the old ratio completely out of the equation and use the (stock tire size / new tire size) x new gear ratio.
So (29.5 / 32) x 4.10 = 3.78
I wouldn't say that is 100% accurate as you are changing both tire size AND gear ratio as well as upgrading to a 3/4 ton axle, etc.
11-29-2012, 09:20 AM #5
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The upgrading of the drivetrain is irrelevant to the calculation in my opinion, as changing control arms and axle design shouldn't affect the gearing due to the fact that 4.10 in a 10-bolt is the same as 4.10 in a 14-bolt, only stronger.
My calibrated seat of the pants assessment is I probably gained a little in the low end torque to the wheels, but not as much as I'd hoped for. I used a stock gear ratio and tire size combo that was available in the 3/4 ton suburban, so I think it should pull my camper like I hope it will. We'll see this Christmas.
11-29-2012, 09:49 AM #6
Regarding this thread, I know it comes up in some threads about tire sizes and stuff but I was hoping a moderator would sticky it somewhere as a good quick reference for people to have. Just sayin...
11-29-2012, 12:29 PM #7
things in my head dont always translate to typing very well sometimes.
11-30-2012, 08:37 AM #8
In the case of above: changing both your tire sizes AND gear ratios, it's hard to say where to start calculating because you don't really have much of a baseline to go off of anymore.
I'm starting to confuse myself thinking about it more and more trying to make sense of it and how to explain it.
11-30-2012, 12:13 PM #9
In the end this formula would make a great sticky
11-30-2012, 01:12 PM #10
I think 4.10 is probably going to give you the best of both worlds. You have a low gear ratio but not so low to kill highway gas mileage.
I have, however, seen some people put lifts and giant tires on their trucks without re-gearing and their overall gas mileage suffers horribly. After they re-geared they got back quite a few lost MPGs because the engine no longer had to work as hard to get the beast moving.
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