Hey all, 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.

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.

Yes that is good info and does come up often enough when people are talking about getting lifts. - - - Updated - - - there are always issues when you completely modify something from stock. In your case converting a 1/2 ton to a 3/4 ton changes things some.

I've been researching and trying to figure out a way to answer this but it appears that "effective gear ratio" is determined using different tire sizes. 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.

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.

Without a doubt you helped your towing capacity. And going from 29.5" to 32" tires isn't THAT substantial. I have a friend who went from his stock approx 30" tires to 35" then to 37" all with a 3.42 axle ratio. After regearing to 4.10s there was a HUGE difference! 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...

this is kinda what I was thinking in my previous post. you would be using the new gear ratio and the tire would change from what was stock (maybe stock tire size of the 3/4 ton, but not sure there) to the new tire. Either way it should get you nice and close. things in my head dont always translate to typing very well sometimes.

Sounds right to me. One thing I want to point out is your gear ratio still stays the same no matter what unless you actually change it. If you have 4.10 then changing your tire size isn't going to change anything but your "effective" gear ratio. 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.

dont you love how that works :lol:. But thats the way I figure it works. I went from from a 30" tire with 3.73 to 35" with 4.10 gears. At that point I used the new gear ratio (as thats what it is) and then used the formula with the tires (30/35)x4.10= 3.51. Which yes gave me a lower effective gear ratio than stock. But at the time I lifted it it was my dd and I did a lot of highway drivig and not much towing so I wanted to go for fuel economy (my mpg stayed the same lifted as stock). If I do more towing I may upgrade to 4.56 or may just throw some stock size tires on it for heavier towing purposes which would look a little funny. In the end this formula would make a great sticky:great:

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.

The thing that helped me get it straight in my head is to think in terms of torque/force rather than "effective gear ratio." The math looks like this (starting with an arbitrary input torque t0 at the driveshaft): ta=t0*gr: ta is torque on axleshaft, gr is gear ratio F=ta/r: r is tire radius (=1/2 diameter) F is Force between wheel and ground (this is what makes you go). combined: F=t0*gr/r solve for t0: t0=F*r/gr for comparing when something changes: t0=F1*r1/gr1=F2*r2/gr2 (remember that t0 doesn't change for our purposes) if we assume 1 is the "stock" configuration, then to compare 2 to 1 F2/F1=(r1/gr1)/(r2/gr2)=(r1/gr1)*(gr2/r2) Example: stock gear ratio 3.55 with a 30 inch diameter tire new is 3.73 with 35 inch diameter tire F2/F1=(15/3.55)*(3.73/17.5)=.90 so the new configuration will put 10% less torque (F2=90%*F1) to the ground and will thus feel more sluggish than stock.

Also some things the math does not take into consideration is if your lifting your truck you will have more wind resistance and also if your adding any extra weight from larger tires and rims. It might not be much but it is a factor. So my recomendation is to always round up to the next available gears or even the next after that. Just my .02.

At first this hurt my brain but once I broke everything down it became very simple. Great formula! Thanks!

wait, im confused... so... (33 / 29) x 3.55 = 4.03 to run 33" tires, i should have at lease 3.83 gears for the axles? i have 265/75 (about 29") and i want to go to 285/75 (about 33") .... i still dont know what gears i should have...

so with the formula you used at a pure conversion to get near a stock gear ratio you would need 4.03 gears. I dont believe they make those so after that it is based on your needs. you have a 4.10 or a 3.73 I believe as options. A 4.10 is going to get you close to the stock ratio but a tiny bit shorter ratio which will give you a little more ability to get the blazer moving. look at it this way as well (29/33) x 4.10 = 3.608 effective gear ratio so real close to 3.55 (29/33) x 3.73 = 3.28 effective gear ratio so taller than stock by a decent margin and a little more effort required to get things rolling.

ooooo-k cool thanks! =) im gonna stick with my 265/75R16s 31" looks, and handles well with my 3.73 axle