What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Gear calcuating 101 & Number use understanding

sponsoredbydad

sponsoredbydad

Dune Slaying Sand Junkie
- In Memoriam -
GMOTM Winner
Joined
Mar 12, 2004
Posts
2,647
Reaction score
1,043
Location
Dana Point "So Cal" "The sue me state"
What is the proper way to figure gearing. Is It pinion times ring?

New ???---So if I'm understanding this right lets say we start with 307. that would be more of a daily freeway gearing? 373's kinda of a semi multi use, 410/411 drag race but crud for freeway, 456's creepy crawly wheeling and towing?
I'm just guessing and learning right now so if this is not correct that would be why I'm asking, plus I'm trying to figure out what I really want for my truck which is going to see some street but not long freeway trips, mostly a 4x4 pre runner of sorts more for the dunes and open dessert.
 
Last edited:
driven divided by drive
or
ring divided by pinion

41/10=4.1 AKA 4.10:1 ratio
 
questions?

don't forget that the tire size is what determines the EFFECTIVE gear ratio......true, 4.56's would seem like a very deep gear for say 31" tires, but would be very "stocklike" with 35" tires. 3.08's were the "fuel economy" setup for trucks that see lots of highway use....3.42's are a good "all around" gear and 3.73's would be used for a towing application on a stock truck from the factory. these are general observations based on a bone stock truck.........for instance, i'll be putting 33's on my otherwise stock 3.73 truck (31" tire), my effective gear ratio will then be 3.5:1. better fuel economy than stock and a tad more gunt than a 3.42......on paper anyway :rolleyes:.
 
So then how does one account for tire size in the equation? I've got 33's but I'm also running a stroked 383 with over 500 HP. I should of not ditched school the day they covered this.
 
Figuring ideal gearing for your rig is a bit of a game. Information you need to supply...

Intended use (sand, mud, rocks, a bit of everything?)
Planned tire size
Transmission
Engine

Lets say you have a mildly wild small block that really wakes up around 3500 rpm and pulls hard to 6000 rpm. Lets say you're running 36" tires and you don't have overdrive. Let's assume sand dunes as primary use, little freeway but does see street driving.

I'd go with 4.56's

Freeway rpm will be 2600-2900 at 60-65 mph...tolerable

You'd be able to pull nearly 90 mph in second gear on a TH350 ( ~5800 rpm) in high range...or 90 mph worth of theoretical tire speed.

Gearing is all about multiplying engine torque. Once you know what your end use is, where your engine makes it's peak tq and hp, and your final tire size you can start plugging in numbers for gearing.

Rene
 
There are many online calulators, and I'm sure someone will post one up soon. I use my handheld calculator though.

Formula is:

RPM = Speed (mph) x Tranny ratio x TC ratio x diff ratio x 336 / true tire diameter

you can plug numbers in anywhere to see various scenarios. For tranny the default is 1:1 obviously, but you can plug in whatever gear you want. If you want to see R's in first gear and you have a TH350 plug in 2.52 into that spot. Same for TC ratio. High range, the default is 1:1 again, but you can substitute that for whatever low range you might have, or might want someday. Let's say you want to see how it is in low range for a NP 208. plug in 2.61 in that spot.

The number 336 is for lock up convertor auto's or manual trannys. For a non lock-up auto use 350 to get real world numbers.

Example, 35's, 4.56 gears TH350 highway rpm at 65 mph

RPM = 65 x 1.00 x 1.00 x 4.56 x 350 / 35

RPM = 2964

Rene
 
http://www.grimmjeeper.com/gears.html

I use this site ALOT when working on gear ratios. It has most transmissions and transfercases ever used. At the bottom of the page you'll find an output of engine RPM at a specific speed. You can use this to compare how your engine will be turning at a certain speed. This thing spits out ALOT of data, so take your time and read each box carefully, making sure you understand what you are actually seeing. Any change you make you have to hit the calculate button to update the figures.

My Jeep on 235/75's with 3.08 gears is very similar in engine speed at 65mph to the 31" tires I run now with 3.55's. 4.10's will bring me up to the optimal RPM for my 4.0L I6 at 65mph to improve my fuel mileage as well as being a little deeper to help with towing and take off with a load.

You can also compare different transmission/t-case setups. Remember though, for crawl ratio, and automatic has about 2x the gear ratio in 1st gear due to the torque converter. If you have a TH700R4 (4L60E) your first gear is physically 3.05:1. Through the torque converter, you wind up with near 6.10:1 effective ratio.
 
In all reality the best way to figure out gearing for your truck is to drive it around how you will be using it.

Note deficiencies and note what you like.

Then figure out how you want to regear.

The only way around this is if you have a buddy with a similar truck. I am not talking Chevy but similar tire size, similar motor, etc.

Finding the perfect ratio is hard to to sometimes.

What is the overall use of the truck? What percentage of time on, street, trail, sand, mud, rocks, freeway, in town?
 
sponsoredbydad said:
So then how does one account for tire size in the equation? I've got 33's but I'm also running a stroked 383 with over 500 HP. I should of not ditched school the day they covered this.
Click to expand...
3.08's and 3.42's are gears for sedans, they never should have been put in trucks. There is a reason why they aren't available anymore.

If you have 33's, I'd go 4.10 gears if you don't have overdrive and I'd go 4.56 if you do have OD. That setup will be a great all around gear/tire combo.

Remember, it's much easier to under gear your rig than over gear it, meaning it's much easier to suffer from the side effects of your gearing being too high, than too low, especially if you have OD.
 
a simple question to you.....what do you like or not like about your setup at this point? RPM range of the cam will play a big roll in how the truck works for the way you want to run it, or any vehicle for that matter. with 500 hp, you have a lot more latitude in choice than most. with 33's and dunes/speed/wheelspin i'd go on the higher side of the spectrum though.......more info is need as Rusty has said already.
 
Well dislikes right now with the 307 gearing is, there is little get up and go untill I reach aprox 50 or so MPH (thats in high gear) I am twin sticked however on the TC and when running in low it is by far a better gearing ratio for the mid range sweet spot of 3000-5000 rpm , just not such a good top speed maybe 65mph) By the way I'm running a TH400 with 2500 stall converter and a 205 TC. I just aquired a factory posi with 373 gearing so I guess I'm going to slap that in for now and see how it compares to the 307. I am also going to check out those links that were mentioned. This is why this site is sweet without it my truck would suck.
 
my feeling, based on what you've just said, is that your cam has some pretty long duration and doesn't reach it's powerband fast enough to compensate for the gears you have. that 3.73 rear should make a big difference though, top end will suffer.....do you have the cam specs?

single biggest problem i've ever seen is an improper cam for the application. i'm not saying that is your issue though.

i know of lots of guys that build motors for top HP, dyno/peak power. all the power in the world won't do most people any good if it's all at 6K........who really runs at that rpm?

my other opinion is that a stroker shouldn't need to rev as high to reach the same power as a stock 350.......piston speed is the enemy here.
 
With 33s and a th400 Id say that 3.73s woulds be a good choice. Especially with the 383. Now if you ever switch to a built 700 or a 4l80e 4.56s would be your best bet.
 
You probably hit it right on the head about the cam thing. It is a very large lope, the intake is also a taller style with a mid range power effect and was originally built for a Camaro to race. I bought it for 1/2 of what one like that would of cost and the lower half was already assembled and the builder did not remember the specs for the cam as a year had passed since he put it together. All I know for sure is that it has a peak power band in the 2500 to 5000 rpm range.

motor 012 (Large).jpg

motor 013 (Large).jpg
 
Last edited:
SBD.....truth be told...it's hard to say what you got going on there without the cam info. that converter could be wrong for the RPM range too. that package sounds like it sure would launch well though.
lopey idle..low LSA for a peaky torque band with a single plane mani is more for a race car (top end) as you stated.
for sustained high rpm running i'd choose a 377 (400 block and 350 crank) or a 302 (327 block with 283 crank). seems like all you really need is a good steady torque band with good AVERAGE torque, not peak for what you want........more under the curve so to speak. see how it acts after the 3.73 rear is in. just throwin out ideas here.


3.08's and 3.42's are gears for sedans, they never should have been put in trucks. There is a reason why they aren't available anymore.
Click to expand...

my friend, the guy i got my K5 from, has a brand new Silverado luxo cruiser with 3.08's in it....with the tow package too :eek1:.....it sure don't ride like a pickup either.
 
sponsoredbydad said:
You probably hit it right on the head about the cam thing. It is a very large lope, the intake is also a taller style with a mid range power effect and was originally built for a Camaro to race. I bought it for 1/2 of what one like that would of cost and the lower half was already assembled and the builder did not remember the specs for the cam as a year had passed since he put it together. All I know for sure is that it has a peak power band in the 2500 to 5000 rpm range.
Click to expand...

I was guessing on most of what I said earlier, other than the dunes and sand stuff as i'd seen some vids of yours before. 3.73's will be better (than 3.07/3.08), but I think it'll still be underwhelming in the dunes with those gears and 3.73's. With the way your motor is built, plus the stall convertor I'd look at 4.56's. You could still do some limited freeway, although it'd be spinning ~3000 at 60-65 mph. The good part is it would really allow the engine to work within it's power range in the sand. You don't want to be spending a lot of time under the stall speed, and 3.73's will have you under your stall for a lot of the time.

I'd say 3.73's would be just fine if it was a DD that saw a lot of highway time, but i really don't think that's your intended use or where you really want it to shine.

Go deep, and be happy.

Rene
 
Man thanks for all the help guys. As far as this 12 bolt goes the 373's along with the posi are going be better than what I now have, as for the right gearing for this truck and it's wants and needs I still have some time to fully figure it out before the 14 bolt comes into play. This just goes to show that if you start a build without a full blown game plan and buy stuff cuz of the deals this is what your going to run into. Funny because that's how I ended up with the engine cage /shock hoops that I now think are sweet was because I bought those 14"- 2 1/2 bypasses way early on because I could get them for 1/2 price . Clear cut case of my eyes were bigger than my stomach so to speak.

O-Yea I was up till 11pm last nite almost have it installed.
 
Deals do affect the build. I ran into a super cheap 4" lift plus a 10/12 combo with 3.42's. Not the best gears, but I had 3.08's.

Run the 3.73's for a while and you'll be much better armed to finalize gear choice based on the difference in performance from the 3.08's...as well as how you find the performance with the 3.73's.

Rene
 
You must log in or register to reply here.

Latest Posts

Top Bottom