How to determine proper gear ratios

131890 Views 176 Replies 78 Participants Last post by davester3500, 44 m ago

Luke300zx

Discussion Starter · Mar 9, 2012

Hey everyone,

Got a new lift kit and want to throw on bigger tires but don't want to lose to many mpgs or put strain on your transmission? Want more offroading power? Look no further! Read this guide and you will know or at least have a very good idea what gears you would need to achieve that.

Method:

Old Tire Size: 31"
New Tire Size: 35"
Old Gear Ratio: 3.42

New Gear Ratio = (New Tire Size/Old Tire Size) * Current Gear Ratio

3.86 = (35/31) * 3.42

Since there is no 3.8 for our trucks 3.73 would be the closest thing but to compensate for the added weight of offroad tires/wheels I would go with a 4.10 to be safe.

For my personal truck I currently have 3.42's with a 31" tire, but since I tow and am soon going to get a heavy toy hauler I wanted more towing power. My plans are a 6" lift with 35" tires and I'm going with the 4.56 gears and here's why:

(35/31) * 3.42 = 3.86 (We'll say 3.73 since that's the closest), So a 3.73 would return me to stock which is not good enough since I'm going to be towing more weight on my tires and on my tow hitch with the new trailer. I want to make my truck equivalent to a stock truck with 4.10's and stock tires. Soo....

If I went with 4.10's it would be like having a truck with stock tires and 3.73's, so instead I'm going with 4.56's to make my truck more like a truck with stock tires and a 4.10 ratio (which was my goal to begin with).

If you have any questions about gear ratios I'd be more than happy to answer them here.

1 - 20 of 177 Posts

jocko463

· #2 · Mar 9, 2012

Cool. And who said you would never use algebra in real life ...

Thanks for posting this. I think most guys, when they talk big tires and lifts, forget about this "little" detail.

Luke300zx

Discussion Starter · #3 · Mar 12, 2012

Yeah I know I did at first....then I was like "GREAT...even MORE MONEY to spend now...."

These lifts add up fast cost wise if you think of lift + gears + cost of M/T tires.

pvoltmer

· #4 · Apr 2, 2012

I'm going from a 31.5" tire to a 32.5" tire...I don't really need to change gears for that small of a jump do I? The numbers are close either way you go, by using your equation.

If later on I want to put on a lift kit (currently just have a leveling kit), I would probably then want to change gears, correct?

druwevr

· #5 · Apr 11, 2012

http://www.4lo.com/calc/geartire.php go here if u need to figure out new ratio

Cuban.

· #6 · Apr 20, 2012

some people dont even go by that rule.

nbs z71's come factory with 3.73's on 32.5's but this method says a 3.73 will make 35's feel stock? But i know that rule applies to the motor itself as well. Go from 3.73's and then step it to 4.56 and you'll see what you've been missing. I have 35's and 4.56's and the smaller ratio is just beyond inferior. There is so much more power and giddy-up. For 33's i recommend nothing lower than a 4.10 ratio.

Luke300zx

Discussion Starter · #7 · Apr 20, 2012

The only problem with the 4lo calculator is that it doesn't take into consideration for weight or rolling mass of the bigger tires. That's why even though you do the calculations and say it puts you right in the middle of 3.73 and 4.10 but you're going from say 32" to 35" I'd get the 4.10's...just because mileage difference would be very minimal and you'd have a bit more towing power if you did tow.

joeyride61

· #8 · May 2, 2012

I just installed 4.56 gears today on my 4x4 2007 NBS 4x4. Soooo much better. I researched for hours on different forums about what to do. In the end, if you want the performance of a 4.10 in a 35" tire, 4.56 is the way to go - IMO.

Also, in all my research and looking at gear ratio charts, it says that I should have gone with the 4.10's too...but keep in mind that the overdrive's final ratio is less than 1:1, its more like 1:.73 or even 1:.69 according to Wikipedia. The charts all estimate based on a 3 speed transmission with a 1:1 ratio. The beauty with the OD on our trucks is it lets you run lower RPM's with a higher gear.

Luke300zx

Discussion Starter · #9 · May 16, 2012

Does your new truck have the 4 speed or 6 speed?

joeyride61

· #10 · May 16, 2012

4 Speed. I have been driving for a few weeks now and its really fun. On the highway, I am turning about 2500 RPM at 75mph...and I will probably not drive any faster than that. But in the city off of stop lights is really great. I am probably averaging around 13 city and like 14 highway at 75...but who cares. The other thing is though I have toyo 35" tires, they actually measure about 33.5" once mounted and installed, so setting a programmer to 35" tires wil be overstating the actual results. The DIC computer will also be over stated because it makes it seem like you are going faster and further than you really are, therefore giving you better average gas mileage.

J.Rieder89

· #11 · Jul 24, 2012

im stuck between choosing a 4.10 or a 4.56 ration. the more I read the more im unsure which way to go. im running 35's on 20x12 wheels, in my mind i think a 4.10 is good but would that put me back to stock or a little better? and for fuel milage. stock is a 3.42 ratio in my truck. thanks for any help

Luke300zx

Discussion Starter · #12 · Jul 24, 2012

4.10 would be like stock. 4.56 would be like having 3.73's. You have to take into consideration the weight of the new big 35" tires compared to the stock ones. Also the size in general takes a little extra momentum to get going. I'm going with 4.56's because I tow a lot so I need a bit extra compared to the stock 3.42s.

J.Rieder89

· #13 · Jul 24, 2012

have you swapped the gears yet in your truck yet? how would that effect the mpg? and also what about a true speed o meter reading?

07BlackOps

· #14 · Jul 24, 2012

This guide isnt exactly accurate. Coming from the jeep community there are a lot of other factors to figure in to this calculation. Horsepower, Torque, Final drive ratio, tire size rim size top speed, vehicle usage. The smaller the gear ratio (higher number) the more low end you will get but the lower top speed you will receive. Example my old Jeep Wrangler 2007 3.8L 202hp and 237lbs/ft tq. Had 3.73 gears stock, but my package had 4.10 gears. When I sold her it had 4.88s and top speed was 70mph but I could get to that speed in 8 secs. And in low range I could crawl on anything! So this guy is helpful, its not 100% accurate! Sorry Luke, just wanted to add my $0.02

J.Rieder89

· #15 · Jul 24, 2012

i plan on keeping it on the road its my DD, so im looking for get up power and trying to increase my mpg. i talked to 4wheel parts and they jumped to a 4.56 ration, im just trying to get a second opinion from some one thats running a similar set up and thier experience on it but thank you for the example its all helpful.

07BlackOps

· #16 · Jul 24, 2012

The lower the gear ratio(higher number) keeps your RPM's higher at 65mph. if you want better fuel economy make your ratio higher. 3.73 to 3.10 in that range lowers your cruising RPMs and increase your top speed but wont give you more get up and go. I would recommend just keeping the stock gears if you want better fuel economy and more get up and go. I hate to say it, but the more you mess with gear ratios the more problem you can create. I have 4.10s in my truck and I can easily go 400+ miles on 26 gallons. If I dont beat on it and let her sit at 65-68mph. and as said im running 4.10s with a 6.0L

J.Rieder89

· #17 · Jul 24, 2012

i would love to get that kinda fuel milage i have a 5.3L and maybe get about 250miles to a same size tank, but my stock gear is 3.42 and have no get up at all. also im still adjusting from my old tuned duramax

retired378

· #18 · Jul 24, 2012

http://www.offroaders.com/tech/gear-ratio-chart.htm

Luke300zx

Discussion Starter · #19 · Jul 25, 2012

07BlackOps said:
This guide isnt exactly accurate. Coming from the jeep community there are a lot of other factors to figure in to this calculation. Horsepower, Torque, Final drive ratio, tire size rim size top speed, vehicle usage. The smaller the gear ratio (higher number) the more low end you will get but the lower top speed you will receive. Example my old Jeep Wrangler 2007 3.8L 202hp and 237lbs/ft tq. Had 3.73 gears stock, but my package had 4.10 gears. When I sold her it had 4.88s and top speed was 70mph but I could get to that speed in 8 secs. And in low range I could crawl on anything! So this guy is helpful, its not 100% accurate! Sorry Luke, just wanted to add my $0.02

I know. It's a general guide. But given the ratios in the chevy silverados there isn't much difference in a final gear ratio unless you change gears or have a 4spd vs 6spd. It's not like a jeep where people swap transmissions, transfer cases, engines, and differential gears daily.

My jeep has like a 6.xx first gear + the transfer case which is like 2.xx i think in low, then my gear ratio is 5.38s. My final crawl ratio is like in the 80s-90s lol.

Luke300zx

Discussion Starter · #20 · Jul 25, 2012

J.Rieder89 said:
i would love to get that kinda fuel milage i have a 5.3L and maybe get about 250miles to a same size tank, but my stock gear is 3.42 and have no get up at all. also im still adjusting from my old tuned duramax

With my 3.42's in my 4 spd silverado I got about 350 to the tank with a 5.3L and stock tires. Now with 35's and the same gears I only got like 275 to the tank recently. I'm going to be regearing since i do a lot of towing, even though this is my DD i have jetskis, a quad, and a Teryx, so I tow my toys like every weekend and the advantages of in-town driving vs my commute to work isn't bad. Since my commute is stop and go anyway I'd rather have the little extra gearing.

I'm expecting my mileage to go back up after talking to other people who have those 4.56's with the same truck as me. You COULD do 4.10's if you're on the highway a ton but it really would only be like a ~100rpm difference at 65mph between the 4.10's and 4.56's. So the call is up to you. About 95% of the people who get 35" tires also get 4.56 gears and every single person has been happy with them. If you had the 6spd transmission i'd say 4.10 would be fine. Since that transmisson has a very high 1st gear.

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