2nd Gen Non-Engine/Transmission What's My Effective Gear Ratio?

[DwayneKelch]

I'm getting ready to purchase new tires and would like to get a set of 285/75-16s. However, I'm worried about the impact to the effective gear ratio.



I have 3. 54:1s, but not sure if that ratio is with 245s or the 265s that came on the truck. I'm guessing that the ratio is with the 245s since the 265s were considered an option.



I don't want to get the 285s if they'll cause my effective ratio to be too tall a gear that it forces me to downshift to 5th gear on a grade that I would normally pull in 6th.



I normally don't exceed 8K pounds when towing and have a TTPM and EDM 1. 5s with stock turbo. Also, don't want to downshift into 5th just to keep the RPMs up to lower EGTs.



Best I can figure, if 3. 54:1 is with 245s, the effective gear ratio with 285s will be 3. 29:1. I'm no expert with gears, so could use some advice/input. Thanks.





Dwayne

 

[KevinLewis]

3. 54:1 is the rear end ratio and has nothing to do with tire size. Tire size does play into your overal gear ratio. Do a search for gearing calculators on the web and you can find all kinds of info. I don't think the 285's will cause you to have to down shift anymore than you do now with your mods. I say go for it and add more power later if you need it.

 

[C. Harlan]

I have always had 285's, and have never had any issues, whatsoever. It is actually beneficial in the long run, saving miles on your odometer. just my 2 cents worth...



-Chris-

 

[B KIRK]

HERE'S a chart I like to use to determine final drive ratio.



Brad

 

[DwayneKelch]

Gentlemen,



I just reread my thread and realized I had two thoughts going at once. Your replies made me realize I needed to separate the two ratios; gear and effective/overall. I did a little research on the internet and found many tire calculators that help determine the impact to gear ratio when stepping up to a larger tire diameter.



Using the tire calculator on the website, I established a baseline with the 245s and compared that to the 285s. The gear ratio would change from 3. 54:1 to 3. 29:1. Would there be a noticeable difference between 3. 54s and 3. 29s? Seems a little tall. Right now I'm happy with the towing capability of my truck and don't want to do anything to jeopardize it.



If I can be assured there's not much difference, I'd switch to 285s. I would like to fill the void inside the fender well and decrease RPMs in an effort to improve fuel mileage at 70 mph. Also, to insure I wouldn't be issued a speeding ticket, I'd have the dealer reprogram my ECM for the larger tire size.



I'm worried about going too tall a tire as I made that mistake before with a chevy pickup I had. Went from stock (not sure of diameter) to 36" diameter and it killed the performance. Don't want to make the same mistake. Guess I need to do a little more research because I'm not quite convinced I'd be doing the right thing.



Thanks for your replies.





Dwayne

 

[NIsaacs]

Dwayne, the 285's will slow down your acceleration a small amount and you will be able to feel it, however don't be afraid to downshift out of 6th to 5th. Overdrive in any standard transmission is always the weak link, so when towing hard up hill it is to your benifit to be forced into a downshift. It has always been my opinion that the trucks having trouble with their transmissions, usually the 1st and 2nd gen. 5spds, are running 4. 10 gears. This allows the mighty Cummins to tow just about any load up any but the largest hills in overdrive. Thus, 5th gear goes bust! With the 6spd, after a down shift to 5th or direct (you can't hurt direct) you can stand on it all day with never a worry. With 285's, 3. 55's and the 6spd, you can run about 65-mph in 5th at about 2300 RPM. With your mod's, this will be good enough to tow at max factory rating any 6 percent grade with ease. I love my 285's, 3. 55's and 6spd.





"NICK"

 

[Gary - K7GLD]

I'm about to make the same switch to the 285's - I have more than enough power to comphensate for the taller tire, and actually PREFER to shift down into 5th gear in steep grades to save the strain on the transmission - the taller effective drive ratio will better allow me to do that, and still maintain excellent road speed. Fuel economy might also benefit with the taller ratio, at least when traveling empty.

 

[nps]

I've got the ETH HO with a EZ, 3. 54s and 315/75R16s.



I pull about 8500 pounds up all but the steepest grades in 6th without losing speed.



When I do downshift into 5th I generally hold it to 60 mph even though I could go faster.



I think 285s would be no problem. You say you are already at 265s so your incremental change is small.



(on edit) The best part about the 315s is that it makes 2nd gear a real gear unloaded. With the stock 265s, 2nd gear was too low of a starting gear even with the 3. 54s.

 

[Graphic man]

Dwayne

keep in mind a 285 will not get you better MPG you will lose some (in most cases)

i travel a lot of HWY's and keep a close eye on my MPG i just took off my 265's and put back on my 285's i went from around 20 to 19 at 60-65 mph

and your egt's will climb a little due to a little more load in OD and due to less RPM's around (1750)

hope this helps



Scott

 

[Gary - K7GLD]

Dwayne

keep in mind a 285 will not get you better MPG you will lose some (in most cases)

i travel a lot of HWY's and keep a close eye on my MPG i just took off my 265's and put back on my 285's i went from around 20 to 19 at 60-65 mph

and your egt's will climb a little due to a little more load in OD and due to less RPM's around (1750)

hope this helps



Scott

Scott, I wonder if you remembered to factor in the new odometer error when calculating MPG with the 285's - your displayed miles traveled will be about 7-8% LESS than actual, and you gotta factor that in when figuring MPG...

 

[DwayneKelch]

All,



Everyone has brought up some very good points; better to be in 5th rather than 6th because of weaker 6th gear, higher EGTs as a result of lower RPM when pulling steep grade, slower offline acceleration, etc. I don't mind shifting into 5th to maintain momentum/speed up a steep grade, actually appears to be the smarter thing to do. Saves the transmission and lowers EGTs.



Scott, as Gary asked, was your fuel mileage calcs taken after an ECM recode? One thing I don't want to do is sacrifice fuel mileage. After all that's the reason I want to go with a taller tire, well, and to fill the vast opening in the fender well.



I talked to a guy at a tire shop today and told him I'd like to put a 285/75 tire on my truck. He tried to talk me out of it and told me to stay with the 265s. He said rolling resistance from the wider tire would negate any gains I'd expect to see... bacisally a wash, no gain or loss. He also stated 285s would put more stress on my drivetrain.



Guess I'll have to sleep on this decision tonight. I would like to purchase the tires tomorrow as I'm leaving for Alaska on 6 Jun. Appreciate all the info you guys have provided.





Dwayne

 

[EBottema]

Dwayne. The best gear ratios to have with those size tires is 3. 73. However, 3. 54 will be fine. I would highly recommend getting either 265 or 285. 285's look much better and are better off roading of course. You cannot go wrong with either, it is up to your and your wallet... good luck...

 

[Graphic man]

Gary,

the difference in odometer was changed

i have a friend in a Dodge dealer that re-flashes it for me

and that figure was on paper, we all know the over head is not accurate,

this is all with a 3:54 gear i wish i would have ordered the truck with a 4:10 gear and then go with a larger tire size,

other people may see a something different, this is just in my case.



Scott

 

[DwayneKelch]

Gentlemen,



I had my new tires put on today. After all the research and input I received, I decided to stay with the 265s, sigh... still have the large void in the fender well. The 265s are a known and the 285s are an unknown to me. I didn't want to sacrifice performance/mileage I currently enjoy. Maybe the next set will be 285s, and if I'm not pleased I'll sell the truck. By then I'll be ready for a new truck anyway.



Appreciate the time everyone took to respond to the thread. It was beneficial to me because I learned the importance of shifting into 5th instead of stressing 6th. The truck is now ready for the trek to Alaska.





Dwayne