8 speed vs 6 speed gears and larger tires.

I have read a lot of confusion over what the new 8 speeds can handle as far as larger diameter tires so I decided to through some math out there going over the first gear ratio of each transmission. The first gear in the 8 speed is 4.71 and 3.00 in the 65rfe. Not to insult intelligence but to be clear that means the engine (assuming the TC is fully locked) will turn 4.71/3.00 revolutions for each turn of the output shaft. If you have a diff gear ratio of 3.21 then the output shaft would need to turn 3.21 times for the tires to rotate once. So if you take 4.71X3.21 you get 15.12 engine revolutions for each tire revolution. Now people say that you need 3.92 or high to run larger tires... Here is where I disagree. If you take a 65rfe's first gear of 3.00X3.92 you get 11.76. Jump it up to 4.56 and you get 13.68. That is still under the 15.12 that the 8 speed turns with 3.21. It would take 4.88s to even get close so now I ask... how many think you need at least 4.88s in a 6 speed to run 35" tires. Most would say that is too high but that is effective the same ratio of the 3.21 gears in an 8 speed. Just thought I would be informative. For any one with an 8 speed and 3.21s looking to move up in tire size you will be just fine. Thanks for reading. Any comments or corrections are appreciated.

If I had a 1/2 ton with 35"s I would love a set of 488"s.. Hell I wish I could in my 3/4 ton.. One day I will upgrade it from 373 to 456.. I have had gears done in many different cars and trucks and I feel a 456 in this 6.4 2500 would be perfect. I have not driven a newer ram with the 8 speed or the 6 speed so I can't speak to that.

Interesting

Thanks for the info. This is something I have been concerned about and at the same time very ignorant about.

i will be coming back to this thread, i need educated in this area as it is something i dont have a good grasp on. i would like to understand more.. so standby for questions!

CRam said:

I have read a lot of confusion over what the new 8 speeds can handle as far as larger diameter tires so I decided to through some math out there going over the first gear ratio of each transmission. The first gear in the 8 speed is 4.71 and 3.00 in the 65rfe. Not to insult intelligence but to be clear that means the engine (assuming the TC is fully locked) will turn 4.71/3.00 revolutions for each turn of the output shaft. If you have a diff gear ratio of 3.21 then the output shaft would need to turn 3.21 times for the tires to rotate once. So if you take 4.71X3.21 you get 15.12 engine revolutions for each tire revolution. Now people say that you need 3.92 or high to run larger tires... Here is where I disagree. If you take a 65rfe's first gear of 3.00X3.92 you get 11.76. Jump it up to 4.56 and you get 13.68. That is still under the 15.12 that the 8 speed turns with 3.21. It would take 4.88s to even get close so now I ask... how many think you need at least 4.88s in a 6 speed to run 35" tires. Most would say that is too high but that is effective the same ratio of the 3.21 gears in an 8 speed. Just thought I would be informative. For any one with an 8 speed and 3.21s looking to move up in tire size you will be just fine. Thanks for reading. Any comments or corrections are appreciated.

Click to expand...

Thanks for that info, very informative. I would have been asking this in the spring.

This seems to make a lot of sense. I think everything you said is correct. By that math these 8 speeds really do make regearing seem unneeded in more cases.

At the higher gears though, wouldn't you still have the old issues? Since the gearing on the higher gears is still closer to the same as the old one, the trans might hunt, etc


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Do the 8 speed trucks have a selector to switch to 7 or 6th gear to be the highest ran gear , like the 6 speeds do ?
If so at higher speeds you could just select a lower gear in the Xsmn , this also disables the MDS so you won't get the dredged droan sound .

How long is your 8 speed in 1st gear though? in a WOT run, the 8 speed is already in 3rd gear within 3 seconds. Do the math in 3rd gear or higher and see how the transmissions and gear ratios compare.

While I am not sure about the hunting, I have researched that the 8 speed has so many gear maps that it would not be that bad. I'm sure members who have actually done 35" tires with 3.21s could chime as I am only speaking hypothetical. Also with the electronic shift you can limit your highest gear like RLK pointed out.

R.L.K. said:

Do the 8 speed trucks have a selector to switch to 7 or 6th gear to be the highest ran gear , like the 6 speeds do ?
If so at higher speeds you could just select a lower gear in the Xsmn , this also disables the MDS so you won't get the dredged droan sound .

Click to expand...

Yeah I believe they do so that's a good point. You could always just induce tow/haul as well to keep it from shifting too soon.


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R.L.K. said:

Do the 8 speed trucks have a selector to switch to 7 or 6th gear to be the highest ran gear , like the 6 speeds do ?
If so at higher speeds you could just select a lower gear in the Xsmn , this also disables the MDS so you won't get the dredged droan sound .

Click to expand...

Yes you can keep it from going into a higher gear. The buttons on the steering wheel. I've kept mine in 6th sometimes pulling the boat.

Hemi450hp I see your point but that can be overcome with the electronic manual shift. Even the 8 speeds 2nd gear is better at 3.14 then the 65 first with a 3.00.

True. For anyone that wants to use their automatic transmission as an automatic transmission for daily driving though, I'm still a firm believer that the 3.92 is the best gear for the 8sp trucks. No need to go higher than that though, and the 3.21 is likely plenty for most guys with stock tires...but with 35"+ tires or guys that plan to race the truck, the 3.92 is still the best gear for the truck. The 8sp is much more forgiving than the 6sp though with the 3.21 gears...no doubt about that.

Yes I agree 3.92 would obviously still be better. My goal was just to point out that when comparing gears the norms have changed with the 8 speed.

CRam said:

I have read a lot of confusion over what the new 8 speeds can handle as far as larger diameter tires so I decided to through some math out there going over the first gear ratio of each transmission. The first gear in the 8 speed is 4.71 and 3.00 in the 65rfe. Not to insult intelligence but to be clear that means the engine (assuming the TC is fully locked) will turn 4.71/3.00 revolutions for each turn of the output shaft. If you have a diff gear ratio of 3.21 then the output shaft would need to turn 3.21 times for the tires to rotate once. So if you take 4.71X3.21 you get 15.12 engine revolutions for each tire revolution. Now people say that you need 3.92 or high to run larger tires... Here is where I disagree. If you take a 65rfe's first gear of 3.00X3.92 you get 11.76. Jump it up to 4.56 and you get 13.68. That is still under the 15.12 that the 8 speed turns with 3.21. It would take 4.88s to even get close so now I ask... how many think you need at least 4.88s in a 6 speed to run 35" tires. Most would say that is too high but that is effective the same ratio of the 3.21 gears in an 8 speed. Just thought I would be informative. For any one with an 8 speed and 3.21s looking to move up in tire size you will be just fine. Thanks for reading. Any comments or corrections are appreciated.

Click to expand...

Now, I understand that you're saying that the 3.21s with the 8 speed are better than the 3.92s with the 5/6 speed and in this case the answer is yes if you compare just the first gear ratio but as Matt has pointed out you have a very quick shifting trans and you will be into 2nd gear very quickly which is a 3.14 ratio and then into 3rd (2.14) and 4th (1.72) which have equivalencies in the 65rfe to 1st (3.0) and 2nd (1.67) and below that you have better combinations in the 5/6 speed over the 8 speed both which end up in a final gear ratio of 0.67 and both will downshift 2 gears to a 1.00 ratio at highway speeds.


At best, the 8speed/3.21s are comparable to the 5/6 speed/3.92s and will suffer the same acceleration degradation when you move up to a larger wheel combo, so your base hypothesis that the norm has changed is incorrect, it has in fact remained the same. A 6 speed/4.56 compared to a 8 speed/3.92 is also comparable through the whole shift schedule and the 8 speed/4.56 is obviously the best combo out there at the moment. If you are getting grief from buddies about having 3.21s then you can tell them it's the exact same as getting the 3.92 combo in the 6 speed but with a better take-off gear. If you plan to just run it on the street with a stock diameter tire then you'll be just fine with the 3.21s.

And to continue the conversation, the math is just fine, but you are now getting into a more indepth discussion that needs to happen if you're going to talk about going to larger tire sizes. Yes, the effective gear ratios you have calculated are correct but there is still a big advantage of going to a higher gear ratio when running larger diameter tires and that is due to one thing: rotating mass.

My stock tires on the OEM 17" wheels weight a total of 65lb each IIRC with the stock 365/70R17 SRAs; that's a total of 260lb that the engine has to turn from the factory. For comparison I'll use a combination that's fairly popular and that's a set of 17x9 Fuel Boost wheels (32lb ea.) and sets of 315/70R17 BFG AT T/A KO2 (65lb ea.) and 35x12.5R17 Nitto TerraGrappler G2s (69.4lb ea.). That is a combo of 97lb and 101.4 with combined weights of 388lb and 405.6lb respectively. As you can see there is a significant increase in the rotating mass (128lb and 145lb respectively) over the stock wheel/tire combo that now needs to be driven, and keep in mind that this is specifically rotating mass. A very common comparison is that adding/removing 1lb of rotating mass is the equivalent of adding/removing 7lb of sprung weight so now that combo difference now becomes an equivalency of 896lb and 1015 plus you have now moved the application length out further which decreases the applied force that the tires will generate.


An easy way to compare that added mass is to throw a quad in the bed of the truck, and aside from how much it sags you will notice a very distinct decrease in how quickly you accelerate. This is now an everyday event because I highly doubt you will be changing the tires anytime soon, big rubber gets expensive! Now, throw a quad in the box and suddenly you start seeing how this can compound issues. If you happen to have a 1 ton to tow then great, you're not going to throw anything heavy into the box, ever, right?


The 8 speed/3.21 combo will be better in the short lived 1st gear but then it's comparable to the 6 speed/3.92 again until it hits 4th gear which will then lag behind as it continues to shift up to 8th with the same final drive ratio between both trans. The reason I bring this up is because of what gears do you normally stay in? On the 6 speed it's 4-6 and in the 8 it's 5-8 where the 6 speed actually has a better ratio combo than the 8 speed. Is the difference significant? not really, and you probably won't even be able to tell the difference between the two. A higher gear ratio is still going to be better if you're going to run larger tires, even with the 8 speed.

Charon. Thanks for the input. You do bring up good points about the comparison and time in each gear. I also agree that for it to be more reasonable you would have to manual shift to hold those gears to make them worth while. Also the higher gear ratio will always be better. I still believe that the effect of running larger tires on an 8 speed are minimal compared to a 6 speed. I think a lot depends on tuning and shift maps. Now I am pulling the statement out of thin air but I would venture to say that under the same circumstances the 3.92 would upshift even faster in the 8 which brings me back to the point of needing to hold the gears.

You guys also have to take into consideration which throws all debate nearly out the window at this point, is that the 8spd has been proven to be a much more efficient transmission over the 65rfe. Means more hp/tq to the wheels at any given moment and its lighter lol.

So 37's should be ok then. I bought a six inch lift kit, and I need new tires.

Oh boy a post like this might keep some people from buying new gears. I've been saying all long how well this 8 speed and 3.21 go together.