To keep her stock you need to know what was stock for your truck. I think they came with everything from a 3:08 to a 4:11. It should say in your glove box sticker what it came with.
I guess I wasn't clear. i want the engine to turn at the same RPM at a given MPH that it did stock now that I have 33's. It came with 4.56s (At least the military version did) stock. So I know I need to gear it lower so what gears should I use?
What you need to do is figure out what gears are in it now. What transmission, and the diameter of your current tires. You need the transmission info so You can get final drive ratio from it. Then use this http://www.csgnetwork.com/multirpmcalc.html to get your current rpm at given speed.
Then fart with it to find out what rear gearing you need to maintain the same rpm. With 33" tires. But you have to have allyoir info first. Not all blazers came with 4.56 so if yours isn't military spec then likely diaric is right and its between 3.08 and 4.11.
For example. Th400 in my truck has a 1:1 ratio in third. 4.10 rear end. And 30.5" tires. So at 65 I'm turning 2936 rpm.
Dan, this little formula will give you a quick and dirty. Just dont think about what the numbers "mean" while your calculating. Take tire height divided by rear end ratio. Write that number down. Take your new tire height divided by the number you wrote down and thats your rear end ratio to keep it the same in final drive. The problem is you might not be able to get the ratio you come up with.
Heres an example. 31 inch tires with 3.42 gear gives a number of ( 31/3.42)= 9.064. You moved to a 33, so take 33 divided by 9.064 will give you 3.64. There is not ( that I know of at least) a 3.64 gearing option. This little quick and dirty should give you what you want to know, transmission wont matter because your not changing it.
TH400 for a military truck, TH700 Overdrive civilian version
Look for a modulator on the transmission, it will be a small can on the passenger side under the cooler lines. If you have a modulator, its a TH400, if not, its a TH700
If he can get a pictue of the tranny pan we can tell right quick. I know that a TH350 will go in place of a TH700 because they have the same spline output shafts ( 27?? i think) and are roughly the same length. I have a good friend who had one of these these trucks with 300k and he was bringing me up to speed on the trans. He swapped his th700 to a th350 and said he didnt have to change anything, bolt and go. His was two wheel drive. He swore up and down these are great little engines.
If your Blazer is civilian it almost certainly has 700r4 as others have said. It also almost certainly has either 3.08 or 3.42 diff ratio. Don't rely on this info tho cause anything can be replaced and often is on a 20+ yr old truck
Okay. I am assuming that the rear end is 3.08 gears. If so and I had 30 inch tires stock and an overdrive gearing of .696 from a 700r4 then my engine RPM would be 1561 at 65 MPH. Does that sound about right for a diesel?
If I went to 33 inch tires I have some axles that already have 4.10s in them. This would give me an RPM of 1889 at 65 MPH. Would that be to high an RPM? How high an RPM can a 6.2 handle for steady speeds without killing it?
I run mine at 3k almost constantly. I have a th400 and 4.10 gearing with 30.5 tires. At 65 I'm at just about 3k. I do mostly highway driving and get 20mpg. These engines love it.
Your RPM would depend on if you have a lock up converter or not ( and if its working). Its always free to just put on your tires and run it like it is to see how it cooperates. Most of these trucks came with 235/75/15's stock. Moving to a 31 is a 2 inch jump in tire size. Most trucks wont even know the difference. In My Opinion you probably dont need to change rear ends unless you have a 3.08 rear gear. If my math is right, the trucks feel a 0.20 change final drive for every shift in tire size by two inches. If your dead set on a change, I would dig around until you find somebody with the combination you are looking at and ask them how it does. Chances are there is not much we can do as far as combo for tires and gears go that somebody has not done before. You can figure and figure and figure, but what matters is where the rubber hits the road.
Are you just driving this truck or are you towing, have future plans for it?
I have the new axles that I am putting under the truck in the shop getting rebuilt. They are torn apart and I have to make a decision on gears. I can't put the tires on the truck as it sits now since the tires are on 8 lug rims that match the new axles and the current axles are 6 lug.
So, I don't have anyway to try things out and I have to get this right the first time. I wish I could put the tires on and try it out before deciding on gears, but I can't. Kind of a pain but I do have to figure this out and buy the gears without any trial and error.
I would be towing a light trailer 2000 lbs max. I will have the truck itself loaded down though, bumpers, winch, fuel, overhead rack and all of that will really add to the weight
If you only have the option of 3.08 or 4.10 and you're putting on bigger ntires I'd go up to the 4.10's. It would put you in the sweet spot for torque and fuel efficiency. You'll have better acceleration and you won't lug the engine. If you had other options tho I'd probably go with 3.73. a little lower cruising RPM for fuel efficiency but still enough torque to move he tires and give decent acceleration.
Diaric, why wouldn't someone want 4.10 gears with 33" tire and a T400. I would have thought that you`d need the torque to turn the bigger tires.
In a thread earlier this year you suggested that my seemingly slow acceleration may be because of bad tire size/gearing ratio. I have 4.10 gears, t400 and 29.75 tires (245/75R16) What gearing would you suggest. I can cruise all day at 100kmh and can bury the needle and the truck doesn't mind (my speedo actually says a little faster than I`m going but my tires are a bit smaller than stock 235/85/16 and i haven't adjusted.) but I sure would like better acceleration. I can beat a semi off the line but I get smoked by cube vans.
"Wow, I was under the impression that high RPM's are not good for a diesel. 3k is close to red-line aint it"
I think the difference is IDI vs DI diesels. From what I read the more knowledgable guys talking about the IDI's can take way higher RPMs. Redline is over 4000rpm I believe. At least that's where the Ip governor hits. (Actually over 2000RPM At the IP which spins half of engine speed)
If you are asking about final drive RPM use the RPM calculator or the formula that was posted here. The 4.10/30"/ th400 combo gives over 3000RPm at 65mph. The 4.56/33"/700r4 give a bit over 2000RPM at the same speed. To get the same RPM at 65 you'd need rear gearing around 6.5 which is not a stock axle and would be really hard to find and I'm sure not cheap. Sure would crawl through the mud and up hills though.
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