3.08 to 3.73?

[drabell13]

Need help. Thinking of having my gears changed from 3.08s to 3.73s. Trying to decide if its worth it and if 3.73 is right. Currently running a Crate 350 with a 350TH, a 4bl carb, and 33x12.5 ATs. Its pretty sluggish know and my gas mileage sucks (about 10mpg). Will 3.73 gearing be real noticeable change and how would it effect my mileage? May eventually go to 35 but not for awhile. The 33s are only a year old. Any advise?

[blazin]

if you planning on running 35's as your next set of tires.. go with 4.11 gears. i run 35's with a 700r4 tranny and im running 4.56. great choice imo

[drabell13]

What kinda mileage are you getting? Also, did you put the 700R4 in yourself, how hard was it? I've been thinking about swaping out my 350TH.

[blazin]

Originally Posted by drabell13

What kinda mileage are you getting? Also, did you put the 700R4 in yourself, how hard was it? I've been thinking about swaping out my 350TH.

me and a friend did it.. it was pretty straight forward.. the 700 is 2.5 inchs longer so just move the transfer case mount back had to get a 700 to a 205 adapter. do the driveshafts. works great.

[blazin]

Originally Posted by drabell13

Need help. Thinking of having my gears changed from 3.08s to 3.73s. Trying to decide if its worth it and if 3.73 is right. Currently running a Crate 350 with a 350TH, a 4bl carb, and 33x12.5 ATs. Its pretty sluggish know and my gas mileage sucks (about 10mpg). Will 3.73 gearing be real noticeable change and how would it effect my mileage? May eventually go to 35 but not for awhile. The 33s are only a year old. Any advise?

i dont think 373's will give you better gas mileage, but more get up and moving off the line and ok top end. the reason i said 4.11 is you will get good mileage and power with 33- 35 tires. 308's is a highway gear with 27's-28's tires. have a look at this gear chart, half way down the page ... http://www.4x4offroads.com/gear-ratio-chart.html

[drabell13]

Thats helpful. How bad do you think hwy driving would be with 4.11 with out OD? (70-75mph). Do you know what type of mileage you are getting with the 4.56 and 700R4?

[blazin]

Originally Posted by drabell13

Thats helpful. How bad do you think hwy driving would be with 4.11 with out OD? (70-75mph). Do you know what type of mileage you are getting with the 4.56 and 700R4?

depends on how soon you are getting the 35 in tires lol. if you do more hwy driving then city or off road, then you should stay with 373's. but if you want more get up and go, do 4.11's. im building my truck for a fun toy and i dont really care about gas mileage. thats why i put a 383 stroker with 465hp/492 tq and i run 35's. if i wanted mileage i would drive a car lol. i put a 700 in it for the overdrive, knowing that it will help out on the gas bill over time. maybe someone else could point you in the right direaction.. but i would go with 4.11 if your planing on up grading your tires to 35's, even if your staying with 33's , just for the fun factor.

[drabell13]

I appreciate the help.

[Nightrider]

I went with 3.42 and 33 inch tires, still gives a good hwy speed.

[Jtrux]

Originally Posted by Nightrider

I went with 3.42 and 33 inch tires, still gives a good hwy speed.

That sounds like the thing to do IMO.

[Critter]

Before I tore mine down I had a 350/th350 with 3.73s and 35 inch tires and I can say it was too high geared even for highway driving. I had plans to swap it to 4.10s. On the other hand, 33 inch tires with 3.73s would be a decent choice with 3.73 gears for the highway. An overdrive would be a good choice with 4.10s or 4.56s but either way don't plan on getting "good" mileage out of one of these vehicles. They are built like a brick which isn't good as far as wind resistance is concerned. You might change over to efi, overdrive transmission, etc only to pick up a couple extra mpg, but it isn't worth the cost to get there. Now if you want updated drivetrain components simply for the cool factor/reliability, then get after it, that's what I'm trying to accomplish.

[AlaskaMatt]

I have the 350/350 combo and 3.73's and 33's. I run about 2350 rpm at 55. Closer to 3k at 65 plus. Average 12.5 mpg. I should note that I rarely run over 65mph in the blazer. It really likes to go 60-65 mph. Snappy enough for me off the line.

Actually this setup and gearing with the 700R4 would be perfect in my mind. The OD should drop you closer to 2K when cruising.

[Critter]

Originally Posted by AlaskaMatt

Actually this setup and gearing with the 700R4 would be perfect in my mind. The OD should drop you closer to 2K when cruising.

I will respectfully disagree. A 700r4 with a true 33 inch tall tire with 3.73s will put you at 1861 rpm according to the calculators. From what I know (which is not everything of course) your tranny will be down-shifting quite a bit. I like to see an overdrive running right at 2000rpm at 70 mpg which is right were you would be with the same setup and 4.10s. But that's just my opinion for street driven vehicles.

I look here for calculations.

[AlaskaMatt]

Originally Posted by Critter

I will respectfully disagree. A 700r4 with a true 33 inch tall tire with 3.73s will put you at 1861 rpm according to the calculators. From what I know (which is not everything of course) your tranny will be down-shifting quite a bit. I like to see an overdrive running right at 2000rpm at 70 mpg which is right were you would be with the same setup and 4.10s. But that's just my opinion for street driven vehicles.

I look here for calculations.

I understand where you are coming from. However, the calculators have never added up to my real world results (and my 33's are pretty much true 33's actual measurement at 32.7). I was basing my estimate on the final drive ratio against my current cruising rpm. I do agree that if you actually ended up down at 1800 you would have tranny hunting issues.

Using the .70 OD gear to figure. At 65 I am currently turning 2800 which puts me right at 1960 for OD rpm. At 55 it would be down at 1645 and certainly could be a problem. I think you could avoid it by staying out of overdrive manually unless on the freeway.

[Critter]

Originally Posted by Mike C

The big issue with trying to calculate theoretical is the torque converter slippage.

I agree, that's one reason I like the gear calculator I linked to, it has an option for "auto tranny OR manual/auto with lockup torque converter". I don't know exactly what correction factor it adjusts, but it does change things a little so it makes me feel a little better about it. Most don't have anything like that.

Originally Posted by AlaskaMatt

However, the calculators have never added up to my real world results...

I know what you mean, they can never get 100% correct numbers, there are just too many variable on a real vehicle, but a good one can get you pretty close. They are one of our best tools for internet discussions, but maybe not much more than that.

[Mike C]

Tire height is a good start for calculators, but what you really need is the rolling circumference of the tire, and if you ever watch top fuelers, you know that tire growth has to be figured in! Admittedly it's nothing like that with a street car, but big tires run at lower inflation levels (I run my Wrangler II's at 24 psi which even then has them wearing towards the middle). So you have the inaccuracy of a stock speedo (possibly REALLY off now that tire and gear changes might have been made), any error in whatever tach you may be running. When you combine each of these errors, even if they are small, you get a BIG error in the end.

[Critter]

Also keep in mind a lot of this is contingent on your ACTUAL tire hight, more importantly the effective radius from the center of your hub to the ground. NOT the size that is stamped on the sidewall.

[Mike C]

The big issue with trying to calculate theoretical is the torque converter slippage. My 69 Camaro with 4.10 and manual trans runs 3100 rpm at about 65 while my brothers car with T350 and 3.73 turns 3500 when cars are running side by side.

I would do no more than 3.73 with the T350 and 33's for sure. 35's and 4.10 are great with my 4 speed, no converter slip! Like most problems, how much money do you want to spend?

Re-gears cost $500 if you do it yourself and more like a $1000 if you have to pay to have it done. The TH700 swap is probably a $2000 swap after you get the driveshafts shortened. So... What are your future goals/plans and how much coin do you want to drop? Once those questions are answered you can get a better idea on what you should do.

[elks]

what is your goal for the rig? Are you talking a commuter back and forth on the highway? A vehicle to put around town in? A trail rig? Or a combination of all 3? I am running a 465 with 4.56 and 36" tires/ High Way speed for me is limited to 65 as it is the speed limit. At 65 I am running around 3,000 rpm. Of course it was not built to run the high way, it was more for trails. So the little bit of high way I run is not a big deal since it is usually getting to or from the trail.

[drabell13]

I'd say my goal is a combination of all three. It's my daily driver and will stay that way for at least the next two years. It's kinda a street queen. I've been cleaning it up for awhile. I take it on the trails but nothing serious. I'm getting a camping trailer in the next couple of weeks so I'm going to need it to be able to pull 3000-3500 lbs a couple times a month without issues, which I know the current 3.08's are going to give me problems. So, I'm looking for a nice median. I will probably always stay with 33's, but have been thinking of putting in a 700R4 for awhile. I know if I go higher than 3.73 I need OD if I want be okay on the hwy. So my delimma is basically do I put 3.73s and eventually add the OD or do the 4.10s and put the OD at the same time. I'm not going to change the axles twice.

[elks]

Originally Posted by drabell13

I'd say my goal is a combination of all three. It's my daily driver and will stay that way for at least the next two years. It's kinda a street queen. I've been cleaning it up for awhile. I take it on the trails but nothing serious. I'm getting a camping trailer in the next couple of weeks so I'm going to need it to be able to pull 3000-3500 lbs a couple times a month without issues, which I know the current 3.08's are going to give me problems. So, I'm looking for a nice median. I will probably always stay with 33's, but have been thinking of putting in a 700R4 for awhile. I know if I go higher than 3.73 I need OD if I want be okay on the hwy. So my delimma is basically do I put 3.73s and eventually add the OD or do the 4.10s and put the OD at the same time. I'm not going to change the axles twice.

From my experience, I have never looked back and said, I wish I had a higher gear ratio. I would lean towards the 700r4 and the 4.10s. Heck maybe even just do the 4.10 and give it a try. You might be surprised at how it does. For me I think the added acceleration with a lower gear is what eats up more gas than the RPMs. Also realize that the world is full of people wanting lower gears for wheeling. If you try it and do not like it odds are you can find someone to swap straight across. I know you do not want to do it twice, but with the 4.10 gears you will be much more likely to find someone willing to pay a little extra if you decide to change. For me 3.73s are just too common. Get in the 4s and the are much more sought after. Also around here towing with a 4.10 is much better than 3.73 due to the mountains...