best rear-end gear for TH-700R4/28" tire combo?

[glock35ipsc]

My 68 currently has a 3.08 rear gear as I was originally going to run a TH-350. Now I am going to use a TH-700R4 and was wondering if I should stick with the 3.08, or go with 3.73, or even 4.11? I have read on some other forums that 4.11 would be too much with the 3.06 1st gear of the 700R4.

I am running a mostly stock 350 crate motor with a small Comp Cams stick, a 600 Edelbrock, and a 28" rear tire. Since it is not a hot motor, I just want good bottom end (for the motor I have) and good highway driveability.

[big_al_71]

I went with "373" and a TH350 not the best combo for highway driving...I think it will be great with the 700r4 and "373"

[Ackattack]

3.73s are perfect for that combo. I put in a 700r4 and it was a dog with the 3.08s.

[SactoJim]

I'm running a 30" tire with 3.73 and a 700............perfect.

[d10s69c10]

i tend to run 4.11s a little too often. and yes 1st gear gets alot of neglect

[PanelDeland]

actually the perfect gear for just a driver(no heavy hauling and no towing) is probably a 3:42 .I only have one basic reason for thinking that.It's the gear ratio in most late maodel 700R4 trucks that I've seen.If they have a towing package it's usually a 3:73

[Longhorn Man]

Being that these trucks have the aerodynamics of a barn door, and you live in hilly country, I would go with the 3.73 in a heart beat. 3.42 may be better in late models cars/trucks with roller cams and reduced internal drag, but on an old school engine, lower RPMs isn't always a good thing.

[my67chevytruck]

I agree. I run 3:73 as well.

[67ChevyRedneck]

Carbed 350/700R/3.73 here as well. Works very well together. I have ~29" tall tires.

[glock35ipsc]

Thanks everyone. Sounds like 3.73 would be the best. I kind of figured that would be the best for my combo, but wanted some reassurance.

[67ChevyRedneck]

I'd love to try a 3.42 in my truck, but don't have money to experiment, so stock 3.73 she stays

[PanelDeland]

The added weight also probably makes the 3:73 a better choice.I was just wanting to illustrate that use plays an important role,as does budget.

[Painter D]

I've got a 28" tall rear tire, 700r4, and 4.10 gears. I wish I would have ran 3.73 gears instead. The 4.10 are just a little too low, if I had to do it again I would run 3.73's

[cparman]

I have a 3.73 also in my 69, and a 29" tire 255/70r15
Being as I am, I am thinking my 72 Blazer is going to get 3.42. Stupid here wants to turn 1500-1800 at 70 mph. There are several sites that have RPM calculators. look for one of those, and plug in the tire size,gear ratio, and OD number and it will give you an idea what your truck should turn

[powell72]

I've been thinking about ditching my 3.73's for 3.42's.

[glock35ipsc]

What's your thinking Chris? Would 3.42 be better on the highway than the 3.73, or 1st being too low?

[powell72]

Originally Posted by glock35ipsc

What's your thinking Chris? Would 3.42 be better on the highway than the 3.73, or 1st being too low?

Both actually, I'd like a couple more mph on the highway and my truck goes through 1st gear really quick with the 3.73's.

[glock35ipsc]

I need to take a ride around the block with you sometime to get the feel of it.

[powell72]

You know where to find me.

[glock35ipsc]

Yez I duz! I might swing by Thursday morning sometime.

[SCOTI]

Originally Posted by cparman

I have a 3.73 also in my 69, and a 29" tire 255/70r15
Being as I am, I am thinking my 72 Blazer is going to get 3.42. Stupid here wants to turn 1500-1800 at 70 mph. There are several sites that have RPM calculators. look for one of those, and plug in the tire size,gear ratio, and OD number and it will give you an idea what your truck should turn

Fuel for thought (pun intended).....

I recently drove from Dallas TX to Pigeon Forge TN for an event. The transportation of choice was a 2002 Collectors Edition Trans-Am. This the 346ci LS1 w/a 4l60e (updated version of the 700r4) & stock size 275-40-17's on 17x9" wheels. The motor & tune is stock GM but the trans is built (Vette internals, 2600 lock-up stall). Calculations were based on round trip to keep the numbers 'consistent' (which should eliminate 'one-way' directional bias).

We started out @ approx 62 - 65mph @ 2000rpm & averaged 20.4mpg. I decided to see what increasing speed slightly would do to the mpg. I bumped it up to 2300rpm/70mph & mpg went to 21.6. I then bumped it again to 2500rpm/75-ish mph & improved to 22.4mpg.

The theory is that the motor is operating within the same 'window' of efficiency from 21-2500rpm's. Since it burned the same amount of fuel @ either speed, the slower I went, the fewer miles I went per hour which was realized w/the driving range achieved per tank of gas.

So don't necessarily equate lower rpm's & lower speed to better mpg. My guess is a traditional sbc will want 2200rpm minimum for efficient operation (that low friction LS1 hyd roller set-up seemed to like it). Anything less & it will likely be struggling to maintain momentum which could actually adversely effect what you're trying to accomplish.

[kwmech]

Easiest way to decide is to figure the final drive ratio. A 3.73 multipled by the overdrive ratio (.72) will give you a 2.68

[crawdadfox]

I would go w/ 3.73 with the 700-R4, it would be fun to drive around town, and still be easy to deal with at highway speed.

[Boog]

Originally Posted by SCOTI

Fuel for thought (pun intended).....

I recently drove from Dallas TX to Pigeon Forge TN for an event. The transportation of choice was a 2002 Collectors Edition Trans-Am. This the 346ci LS1 w/a 4l60e (updated version of the 700r4) & stock size 275-40-17's on 17x9" wheels. The motor & tune is stock GM but the trans is built (Vette internals, 2600 lock-up stall). Calculations were based on round trip to keep the numbers 'consistent' (which should eliminate 'one-way' directional bias).

We started out @ approx 62 - 65mph @ 2000rpm & averaged 20.4mpg. I decided to see what increasing speed slightly would do to the mpg. I bumped it up to 2300rpm/70mph & mpg went to 21.6. I then bumped it again to 2500rpm/75-ish mph & improved to 22.4mpg.

The theory is that the motor is operating within the same 'window' of efficiency from 21-2500rpm's. Since it burned the same amount of fuel @ either speed, the slower I went, the fewer miles I went per hour which was realized w/the driving range achieved per tank of gas.

So don't necessarily equate lower rpm's & lower speed to better mpg. My guess is a traditional sbc will want 2200rpm minimum for efficient operation (that low friction LS1 hyd roller set-up seemed to like it). Anything less & it will likely be struggling to maintain momentum which could actually adversely effect what you're trying to accomplish.

Scoti I agree wholeheartedly. I had similar results with 2 of my trucks. My 88 with 305, 700R4 and 2.73 gear got better mileage running in drive instead of OD. It ran around 1550 rpm at 70 in OD. It was an absolute dog that had to downshift on the slightest incline or headwind.
My 97 K1500 Burb with 350, 4l60 and 3.42 gear consistantly gets better mileage running 75 versus 70 mph. Numerous times it's gotten 19.5 mpg running 75+ mph and 2200 rpm.
I am convinced a stock small block's sweet spot is between 2100 and 2500 rpm. I know it doesn't sound likely that running more rpm can increase the fuel mileage but it can and does to an extent.
One can tailor his vehicle to the way he most uses it to attain the best mpg. If you do more city driving a 4.11 can be beneficial but suck on the hiway. If you spend more time on the hiway then a higher gear will be better. That's where the OD trannys payoff. The 3.42 or 3.73 with OD will be a good all around choice of gears.

[powell72]

I agree with SCOTI and Boog, I think 2200 to 2500 is a good cruise rpm. I'm on the highway twice a day and my truck turns 2500rpm at 70. I want to continue turning it 2500rpm and be able to increase my speed at that rpm.
I have also read that you shouldn't be in OD at less than 1800rpm.