Rear gears

[pontiacvince]

After driving my '71 Suburban with the original warmed over 307, 700r4, and 3.07 ring and pinion gear for a while, I've decided I need to change rear gears. At 70mph, it's only turning 2000rpm's. Looking at the vacuum gauge, it's struggling at that low rpm. If I had a bigger engine with more torque, it wouldn't be a problem, but the 307 wants and needs more rpm's. They sell a 3.42 and a 3.73 for the 12 bolt truck rear with the 3-series carrier. I want to maximize fuel economy, but would the 3.42 still be too low? Opinions wanted.

[CAMAROBOY68SS]

I have 3.42's in my 68, it originally had 3:73's and my small block was a humming on the highway, but I also am running a M21 Muncie 4 speed. I like the 3:42's. I think you need to google one of those rpm calculators where it asks for your tire height and current specs and then see what the 2 options give you and then run that rpm and see where your gauge is at.

[skokie]

Its not a 67-72 Chevy truck but I have the 3.42s in my 01 GMC Sierra with the 4l60e. At 75 the rpms are right at 2100. Been a great highway gear with best of both worlds. Good mileage and power. The plan is to do the same with my 68 Chevy once I get to that point but POSI

[kwmech]

Tire size plays a huge factor.

[hevy_chevy67]

I have 3.73's paried with a 4l60e and a 700r4, I really like that combination.

[HO455]

As an apples to pears comparison. GM put 3.42 gears in most 2wd S-10's. So my 91 Blazer with the 160 hp 4.3 and a 700R4 weighting 3900 lb is a close match to a 307 C10 with a 700R4. I don't believe you would be unhappy with the 3.42 gears. Plus the 25 to 50 mph acceleration is really snappy.

[jocko]

vince what is your rear tire diameter? that will help the conversation.

[pontiacvince]

Originally Posted by HO455

As an apples to pears comparison. GM put 3.42 gears in most 2wd S-10's. So my 91 Blazer with the 160 hp 4.3 and a 700R4 weighting 3900 lb is a close match to a 307 C10 with a 700R4. I don't believe you would be unhappy with the 3.42 gears. Plus the 25 to 50 mph acceleration is really snappy.

HO -, thanks. I just think more RPM would actually HELP fuel mileage and drivability.

[pontiacvince]

[QUOTE=jocko;9142601]vince what is your rear tire diameter? that will help the conversation.[/QUOT

Rear tires are 27" dia.

[jocko]

Here are the numbers to help you choose. I think 3.73 or 3.42 would both be fine for cruising at >60 mph on the highway. If you want a little more accel than that 3.07, both options are improvements, especially with your 700R4, but 3.73 would be the way to go if acceleration is important to you.

[kwmech]

3.73 ratio will be the best for your needs

[pontiacvince]

Originally Posted by jocko

Here are the numbers to help you choose. I think 3.73 or 3.42 would both be fine for cruising at >60 mph on the highway. If you want a little more accel than that 3.07, both options are improvements, especially with your 700R4, but 3.73 would be the way to go if acceleration is important to you.

THANKS! It's not so much acceleration but passing and climbing hills on the highway. I plan to put miles on it. That's the reason for the 40gal. fuel tank. It just lugs with the 3.07.