Rear gears
 

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.

Re: Rear gears
 

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.

Re: Rear gears
 

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

Re: Rear gears
 

Tire size plays a huge factor.

Re: Rear gears
 

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

Re: Rear gears
 

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.

Re: Rear gears
 

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

Re: Rear gears
 

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

Re: Rear gears
 

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

Rear tires are 27" dia.

Re: Rear gears
 

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.

Re: Rear gears
 

3.73 ratio will be the best for your needs

Re: Rear gears
 

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.