Best Diff ratio for 12.50 tires

[Lugnutt]

I'm into a restoration of my 72 Chev 4x4 short bed step side half ton truck. I've got a 350/350 with a 205 TC. When I bought the truck new I messed up and forgot to spec the diff ratio, so I ended up with 307's. At the end of my resto I'm planning on getting back into larger tires (12.50's) and I also want to pull a small travel trailer on occasion. I'm thinking about changing the diff ratios front and back to 373's. I'm concerned about making the most of my engine power at the speeds that I will be driving it with this trailer (55 - 60 mph). I know I can downshift going up steep mountain passes, but I'm most concerned about getting the best fuel mileage on the flats.

What to you guys think would be the optimal diff gear ratio for this application?

Thanks for your advice.

[Dingfodgy]

Well.....you'd get a better response if you stated the height of the tire, not the width. I'll go out on a limb and suspect your thinking in the range of 35" tire. If so, you're looking at about 2200 RPM @ 60mph. If you went with say a 700r4 Overdrive tranny- you could drop that to about 1500 revs @ 60 mph.

[SS Tim]

If you are planning on tires much taller than the original H78-15s, then 3.73 might actually help your mileage some. Without tire height numbers this remains all very general. Gearing to the factory optional ratio might be enough. However the rule of thumb I was taught, and in fact just used again on a buddy's Jeep JK, is to compute for the difference in tire size then drop down a ration to make up for the increased rotational mass and add some performance. Do you really want your truck to perform like it still has 3.07s when its carrying, say 33x12.50x15 tires? Use the calculator with known values and see what looks good then let us know.

http://www.grimmjeeper.com/gears.html

[domeier]

I used to tow my big ol' pig 21' Bayliner with 33x12.5"s and 3:73 gears. It was just right. 3:08 with a stock motor and big tars might be pushing it, depending on how much your trailer weighs. Only one way to find out...

[a2tripp]

i have a nice 355/350 on my k20 running a 35x12.5. I have 4.10 and can cruise 70 no problem. I have a 28' boat coming in at 10k lbs and haven't had an issue so far other than stopping

[Lugnutt]

Originally Posted by Dingfodgy

Well.....you'd get a better response if you stated the height of the tire, not the width. I'll go out on a limb and suspect your thinking in the range of 35" tire. If so, you're looking at about 2200 RPM @ 60mph. If you went with say a 700r4 Overdrive tranny- you could drop that to about 1500 revs @ 60 mph.

You're right, sorry, brain fade. I'm considering a 33" tire.

[Lugnutt]

Originally Posted by SS Tim

If you are planning on tires much taller than the original H78-15s, then 3.73 might actually help your mileage some. Without tire height numbers this remains all very general. Gearing to the factory optional ratio might be enough. However the rule of thumb I was taught, and in fact just used again on a buddy's Jeep JK, is to compute for the difference in tire size then drop down a ration to make up for the increased rotational mass and add some performance. Do you really want your truck to perform like it still has 3.07s when its carrying, say 33x12.50x15 tires? Use the calculator with known values and see what looks good then let us know.

http://www.grimmjeeper.com/gears.html

That's a great website, thanks. With 33" tires and 373 gears it looks like approx. 2400 RPM at 60 MPH. It's tough with my truck as I've never had a tach, what is the optimal RPM engine speed for freeway cruising with load? I'm looking at optimal fuel mileage.

and thanks again all of you for your advice and input.

[SS Tim]

You didn't mention the actual size tire you were running or if you were satisfied with performance and mileage on them. Stock G or H-78s were in the 28" range. So you could use that number for a before and after RPM at 60MPH. As far as best cruise RPM someone else is going to have to give you a real world number. But in the early '80s when people were trying to get better mileage, someone (Bruce Crower?) was quoted in Hot Rod as I remember that the best mileage/power spot was were the torque and horsepower curves crossed. On the GM published '72 chart for the LS9 its about 2600 RPM for a STOCK motor.

[SS Tim]

Here is the chart clip.

[special-K]

I really like 35-37" tires with 4.10s in these trucks. With 35s that gets you to about 3.73s,so you still have power and can roll along with little throttle at 60-ish. If you want to go faster you want o/d or just let 'er roar. If you are thinking 33" tires I'd go 3.73 for an ideal power to decent cruise rpm combo. I've run 33s in a '90 Blazer 350TBI/4spd manual with 3.08s and it was like having o/d. It pulled good enough and I rarely towed anything. It did fine towing and I always had the option of Granny 1st.