Thread: Gear Ratio

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So im going to run a 396 bigblock with a TH400 and a 28inch tall tire in my 64 chevy truck. What gear ratio would be best for me to have? I still want to be able to run the truck on the freeway at like 65mph. I have a 12bolt truck rearend and should i run a limited slip?

Thanks for all your help.

Your TH400 is 1:1 final drive, so with a 28" tall tire at 65 you'll be turning 2400rpm with 3.08's, and 2130rpm with 2.73's. Kind of depends on what rpm you're up on your torque curve, and how low you want your rpm at cruise knowing that you're giving up punch off the line.

It sort of depends on where the cam makes power. With an OEM stock cam, you can run any gear ratio that you want to, 2.29/2.56/2.73/3.07. With an aftermarket cam of the "RV" type, you will want to use a little more gear, maybe a 3.31/3.55 or 3.73. With a lumpy cam, you might want to use a 4.11 at a minimum and more than likely a 4.56/4.88. These are just general guesstimates to give you an idea to start from.

Well the cams operating range is from 2000-6000rpm and asking to use 2500 stall converter.

Your stall converter will not affect anything other than the stall point under load, like against the brake for launch. So the question is, how much "launch" do you want to give up for cruise economy and reduced wear & tear on the engine on the road? Tech gave you the whole range of gear ratios, and you can simply proportion from the values I gave you to get different cruise rpm's - like 2.73/2130rpm = 3.55/X and solve for X ((3.55x2130)/2.73 = 2770 rpm for 3.55's). You also need to consider how radical that cam is, like Tech pointed out.

Here is the cam specs, im looking to be able to jump on it every once in awhile, if i run 4.11=3206 at 65 is that rpm to high for freeway? 4.56= 3557 at 65 And if i run 2.73 will i be able to still jump off the line good?


Cam Style Hydraulic flat tappet
Basic Operating RPM Range 2,000-6,000
Intake Duration at 050 inch Lift 230
Exhaust Duration at 050 inch Lift 230
Duration at 050 inch Lift 230 int./230 exh.
Advertised Intake Duration 280
Advertised Exhaust Duration 280
Advertised Duration 280 int./280 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.520 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.520 in.
Valve Lift with Factory Rocker Arm Ratio 0.520 int./0.520 exh.
Lobe Separation (degrees) 110

Plenty high for cruizin' IMO. You'd probably be a lot happier with something in the 3.50 to 3.73 range, better rpm on the highway and still decent off the line

Back in the late 50's through the 60's the OEM "standard" seemed to be around 3000rpm at 70mph highway speed, but that was also when a 50,000 mile car was considered to be high mileage, and anything pushing 100,000 miles was reaching the end of it's life expectancy, right? I'd much rather be cruising in the 2000 to 2500rpm range myself. Much less wear & tear on the engine, and closer to the efficiency point on most.

I'm using 3.25 gears, 29.1" tires and am turning 23-2400 rpm at 65 mph with a 1:1 ratio C-4 transmission. If I decide that I want to put the 3.79 gears back in, that same rpm will be about 55 miles per hour or about 10 mph less for the same wear and tear. It all has to do with economics - that is, your wallet, gas mileage and what your earlier engine wear will cost.

My favorite set of gears is 3.25:1, I find them to be a great compromise between highway and city rpm.

Originally Posted by 1972chevy

And if i run 2.73 will i be able to still jump off the line good?

Not likely.

With the questions you're asking it seems to me that you need to be looking for a good, strong overdrive tranny; or looking at adding a GearVendors unit behind your TH400. Either way will let you run lower gears, like 4.11's and still get your cruise rpm down. You're wanting great acceleration off the line, but low cruise rpm for efficiency - the definition of overdrive.

I agree with Dave. A set of 3.25 gears will give you the best cruising rpm. You should have enough torque with that big block to get you off the line and provide you with good performance. Everything is a compromise. An overdrive gives you more flexibility--I run 3.89 rear gears with 30 inch tires but I can cruise around 2200 rpm at 70 mph.

Originally Posted by rumrumm

I agree with Dave. A set of 3.25 gears will give you the best cruising rpm. You should have enough torque with that big block to get you off the line and provide you with good performance. Everything is a compromise. An overdrive gives you more flexibility--I run 3.89 rear gears with 30 inch tires but I can cruise around 2200 rpm at 70 mph.

Like Lynn, I run 4.11's with 31" tires, but in 5th (0.64 OD) I cruise at 2000 rpm at 70 mph. You'll be at ~2535 rpm with 3.25's, which is pretty good if you're not looking to change to an OD unit.

Yeah I don’t want to change the trans I just had it built, I just want to make sure that running at 2535 rpms isn't bad for the engine. And one other question I have about the motor I’m running, what would be the max rpm for the engine. It is a 408 with 4.155 bore 3.776 stroke all forged rotating assembly with the cam I listed above. I was thinking of putting a rev limiter at 5000. Is that too high or too low.