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Gear ratios


Well-Known Member
What kind of cam profiles and gear ratios are some of you SB Chevy guys using?

Obviously, if the motor has a lot of camshaft, it will want some gear ratio to make it work. But I'm not sold on the idea of a lot of camshaft being necessary for a vehicle that is so light. I'm definitely not sold on the idea of increasing static compression ratio, to make up for the longer cam profiles, either. The attendant increases in fuel requirements is not a strong selling point for me.

I'm thinking that something around 230 @ .050 is likely more than enough camshaft to get the job done. Even at that, I suspect that loosening up the converter at least 500-750 RPM might be wise, taking into account the curb weight of the car.

Let me hear your thoughts and ideas.
On my 355 blower motor, my cam profile is pretty tame. It a Comp Cam AH268. My static CR is about 8.5:1 and the blower brings it up to 11:1. My Ford 8" rearend has a 3.40 gear in it and the tires are 29" tall. At 3000 RPM I'm running nearly 80MPH. I also have a TCI 3000 stall 10" convertor. The 327 SBC I'm building for my dad will a 268 roller cam in it.
I would like a stock 350/350 trans. What gear ratio should I concider?

I'm making my 440 totally stock internally. Just can't see going wild, the car is so light. I want a smooth running engine with decent gas mileage, well, as decent as a 440 can be. The only real mod I'm making is mounting 2 Edelbrock 4 barrel carbs. I'm not building a show car, I plan to drive it instead of the family car, but I am wanting it to look pretty.

When you say 440, is that the gear ratio?:eek: Again, I must admit I'm not very bright when it comes to Automotive. I am hoping to learn a lot about cars when I get started with my future kit.

thomas said:

When you say 440, is that the gear ratio?

No that's the engine size. It's a Chrysler (Mopar) 440. Another popular Mopar engine, if you're a Mopar fan, is the Hemi 426. I'm using the 440 mainly just to be a bit different. I could have used a Chevy engine, but I'm a contrary type person.

I'm also using a corvette rear-end, and right now I don't know what the gear ratio is that's in it.
Terry said:
I could have used a Chevy engine, but I'm a contrary type person.
I deal closely with Indy Cylinder Heads in my work and I keep thinking that one of their Legend Hemi's would really be the bee's knees. But trying to fit that into the budget would set the entire project back for years. :confused:
It's amazing how much a person could spent on any hobby if they really wanted to, in the name of being cool or unique. Believe me, the thought of adding a blower, aluminum heads, etc. had crossed my mind several time. I'm not made of money, so some lines needed to be drawn.
So, is there anyone out their that could tell me some gear/ratio's that might be good for a nice easy drive that would help give good gas milage. with stock 350/350.

Thomas, there are other factors that come into play, when determining rear gear ratios.

Camshaft profile is one. At what RPM range is the motor going to be 'happy'? If you are running a huge cam profile, you will want to use lower gear ratios (higher numerically), because the engine will be happier at a higher RPM.

Tire diameter will be another factor. Taller tires will kill off gear ratio. I always get a kick out of the 4WD guys that come to us for a camshaft. They will have a truck with a lift kit and 33" tires on it and they want to use camshafts in the 278-284 range. We always try to get them to pull the cam profiles back, but some insist they want a really rough idle. And then they discover we were correct, when we warned them they will have to drive the truck in 2nd gear, just to keep the engine screwed up tight enough to be in the power band. :roll:

I'm thinking I want to try to end up in the net 3.55:1 range. I'm not going overboard with the engine, I can make an honest 350-375 crankshaft HP without much effort and that will be more than enough in a 1650 lb car. Small cams don't need high compression ratios and that means lower octane fuel. I'm trying to build a combination so I can spend my free time driving the car, not working on it. :D
Mike said:
I'm thinking I want to try to end up in the net 3.55:1 range.

Is this the actual ratio in the rear-end, or the effective ratio after you've figured in the tire size?
Great information gentlemen,

I can see now that only should I enjoy building this car,but I should also learn some very important aspects of the engine and drive train. Looking forward to getting my kit.

My .02 cents on cam shaft selection is to call a couple of the cam companies and talk to their tech people. Let them make a recommendation based on what your car weight, engine info, transmission type, and intended desire for the final outcome. Then you can make a reasonable decision. As far as gears are concerned. As mentioned before, tire size should be taken into account when determining an effective gear ratio for your intended outcome. In my opinion, 3:23 to 3:73 ratios give a nice compromise between gas mileage and accelation. 3:90 to 4:10s are a lot of fun but tend to suck up the gas. 4:56 to 5:13 or above should be left to the dragstrip. Those ratios below 3:23 are great for cruising down the interstate and avoiding the gas pumps. Just keep in mind that those rear tires can make a substantial difference. For instance, a car with 3:23 gears and a 1:1 high gear ratio, at an engine rpm of 3000, would be doing 66 miles per hour with 24 inch in diameter tires, while the same car/same rpm with 31 inch tires would be doing 86 mph. When determining the rear end ratio you want to use, be sure to actually measure the rear tire's diameter and do not rely on the tire's listed demensions. They are often quite different.
Hey fox,

Is there a formula for tire dia. and rpm for getting the right rearend gears?

thomas said:
Hey fox,

Is there a formula for tire dia. and rpm for getting the right rearend gears?


Thomas, here is a link I found about 2 months ago. Scroll down to the Tire Calculator and put in the tire size you have or are thinking about and you can compare it to up to 4 different sizes. It breaks down EVERYTHING including speedometer fluctuation! Check out the tech section too. A very valuable site.
The number one factor in determining gear ratio is tire diameter. If you are running "old school" small white wall tires then maybe cam specs will matter, but not with tall steamroller tires. I started out with a 3.89 with the 16/32.5x15 Firestones. It was too doggy. Went to a 4.86 but spun just alittle too much on the highway. Now I am at 4.30 and it is perfect. I run about 2850 rpm at 65 mph. But also my new American Racer tires are 34" tall.

Also the torque converter stall speed will have alot to do with the "doggity" effect. Right now with the 6-71 at 12% overdriven and a full manual TH400 I have a 3300-3500 stall and BOY is it FUN.

With a blower you want around .500 lift, 295 duration and 114 lobe separation. With a regular 108-110 lobe separation you will blow alot of the charge out the exhaust on the compression stroke. Also a blower will tame the radical lope of a 108-110 cam.

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