I was wondering how exactly the ratio of your rear end gears effect your speed at differnet RPMs. I know that something like a 2.79 is great for the high way but bad for low end street driving, and the 4.10 is for brutal acceleration, but you will be reving very high on the highway. Is this because for every 1 revolution of the driveshaft the tires will spin about 4 times? If that was the case, then shouldnt you be able to go faster on the highway at a lower RPM with something like a 4.10. I was just thinking of this becasue i want to upgrade my 2.79 gears to around a 3.80 with a posi or a locker. Can anyone please explain this to me, if you dont mind.
thanks alot
Matt

 

No the driveshaft will turn 4 times for every one revolution of the tire with a 4.11

 

Another way to put it, is the engine will turn 2.8 times for each revolution of a rear tire if you have 280 gears. It will have to turn 4.10 times for one revolution of a rear tire if you are running 410 gears.

 

You are correct that the numerically higher the gear ratio, the higher the engine rpm at highway speed. Several things to factor in when selecting rearend gears is powerband (dictated by camshaft), type of transmission, stall converter size, weight of car, and size of rear tires. You should spend some time with some rpm/gear ratio calculaters to see what gear ratio will result in the rpm you want to cruise at. Summit or Jegs should be able to help you pick out a good gear ratio.

Also, don't forget to recalibrate your speedometer with a different speedo gear or it will be way off.

 

Ok, thanks for the reply, i knew my theory was off, but I am wondering how come you can accelerate much faster with a 4.10 as opposed to a 2.80 ratio. My camshaft calls for lower gears with a 2500 stall converter. What do you think a good ratio would be then? My power band is from around 1500 to about 5500. It provides over 300 pounds of torque at 2000 rpms and stays consistent till about 4500. I wanted a good cam with a lot of low end power so i chose that one. It is comp cams extreme energy camshaft with 268 duration intake with 509 lift and 280 exhasut with 512 lift. The heads are also ported with 1.94 intake and 1.6 exhaust valves. Oh yeah, and it is a 302.
Thanks alot
Matt

 

Basically because the gears are smaller, therefore wind up faster. Here is a chart you might want to use to determine which gears to go with.
http://www.stanginfo.com/Gears/GearOptions_view.php

 

Have you ridden a multi speed bike before Matt? When you are cruising along on a level bike path/road, etc, you are probably in a gear ratio that has a bigger tooth up front, and a smaller tooth in rear for maximum speed. This also causes your legs to spin slower relative to tire speed, but also causes your legs to work harder. You can maintain and achieve great speed once you are moving, but you cannot accelerate very fast. We will call this a 2.80 gear ratio. Now, you come to a steep hill, or decide to venture off road. You switch your gears so you have your chain on a smaller tooth gear in front, and a larger tooth gear in the back. This causes your legs to spin faster relative to your tire speed, and also causes your legs to work less. You can accelerate quickly, and can climb almost anything your strength will allow, but you do not have any top speed at all because your legs are spinning so fast. We will call this a 4:11 gear ratio. This same physics principal applies to your motor and gear ratio. I just used leg power instead of mechanical power.


Does that help you at all, or did I just confuse you?


Mike

 

Hey Mike,
Thanks alot for the info! You are a great teacher, now i fully understand. So i guess that a 3.55 ratio would kinda be the best of both worlds.

"SOARING" Thanks for the website. When ever i seem to have a problem you always respond, you seem to know your stuff!

Thanks everyone!
Matt

 

That was a great explanation.

 

I'd actually go with 3.70 or 3.73.