[Nightfire]

Hey y'all,

looking to learn a little bit more in detail about how gears are calculated. May seem like stupid questions but here they go:

1) When people talk about "I'm runnin' 4.13's" or "3.73's" what does that number refered to? I understand it means how many times the wheels spin for each time the driveshaft turns?

2) How are gears calculated?

3) How do you change your gearing?

4) Will higher gears give you more speed or less?

5) Where is the speed of a car measured. Someone told me at the driveshaft, and other people at the brake discs?

What if you put bigger wheels on it, then the speedo will be wrong right?

Thanks,

Mike

[lluciano77]

The gear ratio refers to how many times the driveshaft spins one revolution, compared to how many times the wheels spin one revolution. So when you have 4.13s your driveshaft is spinning 4.13 times for every time your tire spines once.

Gears are calculated by dividing your ring gear teeth by your pinion teeth.

4.10 gears:

41 teeth on the ring gear
---------------------------------
10 teeth on the pinion

=4.10

You change your gearing by changing gear sets with different amounts of teeth.

Gearing doesn't necessarily give you more speed. It helps load up or unload the engine to give you mechanical advantage and to help keep the engine within its powerband more efficiently. That is a complicated question. You could start a thread just to try to answer that.

The vehicle speed is measured by a drive gear inside the tranny. When you change gears your speedo will be off with a mechanical gauge. Same thing with tire size.

[beau5278]

When someone talks about a higher gear ratio,they are talking lower #,and that means more speed.A lower ratio,higher #,means less speed more power.The speedometer reads speed from the transmission.

[Nightfire]

So a stock car comes with the speedo measuring the speed at the transmission. If you would now change gears and put bigger wheels on your speedo would be inacurate? Is that the only place speed is measured? So when you're building a rod, how do you ajust the speedo and know if it's right or not?

Thanks,

Mike

[beau5278]

You can change the gear that drives the speedo cable,or the speed sensor if electronic.If you aren't changing the tire diameter it won't affect your speedo.There are conversion charts that will tell you how much your tires will change the speedo.When you change gears you will have to change the gear in the trans.To check for accuracy,you can do that on a measured stretch of road.Some states mark a section of highway,and most interstate highways are marked with mile posts that you can check your speedo against to see how far off you are.

[4 Jaw Chick]

[sighs, shakes head back and forth] I'll just never be able to wrap my brain around that concept. Chuck has explained it to me a thousand times and I still have trouble with it. It makes perfect sense when I read it, but if you were to ask me 5 minutes from now, all that great info would be gone (zip) right outta my head. [sighs again. hangs head in shame].

BTW Nightfire, the only stupid questions are the ones you don't ask.

Chickie.

[Nightfire]

So to change the gearing in a car, you would bolt the drive shaft of the rear end, get the diff outta the car and change the pinion and ring to either higher gears or lower gears. Than you just bolt the whole rear end back in right? So, let's say I built a car with a 350 and a 700R4 trans with a ford 9 inch rear-end. How would I know what type of gears I'm running. Would I take it to the shop and they would measure how many times the driveshaft spins for every time the wheels spin? Thanks

Originally posted by 4 Jaw Chick


BTW Nightfire, the only stupid questions are the ones you don't ask.

Chickie.

Mike

[beau5278]

You have to read your replies here nightfire.It was explained a few posts back.Youtake the number of teeth on the ring gear and divide the number of teeth on the pinion into that number.40 teeth on the ring gear,10 teeth on the pinion,gives you a 4.00 to 1 gear ratio.Your drive shaft and pinion gear rotate 4 times for every turn of the ring gear,axle,tires.

[Kevin45]

Hey Nightfire....not a stupid question at all but an intelligent question. Check this out in the Knowledge Base http://hotrodders.com/kb/transmissio...lculators.html There are a few different articles you may want to look at to grasp the concept. And what Beau5278 said is correct but confusing. For example a ratio of 2:73 would give you a higher speed on the highway with lower engine rpms and lower torque on takeoff. A gear ratio of 4:11 will give you the power at takeoff but slower highway speeds but high engine rpm. So say at a cruising speed of 55 mph the 2:73 might be running the engine at 2500 rpm's whereas the 4:11 will increase the rpm's significantly.

Kevin

[Nightfire]

Thanks Kevin, so for a drag-strip car you'd rather want something like 4.11's to get of the line fast right? As to a daily driver street-rod where you'd better of going with 2.37 or 3.53's right?

Thanks to all,

Mike

[chrisflock]

Another great way to find out your rear ratio is to jack the back of your car up (support securely on jackstands!) and put it in neutral. Use a crayon or chalk to mark a line on your driveshaft and on your tire, then have 1 person spin the driveshaft and one person watch the tire and see how many times you have to spin the driveshaft to spin the tire. FOr an accurate reading, you probably want to count how many rotations it takes to get the rear tire to spin 10 times, so instead of getting tenths of a turn (often hard to tell how many tenths of a turn you've spun the shaft) you'll get whole numbers.

37 turns of the axle to 10 turns of the tire
means 3.7, and so on.

[toddtheodd]

Or you could use one of these.
Gear calculater
Or you could use this link to figure out what you gear ratio is.
Whole lot easier than spinning a driveshaft. No offense meant chrisflock, this is just a lazier way to do it.

[36scsc]

OK Nightfire,

Lets apply some of this new found information... nothing like a practical test to seperate the men from the boys.

YOU tell US what the ratio's are on that cool bike of yours!