differential gear ratios

[debi]

Differential ratios are like Greek to me. Could someone please explain how these gear ratios relate and what they mean in simple, simpler, simpliest terms - I need to understand.

[Arthur Dalton]

I/ll give it a try.

A differential is basically a gearbox with different size meshing gears where the input shaft [ drive shaft ] has to
turn a certain number of revolutions in order to turn the output shaft [ axle/tire ] one rev.
Now, if the drive shaft has to turn, let's say, 3 and 1/4
turns to make the wheel turn 1 turn, we have a ratio factor of 3.25 to 1. This would be referered to as a 3.25 ratio differential.
From here you can see how tire size and different types of transmissions affect engine RPM's at given speeds.

The basics, anyway.

[Mark V]

it is a ratio of turns in to turns out.
Or for so many turns of the driveshaft you get so many turns of the wheels. For example a differential of 3.92:1 turns the rear wheels once for each 3.92 times the driveshaft turns. Now imagine a 3.69:1 diferential the ratio works the same way, but the difference in the real world is can be best illustrated by this:

At engine 2000rpm with the manual transmission in 4th turns the driveshaft 2000 times in one minute. (4th is a 1:1 in mine and I think yours) the wheels then with a 3.91 will turn 511.5 times. With the 3.69 differential however the wheels will turn 542 times. With an estimated circumference of 6.25 feet for a 14" wheel, the 3.69 will travel 190 feet 7.5 inches further the same car with the 3.92:1 differential. Alternatively, the engine turns less turns to go the same distance with the 3.69 or at a fixed speed will have a lower engine speed than the 3.92. Keep in mind the lower the rear ends ratio the more oomph your engine will need to produce to make the car move. So off the line the 3.92 will be quicker because there is more torque at the rear wheels.

I hope all this got through the keys alright, I have a cold thats turned my brain into a throbbing mush...

[debi]

Hey Guys: Thanks for excellent info... you both gave me a much better understanding of the process. Tell me more..
Wonder if different gear ratios cause a noticable effect on gas mileage? i'm geared very low - i need to find out my number - and get about 11-13 mpg. My question: would a higher gear significantly lower amount of gasoline used?

Tune first. before you alter the design of the car make sure the engine is operation within all specs. Thats the best place to target mileage problems.

That said the rear end can make a big difference depending on your driving patterns. If you drive mostly highway, you'd be helped by a lower ratio because the engine will turn less revs in one mile, generally less revs less gas. If you are prone to putting your foot into the carbs to accelerate quickly, you will use more gas with the lower ratio, because you have comparatively less torque, and torque is a requirement to get that jump. But, under normal highway driving the lower ratio will be better. In town where you need to accelerate often, the higher ratio is the one to have since you'll be able to go easier on the throttle since you have more oomph at the rear wheels.

Thats the textbook analysis. As I pulled from the changing light in 2nd this morning (I had a coffee in hand and had to turn, no cupholders)I was thinking, "1st in this car is almost useless unless you have a steeply sloped driveway". That tells me in a practical sense, it likely won't hurt to have the lower ratio.

So thanks Debi, now theres something else for my 'to do list'. But seriously, the tunings the thing here first.

[cth350]

The amount of money a rear differential will cost to replace is going to be many, many, many tanks of gas, unless you DIY from a parts car.

A loose carb will give you horrendous milage. With everything under the hood being perfect, you're not going to pass the 17mpg mark. If you drive 5,000 miles in the year...

5000 / 13 mpg = 384 gal gas
5000 / 17 mpg = 294 gal gas

You're talking 90 gallons of petrol, or under 200$ here on the east coast AND that's your maximum saving. Your milage may vary.

-CTH

[Mike D]

Sigh. How times change. I came across this post while searching for differential ratio markings.

Whaddya think Charles? 90 gallons of petrol for under $200.00?