Gear Ratio Math

[Fat Fender]

When I read about a 30% improvement with an over drive, what does that compute to for a 3.78 rear ratio? Is the final drive 70% * 3.78 (= 2.45) or am I not doing this right? Does the 1:1 become 1:0.7?

Thanks for any math lessons, I don't have enough fingers and toes.

Fat Fender

 

[Mart]

Yes, the engine revs will be reduced by 30%, or your effective rear end ratio will be 3.78 *0.7 which I get to be 2.646.
In od the drive is effectively 1:0.7 as you state.

Mart.

 

[my4dv8]

This is the basic formula to work out desired RPM at a given speed and work out rear end ratio

TD x RPM divided by AR x 336

TD = Tire Diameter
RPM = you know this one . . . and if you don’t, maybe you’re in the wrong place.
AR = Axle Ratio
336 = K or the constant
(Tire Dia. x rpm) Divided by (336 x desired mph) = gear ratio

example

(30" dia. x 2,100 rpm = 63,000)/(336 x 70 mph = 23,520) = 2.68

 

[CajunCrawfish]

When I read about a 30% improvement with an over drive, what does that compute to for a 3.78 rear ratio? Is the final drive 70% * 3.78 (= 2.45) or am I not doing this right? Does the 1:1 become 1:0.7?

Thanks for any math lessons, I don't have enough fingers and toes.

Fat Fender

Here is a cheater site for your calculations
http://www.rocky-road.com/calculator.html

 

[40cpe]

I think that site has it's tire sizes reversed in the gear calculator. But it tells me that increasing tire diameter by 1.9 inches is equivalent to changing from a 3.78 to 3.54 gear.

 

[oldford2]

? ? ? ?

 

[Bassman/NZ]

I think that site has it's tire sizes reversed in the gear calculator. But it tells me that increasing tire diameter by 1.9 inches is equivalent to changing from a 3.78 to 3.54 gear.

I agree. It told me that my 3.78 rear with 31" high tires gave it a ratio of 4.11. Impossible.

 

[CajunCrawfish]

I think that site has it's tire sizes reversed in the gear calculator. But it tells me that increasing tire diameter by 1.9 inches is equivalent to changing from a 3.78 to 3.54 gear.

OOPS! BAD SITE
Well at least we know that there are some helpful sites out there for us. Apologies for finding a crap site.

 

[Kahuna]

Here's the best calcs: http://www.wallaceracing.com/Calculators.htm

 

[40cpe]

Good stuff in there, Kahuna

 

[CajunCrawfish]

here's the best calcs: http://www.wallaceracing.com/calculators.htm

fantastic :d

 

[THE "A" FAMILY]

Kahuna, that's a good site. thank you. I would never find this stuff with out the knowledge of the people on the Ford Barn.

 

[-Big Al-]

http://www.csgnetwork.com:80/multirpmcalc.html

 

[Flat Ernie]

Here's an old post I did crunching numbers showing RPM drops:

http://www.fordbarn.com/forum/showthread.php?p=197686#post197686

It may not be exact to your combination/scenario, but it will give you a relative idea what it does for you and some of the numbers involved...

 

[32phil]

If memory serves a Columbia with a 4:11 R&P will switch to 2:92(?) if the unit is equipped with a 3:78 R&P it will switch to 2:73(?) Perhaps one of the Columbia experts can enlighten us.