Overdrive Confusion and Quiz

[TWChamp]

Abe wants an overdrive, so he orders a 30% overdrive unit.
Ben comes along and says that's great because it will slow the engine 30% for the same road speed.
Abe says no it won't, it will make the car go 30% faster for the same engine speed.
Who is right, or are they the same thing and they are both right, or is neither one right?

Now to make it more confusing Chuck comes along and says he has a tranny with .70 overdrive and that would be the same as Abe just ordered. Is Chuck right?

[PackardV8]

All saying essentially the same thing, just none clearly or completely defined.

jack vines

[Hawkowner]

Jack is correct
Hawkowner

[JoeHall]

Abe is planning to add an overdrive unit, between the tranny and rear end. Chuck has an overdrive tranny, with the OD unit built into the tranny. With either setup, slowing of the motor/speeding up the car by 30 percent will occur, so Abe and Ben are both correct. With either setup, the end result, "would be the same", so Chuck is correct in that aspect also.

[jeds55]

All the above....

[garyash]

Not so fast, gents. A surprise awaits!

Let's pick a normal Studebaker using modern 205-75/R15 tires with a rolling diameter of 27.1 inches. At 60 mph, we're covering 5280 ft/min. The circumference of the tire is pi*27.1=85.1"=7.095 ft. That means the wheel has to rotate 5280/7.095=744.2 rev/minute. With a 3.31 rear axle ratio and no overdrive, the engine must turn 3.31*744.2=2463 rpm.

If we install Chuck's 0.7 overdrive, in order to turn the driveshaft at 2463 rpm, the engine now turns at 2463 x .7 = 1724 rpm. So, the engine rpms are reduced by 30%.

However, if we now speed the engine up to the original 2463 rpm, then the driveshaft turns at 2463/0.7 = 3519 rpm. The rear axle gearing reduces this to 3519/3.31 = 1063 rpm and the tire covers 1063 x 7.095 = 7541 ft/min = 1.428 miles/min = 85.7 mph. This is 42.8% faster for the same original rpm.

I'm not sure what "30% overdrive" means, so it's safer to stick with the actual ratio numbers for each gear. Modern 5-, 6-, and more-speed transmissions can have several of the highest gears at values less than 1.0. See this about the ZF 8HP transmission used on a number of cars: https://en.wikipedia.org/wiki/ZF_8HP_transmission.

Class dismissed!

[TWChamp]

I agree with Gary.
I read once that my 1950 Stude tranny has a 33% overdrive, which to me means the shaft OUT is overdriven by 33% over the shaft going IN. So if the shaft in makes one turn while in overdrive, then the shaft out makes 1 1/3 turns.

Now, if someone says the tranny has .7 overdrive that tells me they are referring to the output shaft as having ONE turn while the input shaft makes .7 turn while in overdrive.

So, I'd say only Abe is right. Wish the salt hadn't been dumped on the roads, then I'd connect a tach and do some checks.

[GrumpyOne]

Abe wants an overdrive, so he orders a 30% overdrive unit.
Ben comes along and says that's great because it will slow the engine 30% for the same road speed.
Abe says no it won't, it will make the car go 30% faster for the same engine speed.
Who is right, or are they the same thing and they are both right, or is neither one right?

Now to make it more confusing Chuck comes along and says he has a tranny with .70 overdrive and that would be the same as Abe just ordered. Is Chuck right?

All things mathematical are reciprocal...

[jclary]

Not so fast, gents. A surprise awaits!

Let's pick a normal Studebaker using modern 205-75/R15 tires with a rolling diameter of 27.1 inches. At 60 mph, we're covering 5280 ft/min. The circumference of the tire is pi*27.1=85.1"=7.095 ft. That means the wheel has to rotate 5280/7.095=744.2 rev/minute. With a 3.31 rear axle ratio and no overdrive, the engine must turn 3.31*744.2=2463 rpm.

If we install Chuck's 0.7 overdrive, in order to turn the driveshaft at 2463 rpm, the engine now turns at 2463 x .7 = 1724 rpm. So, the engine rpms are reduced by 30%.

However, if we now speed the engine up to the original 2463 rpm, then the driveshaft turns at 2463/0.7 = 3519 rpm. The rear axle gearing reduces this to 3519/3.31 = 1063 rpm and the tire covers 1063 x 7.095 = 7541 ft/min = 1.428 miles/min = 85.7 mph. This is 42.8% faster for the same original rpm.

I'm not sure what "30% overdrive" means, so it's safer to stick with the actual ratio numbers for each gear. Modern 5-, 6-, and more-speed transmissions can have several of the highest gears at values less than 1.0. See this about the ZF 8HP transmission used on a number of cars: https://en.wikipedia.org/wiki/ZF_8HP_transmission.

Class dismissed!

How 'bout that! Gary already has a thread about "PIE FOR BREAKFAST"...then he comes here and serves "PI" for an after dinner snack!

[Studebakercenteroforegon]

To further confound this discussion - rather than consider the published tire diameter - one should instead know the "loaded radius" of the tire, since that is the real distance to the contact surface.
This is what we used for oval short track racing calculations, but then we didn't have much air pressure in the right rear, either.

[spokejr]

To further confound this discussion - rather than consider the published tire diameter - one should instead know the "loaded radius" of the tire, since that is the real distance to the contact surface.
This is what we used for oval short track racing calculations, but then we didn't have much air pressure in the right rear, either.

For that to work, the tyre's circumference would need to change. Deflating a tyre has no means to remove material.

Am I missing something?

[greyben]

The 1955 shop manual says the overdrive ratio is approximately .70. A 1956 Motor manual says the same. Has anyone calculated the actual ratio? Maybe someone with a disassembled od unit will count the teeth on the gears and then calculate the actual ratio so that this vital issue can be discussed more precisely.

[ddub]

I always hated story problems.

[JoeHall]

The 1955 shop manual says the overdrive ratio is approximately .70. A 1956 Motor manual says the same. Has anyone calculated the actual ratio? Maybe someone with a disassembled od unit will count the teeth on the gears and then calculate the actual ratio so that this vital issue can be discussed more precisely.

I agree, we need to be more precise here. I have heard the actual ratio is .70003, due to film lubrication altering the input shaft's ball bearing circumference. This is beyond my pay grade, but I sure would like to get to the bottom of this, and as precisely as possible please.

[Son O Lark]

What about coefficient of friction. Wouldn't it be harder to go 30% faster due to increased friction of the air?