Technical Ok, This is a Weird Question...

So I’ve finally gotten my 37 Hudson Coupe back on the road after close to 2 decades of sitting. During that hiatus one of my rear tires sidewall bubbled & thus I threw on a trailer tire to move it around with. Now I have one tire at 225/75/15 (28.3” diameter) & other is a 205/75/15 (27.1” diameter). Since it has an open differential (Ford 7.5”), I said to myself I have to test drive it & did yesterday.... Greatest day EVER!

So my question is....does anyone know which size tire I’m getting my speedometer & rpm based upon? (and yes new tires before I drive it again, I promise!)







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Average them...that's what the differential does.

 

Dam Squirrel you always have the answers, I should just call you 1st!!!!

A quick video of the maiden voyage. (I took off in second gear... still working on getting the electric hand fully functioning in all gears.)




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That’s a bad ass looking car!!
Must feel fantastic having it on the road again

 

Terraplane Blues... errrr… Yellows !
welcome back husker nation...

 

Congrats

 

Awesome car! Congrats!

 

Get's the speed off the tranny, right? Verify against your phone GPS...take the guess work out of it

Oh, and yes, very sweet ride you have!

 

Suuuuuuuuuuuuuuuuuuuuuuuuuuuuhweeeeeeeeeeeeeeet ride!! Don't think I've ever seen one before. Purdyer than a Ford!!!( and that's saying a lot)
6sally6

 

...who really cares what the actual speed is?.(check it against some mileage markers on an interstate,....that car looks fast sittin still.

 

I love it when the wife says, "You're gonna wake up the neighbors!"......Isn't that the plan after all.

 

Congrats. That grille is pure art deco.

 

If this place was Vegas I could make a good living betting on Squirrel's answers. And I know he would be the smartest man in the county if he lived in my town.
So it is with fear and trepidation that I disagree with his statement. I believe that the taller, slower rolling side is that which is shown on the speedo. For the 'tall', slow side to roll below the "average" input, which is faster than the highway speed, the tire will have to slip on the pavement. What I do think is, the 'tall', slow side clutches will be in sync with the input, speedo, highway speed, and on the 'short', faster-rolling side the clutches will be forced to do all the slipping. Another way to say it is, it's possible for the 'tall' side to slip but only in the same direction as the 'short' side. It's not possible for the 'tall' side to slip in advance of the input/highway speed/speedo.
There will also be some heat issues but since I'm probably educated beyond my intelligence, I can't go there.

PS: Love the car.

 

You would lose that bet. Sq is correct.

 

One tire will be moving slightly faster than the ring gear carrier, and one will be moving slower.

 

I think you must have missed the part about being an open differential. There are no clutches to slip...and besides, the way that positraction differentials work, they are designed to slip at a lower torque than it takes to slip the tire on pavement.

 

Not remotely interested in getting into the science of hair splitting conversation that is about to ensue... I am very interested in complimenting your car. Can't say I've ever seen a hotrodded Hudson of that type before. It's beautiful! I need one.

Congrats on getting it back on the road, and prepare to be bombarded with incorrect information about your Ford.

 

Cool ride - no matter how fast it is going

 

Oh, I'm sure I have all the details wrong. But the principle I'm trying to relate just seems obvious to me. You can't roll down the road and have one half of the rear turn faster than the momentum over the pavement and the other half slower than the momentum to come up with an average over the pavement. One side has to be exactly the same as the input which means the other side has to slip for the tires to both match the road speed.
I'm probably not writing as clearly as I should but it makes sense to me in my head. Hey! If I'm wrong, I'm wrong. I just don't see it yet.

I don't know about a "science" discussion. I'm just replying to the OP's question.

 

Roll a cone a see how far each end travels. Now divide that by half.

 

Having never crawled into an open differential as it is going down the road, I infer that it is magic. However, a different weird question : Do these come in other colors than yellow?

 

I'm not sure I understand that you mean by "other side has to slip". Both tires get full traction. Each axle turns at a different speed. One side gear turns faster than the other side gear. The spider gears turn slowly relative to the differential case, on the axis of their pin. At the same time, they are driving the pin to rotate around the axis of the axles. This rotation speed is the average speed of both axles.

Explaining differentials is really difficult, but there's an old video about it....and you have to consider that when you have two different size tires, it's like you're always going around a gentle bend in the road, as far as tire speed is concerned.

 

That video is probably one of the best explanations of how a differential works, as squirrel said both sides receive equal torque but can absolutely turn different speeds.

 

OK let's get even weirder. How do humans go around a corner when both legs are the same length?

 

Some humans have one leg that is shorter than the other. If it's the left leg, they probably excelled at running around the track in high school.

 

Hey, Jim. I hope it's clear to you, and the rest of the gallery, that I have too much respect of your knowledge and experience to ARGUE with you, but by the same token, I have the same respect to want to have the DISCUSSION with you. My goal is understanding, not winning. So far in our discussion, I'm thinking that the speedo registers what the 'tall' tire/axle half does and you're saying the speedo will register an average blend of the 2 axle halves and you're avoiding the "Which tire?" question altogether. Not to put words in your mouth, but are you saying that if both tires are the same size, the speedo can register accurately but if the tires are mismatched the speedo can't and will register a random number in between? That seems fuzzy to me but sometimes the answers are fuzzy, I guess.

 

Nothing about what the speedo registers is will be random. It will be related to an average of the two tire sizes. If the speedo is accurate with two 27" tall tires, it will also be accurate with one 26" and one 28" tire. It will always read the average speed of both tires.

This is assuming a gear type differential in the car. If you have a ratcheting differential, such as a Detroit locker, then it will read the speed of the taller tire. If you have a spool, it will probably be the average, assuming both tires slip the same amount.

I hope it looks like I'm discussing, because I don't mean to be arguing. I just like facts, is all

 

Did he become a Sprint Car racer later in life?

 

That would be the guy with the shorter left arm.