scott12180

larger size tires on the rear of a 1930 car

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Without explaining all the reasoning, here's a hypothetical question.

Suppose you have a circa 1930 large-car Sedan.  Packard, Cadillac,  Buick, Pierce, etc.

Could you put larger tires on the rear to get better speed while leaving smaller tires on the front because they look better . . . . or because you don't want to buy four large tires. The large tires on the front really would look truck-like. But on the back they are mostly hidden under  the fenders.

The question came up to put say, 650x19 on the front and 750x19 on the rear.  The two-inch diameter increase on the rear gives about a 10% better speed for the same engine rpm.

 

If the answer is "no, you can't do it", then why?  What would happen?  Have you done it to know for sure?

 

No flames or insults to my intelligence or ancestry, please.  This is just a question.

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You can certainly do it . I have done it , and I assure you I am not the only one. I rather like the look , some who don't are particularly welcome to criticize , or just look away. Tell 'em it was common practice 80 - 90 - 100 years ago ? !   - Carl 

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If you were to measure the loaded/mounted/rolling circumference of some of the same size tires you will find as much or more difference than the 650 to 750 will be.  Going from 475/500 to 500/550 gave me 4" more per revolution.  By checking different 550's I was able to gain 8" per revolution over my original tires.  That translated to a decrease of 207 rpm at 50mph.  (2539 down to 2332 or an 8% decrease) 

Edited by Tinindian (see edit history)

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It was common practice 50 years ago too and not just on hot rods. Lots of cars in the north used heavy lugged snow tires in winter that were significantly larger in diameter than the stock fronts.

 

And, you are correct that it is a cheap way to raise your hiway speed without increasing RPMs. Just remember your speedo will not reflect the new gearing. Use your GPS or time some mile posts and get an idea how far off you are at 60 MPH.

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The speedo difference and effective gear ratio differece is simply the ratio of the tire diameters.  If the old tires are 26" in outside diameter and the new ones are 27", the difference will be 1/26, or about 4% higher effective gear ratio (and thus about a 4% slow reading on the speedo) with the taller tires.  Of course, the 4% higher gear ratio means slower acceleration.

 

Of course, these are theoretical numbers assuming a rigid tire.  The tire is never at the nominal diameter, as it compresses a little under load.  This naturally reduces the effective radius, which makes the tire seem "smaller" in diameter.  Bias ply tires flex less than radials due to stiffer sidewalls. The dimension that really matters is called "rolling radius", which is the effective radius under load.  Big truck tires usually have a revs per mile spec, which accounts for this reduced rolling radius.

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I was running bigger tires on the back of my 1930 Dodge Brothers Eight. The fronts were 550x18, the rears 600x18. The odometer read 7% under and the car didn't perform so well on the hills. It also gave me 13.5 mpg (UK) = 11.2 mpg (US) = 20.9 l/100km. I have recently replaced them with a set of 550x18; it will climb anything and accelerate as well, but the fun is such that the fuel lack-of-economy is now 10.5 mpg (UK) = 8.7 mpg (US) = 26.9 l/100km. I also have a 600/650x18 tire that is smaller than all the old 550x18s I have.

 

Most would not notice it hard larger tires on the back, I suppose because the 550s on the front were not so small either.

 

I replaced the 700s because it is a locking ring wheel and the tires were old and very stiff. I had already had a crack in a locking ring land on a rear wheel.

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