Model A rear tire size verses cruise RPM

[41blueoval]

I have a stock 31 A Tudor with 5.00x19 tires on it now. I would like to lower the overall RPM at cruising speed by changing the rear tires to either a 6.00 or 6.50.
In stock form is there enough power to use tires that size? Eventually I’m adding a different head and intake.
I’m running excelsior comp H’s, Coker lists the diameter as 30” but they actually measure 29”. Does anyone know the exact size a 6.00 or 6.50 is on a stock wheel?

[jeepguy1948]

There is lots of room in there. For starters, remember that 28-29 used 21” wheels/tires. It’s interesting to me that when Ford switched from 21” to 19” wheels they did not change gear ratios so in effect the 30-31’s have a lower final drive ratio. I have tall wide tires on mine and it’s also lowered, you can fit anything in there. That said, I don’t think that you will gain much just by changing tires.

[jeepguy1948]

I think that maybe 150-200 rpm gain max

[katy]

Originally Posted by jeepguy1948

It’s interesting to me that when Ford switched from 21” to 19” wheels they did not change gear ratios so in effect the 30-31’s have a lower final drive ratio.

The 19" tires are fatter than the 21" so the 21" and 19" tires have basically the same OD, and the final drive ratio didn't change.

[Y-Blockhead]

Originally Posted by katy

The 19" tires are fatter than the 21" so the 21" and 19" tires have basically the same OD, and the final drive ratio didn't change.

I agree, they are basically same diameter. 6.50 tire would be pushing it to fit properly on a Model A rim, IMO. The rim is only ~3"wide.

[BRENT in 10-uh-C]

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Originally Posted by 41blueoval

I have a stock 31 A Tudor with 5.00x19 tires on it now. I would like to lower the overall RPM at cruising speed by changing the rear tires to either a 6.00 or 6.50.
In stock form is there enough power to use tires that size? Eventually I’m adding a different head and intake.
I’m running excelsior comp H’s, Coker lists the diameter as 30” but they actually measure 29”. Does anyone know the exact size a 6.00 or 6.50 is on a stock wheel?

Use this CALCULATOR to answer your question. The gear ratio number will be 3.78, insert the height number that you feel is appropriate, the RPMs will generally be around 2000 for the average engine whereas a nicely built & balanced engine can withstand sustained 2500 RPMs. The transmission gear ratio is 1.00 because it is not an overdrive.

You did not mention what wheels you are using, but a wider width rim will decrease overall height, as will under-inflated tires. Also, the overall height is measured without any loading or weight on the tire.

Hopefully, that helps you figure it out.

[rotorwrench]

It wasn't till early in the modern era when the aspect ratios became part of the tire size. As rims became available in more width and diameter choices, it really started to affect the overall circumference of the tires. A tire with a substantially larger aspect ratio will affect the speed of a particular final drive ratio.

The model A tires generally all stay within a narrow range of aspect ratio so it doesn't change the top speed capability all that much. Put on some wide 16 inch wheels from a 40s Ford and use some high aspect ratio tires and it will likely do more than a stock wheel/tire combo.

[Y-Blockhead]

Originally Posted by BRENT in 10-uh-C

Use this CALCULATOR

You did not mention what wheels you are using, but a wider width rim will decrease overall height, as will under-inflated tires.

Originally Posted by 41blueoval

I have a stock 31 A Tudor with 5.00x19 tires on it now. Does anyone know the exact size a 6.00 or 6.50 is on a stock wheel?

He did mention the wheel size, stock 19", that is why I stated I didn't think a 6.50 would work so good...

[BRENT in 10-uh-C]

Originally Posted by Y-Blockhead

He did mention the wheel size, stock 19", that is why I stated I didn't think a 6.50 would work so good...

Its all good and I agree with you. And thanks for pointing that out. I read his post meaning where he said he had 5.00x19" tires, ...but because he asked what it would be if he put them on a stock wheel, it made me wonder if he was contemplating changing to a 6.00 or 6.50 on a stock wheel. Again, all is well and thanks for mentioning that.

[41blueoval]

I am running stock wheels. I was using a different calculator. A gear change to 3.54 would yield around a 147 rpm drop. So if the next size up tire 6.00x 19 comp H is truly a 31 inch tall tire from the 29 I have now it would yield about the same rpm drop. Has anyone ever measured a 6.00 or 6.50 comp H Diameter on a stock wheel? From what I’ve read a stock banger would not pull a 3.25 ratio is that true? Thanks for the replies

[Oldgearz]

I think your idea of going to an HC head will pay off far more than larger tires. Regarding cruising speed (55 mph for me) and RPM, I run a Synders 5.5:1 head, a B carb and Stipe cam, with a Mitchell 26% overdrive and I turn about 1800-1900 rpm at that speed. Can't beat that for "cruisin"!. I realize however that some folks don't like to change their A's that much, but with todays crowded roads I personnally felt unsafe at 45 MPH.

[jack backer]

Use a tire calculator

https://tiresize.com/calculator/

Tire size might not be your issue.. engine efficiency is sometimes the cure, By increasing the engines efficiency I can effortlessly run a sport coupe body on 31 inch rear wheel with a 2.91 final drive ratio..

[The Master Cylinder]

Originally Posted by jack backer

Use a tire calculator

https://tiresize.com/calculator/

Tire size might not be your issue.. engine efficiency is sometimes the cure, By increasing the engines efficiency I can effortlessly run a sport coupe body on 31 inch rear wheel with a 2.91 final drive ratio..

I sure having a lightened car with no fenders or interior and all doesn't hurt either?

[jeepguy1948]

I am confident that a Banger with an HC head and possibly a down draft carb would pull the 3.24 gears you mentioned. It would not be able to do it totally stock (a touring cam would wrap it up for sure.

[41blueoval]

Update on tire diameter. Excelsior comp H 5.75/6.00 tires measure 30.5 on stock model A 19 inch wheels. Coker spec lists 31.