16" wheels on my '28?

[saltman]

I have 16", 1935 spoked wheels on my '28 Coupe. They came with the car when I bought it. And although my initial intention was to replace them, after some time, I began to like them... and still do. BUT, now that I have got the car running so well and want to drive it more and more, I fear that the engine (bearings) is really going to take a beating. About 40mph is a comfortable 'max' speed although it will run 45or so. But, at 45mph, I feel that the engine is really working too hard and I am not having near as much fun because of this. I have checked the math and IF I were to use the original 21" wheels, I would then be able to cruise at almost 50mph (about 24% faster) rather than the 40mph. And, with 19" wheels, I could cruise at about 46mph (16% faster). Is my math correct and is my assumption correct that if I stay with the 16" wheels, I will overwork the engine just trying to keep up with other traffic or staying out of their way? Would really like to have some comments regarding this since its going to cost a great deal of money to make the conversion and don't want to ruin my engine. Also, would the larger diameter, thinner wheels make it less or more difficult to turn? Thank You so much in advance... SALTMAN

[johnbuckley]

I find the only way to check perimeter/circumference of tyre is to have it on the car, put a chalk ( or paint) mark on it and drive the car one wheels revolution then measure garage floor!

[saltman]

Does anyone know the outer diameter (or the circumference) of the 'tires' on 21" wheels? Same question for the 19" wheels.

[Cajun]

Go to Coker.com and you can get the circumference of each size.

[saltman]

Using the tire specifications that Coker tire gives, going from a 6.00/16" (wheel) tire to a standard 21" (wheel) tire, would only improve the speed by about 6.5% Therefore, improvement at 40mph would only be stepped up to about 42.5mph. My initial calculations were way off because I did not consider the thickness of the tires. Regardless, I would still like for someone to agree or oppose this. Thanks Always... SALTMAN

[mccsix]

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My 21" wheels measure 31 1/4" dia.

[BRENT in 10-uh-C]

Originally Posted by saltman

Does anyone know the outer diameter (or the circumference) of the 'tires' on 21" wheels? Same question for the 19" wheels.

Well, tire manufacturer is going to make this vary some but for the most part it will be 30" tall for a 21" tire, and 29" to 29¼" for the 19" tire.

Basically what you will do to find the roll-out is multiply the tire height X Pi π which is 3.14159 to get your one-revolution roll-out. Then convert 5,280 which is feet per mile into inches and divide that number by the amount of your tire one revolution roll-out. Now you need to do the same for your 16" tire and see what the percentage of comparison is.

I will do the 21" tire for you.

30 X 3.1459 = 94.2477

5,280 X 12 = 63,360

63,360 ÷ 94.2477 = 672.271

Therefore your 21" tire will turn approximately 672 ¼ revolutions in a mile.


To be honest, I suspect a 650-16 tire will probably be very close to what a 19" tire will be (-around 29.250) so when I did the comparison for you, it comes out about 689 ½ revolutions. That only comes out to about 2½% difference between the two --or about 30 RPM. Not enough to make a difference. I would suggest you pull the engine and the flywheel down and balance everything. I suspect then you will find 50-555 mph much more obtainable.

.

[Bob C]

Are you going by the speed the speedometer reads or GPS.
Some speedos are way off.

Bob

[BRENT in 10-uh-C]

Originally Posted by saltman

Using the tire specifications that Coker tire gives, going from a 6.00/16" (wheel) tire to a standard 21" (wheel) tire, would only improve the speed by about 6.5% Therefore, improvement at 40mph would only be stepped up to about 42.5mph. My initial calculations were way off because I did not consider the thickness of the tires. Regardless, I would still like for someone to agree or oppose this. Thanks Always... SALTMAN

Well I was calculating and typing while others were posting too and after the good suggestion of obtaining actual numbers on Firestone 600-16 and 450-21, there is actually about a 10.250% difference. In other words, 40mph now would equate to about 44mph with the 21", ...and 45 mph now would allow you to travel at nearly 50 mph at the same engine rpm.

[Louis]

I don't know about the diff. on the 21" wheel and tire, but I replaced the org. 19" wheel and tires with 16" 35 ford spoke wheels, The 19" tires were new 4.75/5.00-19 firestones, and the 16" were new 6.00-16 coker/classic, I measured both tires side by side with a straight edge, the 16's were 28" high, the 19" were 29-1/2.
I could not tell the diff. in speed or sound of the eng, the 16" tires did ride a lot smoother, very little diff. in steering.
I did go back to the org. 19" last week just to keep the org. look, plus the 16" wheels are a real pain in the a## to line up when mounting.
[mine is a 31].

[christian]

I have 16's on my 31 fordor, eyeballing it the outside diameter looks identical to a 19" with tire I have laying around. Also, the speedo was unaffected by the wheel change, which tells me that the change in roll out is negligible at best.

[Bruce Compton, Canada]

If you are attempting to do really accurate calculations, you should use "loaded radius" and not diameter. Measure the loaded radius from the centre of the wheel/hubcap to the ground after checking inflation pressure and with the weight of the car on the wheel. Cheers : Bruce

[Tom Wesenberg]

I agree with Bruce, and that's why Johnbuckley's #2 post is also a good method.

[johnsor]

My Firestone 6:50 x 16 are the same diameter as my previous 19's. Same RPM's and didn't have to change the speedo gear. Love them, nice soft ride.

[saltman]

Regarding this question... I have calculated the speed difference between 16" wheel/tires and 21" wheels/tires on my 1928 and, considering the outer diameter of each tire, loaded or unloaded (since they tend to cancel ech other out). Even considering tire pressue, etc. The speed gain or reduction would only be about 2-3mph at 40mph.

[Bruce,Upstate NY]

You could compensate with a different rear end ratio.

[Mike/Nebraska]

I've got 7.50x16's on the rear of my 29 and they're very close to the same height as my 21s were.

[Purdy Swoft]

Looking at my Coker tire catalog I see that the Firestone 5.25 X 21 has an over all diameter of 31.50 inches. The more common 440-450 X 21 has a diameter of 30.79 inches. The 5.25 would gain over an inch and a quarter in diameter and would gain additional speed while using the correct 21 inch wheels.

[Cajun]

Most Great Race rally cars run with a very accurate speedometer which you calibrate over a 10 mile or so course daily. It uses a magnetic pickup on a front wheel. I have experience doing this with Coker Excelsior radials in both 16 and 19 inch size.

The 19 inch 600R19 radial will run 5.5% faster for the same tire RPM as a 16 inch 600R16. At 50 MPH on the 16, the 19 will be running 52.75.

Tires are not an good fix to lower RPM's. Overdrive or rear end ratio change will do the job if you have the horsepower.

[Purdy Swoft]

Taller tires will help some at a much lower cost.