VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 tire sizes

until i can locate the 3.55 gears for my 1940 special deluxe.
a friend of mine said to put larger height tires and wheels on the back. he said that's is the way he used to get more speed out of his cars when he was into old cars.
is this true or is he blowing smoke?

No smoke, he's correct. It's relatively simple math. Calculate the changes on this site by plugging in the numbers..........

http://www.richmondgear.com/101032.html

Say Bob, that looks pretty cool. Question: For "height" do you actually measure (like with a tape measure) from the ground to the top of tire --- or --- do you use one of the numbers stamped on the side of the tire???? Thanks again for posting this informative info.

Bill.

If you install 6.50 tires on the rear it will reduce your RPM's by about 2 percent. That is the largest size that will fit on the narrow rim.....not worth the bother. Would be equal to a 4.00 ratio.

I use the measurement from the ground to the center of the axle and double it. The radius of the tire to the ground is all that really matters and the actual measurement with a load on it is different than a mounted tire with no load. For example, a 29" tire (measured with no load) may only have a radius, from center of the axle to ground, of perhaps 14" because the tire is somewhat flattened out where it meets the asphalt. All that is dependent on the load, tire pressure, type of tire (radial or bias ply), etc. I double the measured figure, since the calculations call for diameter, not radius.

Gene is correct, however. The difference of height of available tires for your application will have a miniscule effect on the rear end ratio.

You could also use the roll out distance but I dont know the formula for calculating it this way.
Tony

Tony,

I think the roll out distance would be the most accurate. After all, loaded or unloaded, the distance traveled per revolution should not change. The formula is: divide the circumference by pi. The roll out distance would equal the circumference and pi is 3.14159, that would give the diameter.

Rich

Not all brands of tires are the same diameter. A worn tire will be small and the pressure will also have some effect.
The following is from the 1960 truck data book.
A 6.00X16 tire has an out side diameter of 28.4" inloaded. Makes 739 revs. per mile.
Te 6.50 tire is 29.8", makes 703 revs per mile.
This is fewer revs than a 7.10X15 tire.
I don't have the numbers for a 7.00X16 tire which is too wide for the stock rim.
From the parts book the 1940 car with the standard 6.00 tires used the same speedometer gears as with the optional 6.50 tires because the difference was so slight.
With the 6.00 tires the speedo was usually about 2 MPH fast at 60 MPH and with the oversized 6.50 tires was about accurate.

i discovered that my car has 225r14 tires on it, diameter 26". i thought they were stock 16s.
with the 14s and the 4.11 rear end. its no wonder i can not go faster than 45 mph. i guess i am stuck with them until i can obtain the correct rims and tires.

The smaller diameter tires will make your speedometer read fast. When it says 45 you are probably going only 40 MPH....and the error get worse as you go faster. If your not accustomed to old cars 50 MPH feels like 80.

I just went out and measured the distance for one revolution of my tires (I have 6.00-16 Allstate tires.) I measured 7'-3/8" for one revolution of the tire; this calculates to 751 revolutions per mile. I have a GPS program for my laptop computer. I took a drive and compared indicated speed with the speed shown on the computer. I was a dead nuts match right up to 50 MPH.

If you take the 751 revolutions per mile and multply that times the final drive ratio (4.22 for me) that should give the engine turns per mile in 3rd gear, also the RPM's at 60 MPH (3,169.)

Gene, what is a safe max engine speed for my '37 216 with about 52K on it?

Thanks,

Rich

I would keep a 216" engine under 3000 RPM's due to the cast iron pistons and the babbited bearings. Had a fellow in our local antique car club that was working as a mechanic back in the 50's when the NYS Thruway open with a 60 mph limit. He said he had a noticeable increase in babbit bearing failures on cars running down the T-way. State speed on 'country' roads back then was 50 mph.

From the 1937 Engineering Features-
At 60 MPH the Master with a 3.73 ration is turning over at 2766 RPM.
The Master DeLuxe with the 4.22 is 3145 at 60 MPH.
The safe engine speed for steady high speed driving would depend more on the condition of the engine. i.e. bearing fit and condition of rod bearing babbit. When these cars were newer they could be driven at 65 MPH (4.22 ratio), with 60 being really safe. I drove my '39 at 65 MPH often and later did switch to aluminum pistons for extra saftey. With its present 3.73 ratio I have held it at 65-70 on the inter-states with no problem. Ocassionally will run it up to 75 or 80 for short distances.
After WW II they had more problems. Due to the non-detergent oils the sludge would form in three or four years, the oil pump screen would plug up and starve the bearings.
As I said, the safe speed depends on the condition of the engine and if your not sure 3000 RPM (55 MPH) would be best for the long steady speeds.

Gene,

Thanks, this information is very helpful. I have been wondering what my engine speed was while driving on the highway and now I know. (My calcs. are less than 1% off from factory specs and that is likely because of the actual size of the tire, plus any small error in measurement.) I have driven 50 - 55 MPH for about 30 miles at a time on the interstate and all seemed just fine (55MPH = 2904 RPM.) Next week I will take delivery of a new set of Diamond Back Radial tires for the car. Hopefully, they will be very close in size to the tires that I now have.

Later this winter, I will remove the oil pan and examine the lower end of the engine to see just how things are. This will be a good time to clean any crude that may be down there as well as a chance to make adjustments to rod and main bearings.

Thanks,

Rich

Rich. What size radial tire did you buy. I have a '41 with 6:00 X 16 bias tires. You apparetly researched suppliers. Who did you order your tires from. Thanks JIM

Check patrick's in arizona, I got the gears and the rebuild for the rear and torgue tube, plus a corrected speedo drive a few years ago advertized in hemmings

Jim,

Yes, I have done a bunch of internet research. I have found several sites where people have made comments on tires plus a local VCCA member who has 4 cars and some of the cars have the tires that I have ordered.

Here is a link to Diamond Back tires: http://www.widewhitewalltires.com/diamondback16inchradials.htm

I have purchased the 205-75R16 3" wide white walls (look on page 19 - shipping to Denver is right at $25 per tire.) I spoke to the salesman and I read through their catalog. I found all of their information very enlighting.

For example. If you buy a new 6.00-16 tire that says BFG or Firestone on it, they are made in the original molds, but the molds are now owned by other companies and the quality of the tire made on these molds may, or may not, be good. Also, I have found several sites where the Coker tires were given negative comments. I have seen the tires that the local VCCA member has and they look great. If you are going to really drive the car, all of the information that I have seen really emphasises using radials.

A previous owner of my car purchased new Allstate tires, but never drove the car. They have a date code for 1998 and I don't really feel that they are safe for the type of driving that I will be doing. I have put 800 miles on these tires and they still look new. I hope to peddle them to someone that will use them on a display car or will drive them at low speeds.

Rich

My new tires were delivered today. I stacked them up next to the old ones and they seem a perfect match as far as height goes. The tread is about 3/4" wider and the tread pattern is quite a bit different from the old bias tires. At the time of purchase, I told the salesman that I a little concerned that the new tires might be too wide and could touch part knee action suspension, but he said that they have never had anyone report any problems with the tires. I will get them mounted tomorrow and find out for sure. If anyone wants any photos or other information, let me know.

Rich