Need a Math Wizard

M

mleder

Senior Member
If I run 600 x 16 tires on my Model A and if I were to hit 50 mph while knowing the rpms that the engine runs, if I were to switch to the standard 19 rims and tires what speed would I hit at the known rpm in the above example? Is there a simple multipy by or divide by number you can give me? Thanks to all the teachers out there.
 
Divide RPM by rear axle ratio giving wheel revolutions per minute. Take the circumference of each tire you are testing and divide 1 mile by that amount. that gives revolutions per mile, OH Darn I lost my train of thought!
 
To calculate this correctly the circumference of both tires must be known. The wheel size is really not relevent in this calculation.
 
Let me try to make it simpler for me. Assume the engine is floored (constant) engine rpm in either example. In the first case floored is top speed of 50. How much more speed will I pick up if I went to stock wheels and tires. Thanks
 
I think for all practical purposes you would go about the same speed because even tho you have a different wheel diameter the tread diameter is very close to same due to side-wall profile.
 
If you want accuracy then you have to measure the distance from the road to the center of the hubcap, multiply by 2 x pi and you will have the distance traveled for each revolution of the tire. Divide that figure into 5,280 and multiply times the rear end ratio (usually 3.78) and you will have the engine revs per mile traveled. If you are going 45 MPH then multiply the engine revs per mile by .75, because you will travel .75 miles in one minute. This will be the engine RPM while going 45 MPH.

If you are going 50 MPH, then multply by 5/6. In other words multiply by 5, then divide that number by 6 to get your engine RPM.
 
Last edited:
And don't forget to add in the centrifugal force making the tyre bigger and after a mile, the tyre will be hotter and both will increase the rolling radius.

(insert giggle).
 
Post the distance from ground to wheel centre for one of each 16 and 19 rear wheel.

Must be on car and off jack with weight on each and correct air pressure.
 
Dang, you guys make it hard. Measure the rolling radius of each tire-wheel combination. Now multiply your top speed by the radius of the 19 " wheel-tire and divide by the radius of the 16's on the car. The result is the speed you'd be going at the same rpm if you put the 19" wheels and tires on the car.
 
Geo. H said:
Dang, you guys make it hard. Measure the rolling radius of each tire-wheel combination. Now multiply your top speed by the radius of the 19 " wheel-tire and divide by the radius of the 16's on the car. The result is the speed you'd be going at the same rpm if you put the 19" wheels and tires on the car.
Click to expand...

Yep, it can be simple, and if a guy is wondering about speed changes with different ratio rear ends, it really doesn't even involve any tough math calculations.
If you are changing a 3.78 to a 3.27 ring and pinion, then the engine RPM will be the same at 32.7 MPH with the 3.78 rear end as it will be at 37.8 MPH when using the 3.27 rear end.
 
Used Coker tire for tire diameters and kahuna's link to calculate.

Original: 1928' - 21" wheel and tire - 30.79"

New: 1935' - 700x16 - 30.39"

Speedometer reading 50mph, actual 49.35

Dang it, I lost .65 mph...
 
Oops you want to lose the 16's for the 19"

600x16 ford script tire from coker - 28.36"

19" firestone tires (thanks Steve) - 28.80"

Speedometer reading 50mph, actual 52.54

Question, Wouldn't the speedometer be correct then with the original size wheels. (as long as the speedo gear hasn't been changed) Right?
 
Last edited:
Here's some related info I put together a while back that may be of interest.

Steve
 

Attachments

  • Tire Data.JPG
    Tire Data.JPG
    65.5 KB · Views: 73
You cannot go on total diameter.

You must measure from ground up to centre and double it.
 
How about this?

Just to complicate things even more, how about rolling each tire to establish distance per tire revolution?

Then crank the engine in gear in third, plugs out(?) to see distance per engine revolution?

Then do your math.;)
(revolutions per minute times 60 minutes, times distance in feet per engine revolution, compared to 5280 feet per mile?)


Would that work? :rolleyes:

Sounds like fun for a math major.
 
Did anybody actually read my first post? I think you can find your answer. By the way the circumference will not change in the air or under load.
 
J Franklin said:
Divide RPM by rear axle ratio giving wheel revolutions per minute. Take the circumference of each tire you are testing and divide 1 mile by that amount. that gives revolutions per mile, OH Darn I lost my train of thought!
Click to expand...


Here's your first post, J Franklin.

Interesting, but not so overly informative that you would request us all to read it again. :)
 
You must log in or register to reply here.
Back
Top