Drag Racing

**BadBird** · December 13th, 2015, 08:24 PM

According to this site, my RPM would be 2481 @ 70MPH
with 265 60R 14 tires and .68 transmission ratio with the 4.11 gears.
That works well for me.

Larry

http://www.csgnetwork.com/multirpmcalc.html

**dhbfaster** · December 13th, 2015, 11:04 PM

That tool is definitely simpler. I plugged your tire size into the other one just to see if I would get the same answer and I did. It is kind of cool to push the throttle up and see what happens. With 400hp, tach at 4237, if your car is 3000 lbs, you're running 119.52.

I have no idea if I did any of this right but I'm starting to think after I finish Granddad's falcon, I'm going to need another one so we can race each other.

**MacDee** · December 14th, 2015, 11:38 AM

Okay... I ran the numbers.
From the best I can figure, based on the 235-60x14 tire size, the tire diameter should be about 32.5 inches. That computes to 102 inches per revolution (pi x diameter). At 70 mph, that would be 724 rpm at the wheel. Multiply by 4.11 for the rear end gear, and then again by .68 for the transmission (overdrive), I get 2023 rpm at the crankshaft.

This seems inconsistent with the previous calculations. Can't explain it....

**dhbfaster** · December 14th, 2015, 11:55 AM

Humm...we seem to have two tire sizes in the thread (but I'm not sure that's it alone), but the tire calculator I have been using is giving a different diameter and circumference than what you are showing.

According to: https://tiresize.com/calculator/

235/60 R14 should be 25.1" diameter, 78.8 circumference
265/60 R14 should be 26.5" diameter, 83.3 circumference

If this tire calculator is correct, that might be the difference.

**MacDee** · December 14th, 2015, 12:37 PM

Yeah, I forgot the aspect ratio.
Tire diameter should be 25.1 inches.
937 rpm at the wheel;
2311 rpm at the crank.

I stand corrected... as usual....

**MacDee** · December 14th, 2015, 12:42 PM

I am absolutely no good at arithmetic. I goofed again!!

This time I used .60 at the trans instead of .68!!

Should be 2620 rpm at the crank!

**BadBird** · December 14th, 2015, 01:04 PM

Okay you guys, this is getting over my head again. On the site I sent it said that my tire size equates to 26.5" tall. They also mention that the tire will grow in circumference as the speed increases, but this is supposedly incorporated into the equation.

If I enter in 26.5 in the charts of most of the different sites it comes out almost exactly the same at 2480 RPM at 70 MPH. Let me know if you find something else and thanks for all your help.

NOW..... Lets get more of our cars at the drag strip this year and see what kind of fun we can have. The cars don't have to be fast to have a blast. My car won't come close to the times and speeds that are listed. I would be happy if it ran in the 12's at around 105MPH. This would be a great trip for our club to go to the drag strip!!!!!!!!!!!