# Automotive 100 MPH/Rear Gear 1/4 Mile Drag Calculations

1. Jan 17, 2012

### average guy

i want the gear so the car is going 100 mph at 6000 rpm 1:1 drive
at end of 1/4 mile.
have figured out this much for dry run.
32 inch DIAMETER tire x pi 3.14 = circumference.
let's call it 100 inches.
one mile is 5280 feet.
engine will turn at 6000 RPM.
RPM is in minutes.
MPH is in hours.

that's as far as i've gotten.

Have A Nice Day!

Last edited: Jan 17, 2012
2. Jan 17, 2012

### xxChrisxx

What exactly is the context of this question?
You can easily work the engine speed to wheel speed ratio from what you have given.

But it's not going to tell you anything useful in the real world.

3. Jan 17, 2012

### Staff: Mentor

Doesn't the problem have conflicting constraints, specifying a 1:1ratio?

4. Jan 17, 2012

### S_Happens

The 1:1 is referring to engine:trans output.

Maybe it's just too early in the morning, but the 32" tire makes me think it's right. I'm coming up with 5.71 (dang).

That seems a little crazy for something that is only going to trap 100 mph in the 1/4. I mean you're probably talking in the 14 second range. What the heck are you driving with a 32" tire, that you care about gears in, that you want to take to the track running those kind of times?

5. Jan 17, 2012

### average guy

happens
how are you doing that?
i'm still back at converting circumference to feet.
changed 100 inch circumference to 96 inch or 8 ft.
96/3.14 equals a 30 inch diameter tire, good enough.
5280 feet in a mile divided by 8 = 660.
the tire has to rotate 660 times to cover 1 mile.
phew!
i think it's a good idea to change RPM to 5000.
change MPH to 120 that will be 2 MILES/MINUTE.
matches up with RPM better.
what's next?

Have A Nice Day!

Last edited: Jan 17, 2012
6. Jan 17, 2012

### S_Happens

Sorry, internet was sketchy today so I was able to log on, but not reply.

I used the circumference of the tire (100.5") and calculated the rpms of the tire at 100 mph (1050), then divided into engine rpm. I don't calculate step by step, but list out the conversions in a row (I don't know what the technique is called, but it's pretty common) to know my units are correct then simply punch it into a calculator. That way it doesn't have to be pretty.

If you don't want to do that you can google for an online calculator. I happen to also have one on my desktop along with a slew of automotive calculators (most never used), but have been on campus all day. Both the online calculators, and now my desktop application agree with me.

OK, 5000 rpm at 120 mph is a much more realistic, and common, gearset which would be 3.96s (technically I rounded down, but I liked saying the common 3.96 rather than 3.97).

That's much better. I was trapping 100 mph in my first ever trip down the track in my stock Z28.

7. Jan 17, 2012

### mender

Okay, 60 mph results in 660 tire rpm; 100 mph would mean 1100 rpm (100/60 x 660 rpm)'. Divide your target of 6000 engine rpm by the wheel rpm (1100) and you get 5.45:1 gear required.

8. Jan 17, 2012

### average guy

mender & slideways
here it is!
the goal is to have the car going 2 miles/minute
1 wheel RPM = 8 feet
2 miles = 2 x 5280 = 10560
10560/8 = 1320
when the car is going 2 miles per minute or 120 mph
the wheel is spinning at 1320 rpm.
now to match engine rpm to rear wheel rpm
by changing the rear differential ratio.
6000 rpm/1320 rpm = 4.54
5500 rpm/1320 rpm = 4.17
5000 rpm/1320 rpm = 3.79
this translates to these mopar 8 3/4" ring & pinion sets
6000 rpm 4.56 ratio
5550 rpm 4.10 ratio
5000 rpm 3.90 ratio
you can think of it the other way
the spinning wheel driving the driveshaft
( which is 1:1 to the engine in direct drive ).
wheel rpm x gear ratio = rpm's

Have A Nice Day!

9. Jan 19, 2012

### Bihili

10. Jan 19, 2012

### average guy

bihili
can't. need the money for welding rods, steel tubing,
NAPA parts and diet pepsi.

Have A Nice Day!

11. Jan 20, 2012

### pantaz

No money involved -- the dozens of calculators on wallaceracing.com are free to use online.

12. Jan 21, 2012

### average guy

pantaz
thanks for looking for that.
there's 3.73 ring and pinion for the Mopar 8 3/4 also
that can be used to get close to what calculator
comes up with.

Have A Nice Day!

13. Feb 9, 2012

### 2milehi

Do you have an auto or manual tranny?

P.S. - Can't you be considerate and construct paragraphs like the rest of the world does?

14. Feb 10, 2012

### average guy

2 mile high
oh i get it denver!
trans type doesn't matter it's a formula.
I've told people, I'm working on it.
this is only forum where anybody has said

Have A Nice Day!

15. Feb 10, 2012

### 2milehi

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