Thread: Calculator to find the right rear end gear for my car.

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Can anyone tell me the calculator to figure out the right gear for my car.
I want my top speed at 160 and I don't want to run out of gear before that.
My motor can turn 7,000 rpm's and my 3rd gear is 1.00:1 and tire 31 X 18.5 X 15.
If anyone know's how to calculate this, I would sure appreciate it.
Thanks Kurt

I'm thinking convert miles per hour to feet per minute, divided by tire circumference in feet, will give you rpm of tire. Since trans output is 1:1 it drops out of equation. Then your 7K engine rpm will form a ratio to your wheel rpm.

Kurt, I built a little Excel spreadsheet to plug the numbers into. The formula is:

Cruise RPM = (mph x Gear Ratio x 336 x OD Ratio)/Tire Diameter in inches.

The "336" is simply the required unit conversions melded into one value. Using your numbers shows that with a rear gear around 3.73 you'll be cranking 6469rpm at 160mph, and those tall tires are what lets you run the big gears. You won't run out of gear, but you'll have to have enough horsepower and torque at the top to push through the air and overcome friction at that speed.

Ok, here is what I got:

1 mph equals 88 fpm

160 mph equals 14,080 fpm
31" tire equals 8.1 feet circumference
14,080 divided by 8.1 equals 1736 rpm
7,000 rpm : 1736 rpm equals .248
1 divided by .248 equals 4.032

SO , 4:1

Calculated, 7,000 rpm times .25 yields 1750 rpm.
1750 rpm times 8.1' equals 14175 fpm divided by 88fpm yields 161 mph.

Spears beat me to it, and I'd lean towards his #s.

Originally Posted by firebird77clone

Spears beat me to it, and I'd lean towards his #s.

Your's are there too, Firebird, it's just that I had a couple of more decimal places on the value of pi or something. And for those who question the "336" it's:

(5280'/mile X 12"/ft)/(60minutes/hr X 3.141593 diameter to circumference), or 63,360 divided by 188.4956 yields 336.135240, so I round to 336 and call it good.

I did a lot of rounding.

One of my instructors at tech. Told me ( while grading a test )" One of these days, your rounding is going to get you in trouble".

www.randysringandpinion.com

use the calculator pages it will do it all for you.

Godspeed
MrC.

Thanks Roger, Firebird and Mr.C. I figure it was gonna be about a 3.73 but wanted to be sure.
I used to run a 4.11 in my old big block Dodge but I spent more time replacing cranks and bearing in that motor then racing.
This Stock 351 Cleveland block won't turn the kind of RPM's my old 302 Boss would, The Cleveland's like to crack in the valve galley
at about 7'500 rpm's. The Bottom end is well built and can take the RPM's but the blocks can't.
This Cleveland should put out a lot more HP and Torque then it's little brother, so I'll keep the RPM's down a little.
The car should weight just a little over 2,000 pounds so I think it will be alright in the 1/4.
Trying to get it ready to compete with my buddy's fast Chevy his car weighed in at 2,900 but he's running a 408 motor.
Thanks for your help again guy's.
Kurt

Kurt, if you're building for the strip and have dialed in the 160mph then consider 3.70/6417rpm, 3.89/6746rpm, and 4.44/7128rpm, all at 160, 1:1, 31" tire.

Thanks Roger.
Kurt

One thing to keep in mind about figuring potential top speed, is that the important engine rpm is the peak power rpm, not the rev limit. For example, my LS3 has a peak power of 430 hp at about 6000 rpm, but it can be revved to 6500.

If you are performing a true top end run, most often you will either be over geared and not be able to reach the peak power engine speed before wind resistance limits the car's speed, or you'll be under geared and rev over the peak power engine speed. If you hit the rev limit, you're quite a bit under geared (too low).

With a modern overdrive 4 speed automatic, you should also figure the top speed in 3rd gear - usually the normal 1:1 ratio.

When I selected my gearing, I took into consideration the engine's rpm at a cruising speed of 70-80 mph, in the .7:1 overdrive 4th gear, in addition to theoretical top end in the 6000-6500 rpm range. With poor aerodynamics from a '37 style car, I might find that wind resistance limits my engine speed to far less than that. If not, it would be in the 160-165 mph range.