I HAVE A 70 F100 WITH A BUILT 302 THAT I'M RACING AT THE LOCAL DRAG STRIP. THE TRUCK IS RUNNING IN THE MID 9'S IN THE 1/8 MILE, BUT MY SPEED IS THE SAME AS SOME OF THE 7 SECOND CARS (~75MPH). I NEED HELP IN DECIDING WHAT RATIO TO RUN TO MAX OUT AT 6800 RPM IN SECOND GEAR WHEN I CROSS THE LINE. IM USING A C4 AND WANT TO RUN A 28 TO 30 IN TIRE. THE TRUCK IS FAST, I JUST NEED HELP GETTING IT MOVING (A 2.65, 60FOOT TIME SUCKS!!!).
ANY HELP WOULD BE GREATLY APPRECIATED.

JEFF

 

I use these formulas to decide how to set up a car or truck to do specific things. Keep in mind that auto trans have nominal slip around 15% which needs to be factord in to the final answer. RPM x Tire Diameter divided by MPH x 336= Gear Ratio.
Gear Ratio x MPH x 336 divided by Tire Diameter=RPM
The more stall you have in the convertor the less slip you have. Example is I want to go 130 at 6800 rpm and I run 31 inch tires and a C6 with 15% slip I need 4:10s (6800x31/130x336=4:82) but I can subtract 15% for slip in trans and that is 4:10. This is the rough guideline for setting up the right gear choice but you have to remember that in racing there are many other factors. These formulas just get the right gear for the right speed at a given rpm and tire size. Its the formula used by most gear manufacturers.

 

After posting I reread your post and saw you want to know about second gear. You need to now the ratio of second gear in your trans, find what percent lower than final is and subtract that from the answer.

 

JEFF,

Greasball's formulas are correct, and should point you in the correct direction. Here's the ratios for a STOCK Ford C4 automatic transmission:

1st gear.....2.26:1
2nd gear....1.46:1
3rd gear....1.00:1

I hope this helps, and good luck with your racing!

Sidewinder

1979 Ford F-350 Ranger XLT Crew-cab

 

I don't agree with the other posts, the 336 factor is for manual transmissions, use 355 for automatics to account for slippage. The 1.46 second gear ratio for a C4 is correct. Using that factor, a tire dia. of 28" and a 75MPH top speed, you would need a 4.90 axle ratio; 31" tires would result in a 5.42 ratio. Looking at Summit's catalog for a Ford 9", you would have a choice of 4.86, 5.14 and 5.43. Since tires wear out and smaller dia are cheaper, you could start with the 4.86 and 28" tires. You could then dial in a higher top speed with taller rubber. Evaluation of your converter's stall speed may also be in order for that all important launch.

 

As much as I would like to take credit for coming up with the formula I did not. These formulas are used by "Richmond Gear" and other respected Gear manufactures to aid in the proper choice for a specific application. True the formulas are for a 1:1 final ratio, they also specify, if you run an overdrive you need to subtract the % of overdrive from your answer or add an average of 15% for slippage in automatics. I used your formula and found it only added approximately 5% for slip. I cant say your formula is wrong but it doesn't jive with the formulas provided by the gear manufactures for correct gear choice, or the transmission experts oppions that automatic transmission slip on average 15% unless you are running one of the newer full lock up transmissions.

 

I also won't take credit for the formula but I do take issue with Richmond Gear et al about a 15% slippage. I have used 355 numerous times on mine and other vehicles and it is not a bad starting factor. The worse slippage I came across was over 9% in a well worn C-6 (mine). I will admit that 5.6% appears to represent a new or recent quality rebuild. 15% slippage is an automatic ready for a rebuild. Just think about the number of rpms that a 10% slippage represents and the amount of heat that gets generated by that rpm loss inside the case. Record rpms vs mph and try it on one of your vehicles, calculate your own factor and I think you will see what I mean.

 

The truck I currently drive had a fresh C6 using mostly new parts, and I dialed my speedo gear in to within 1/2 a teenth in 10 miles. I got out on the hiway and checked speed versus rpm using 4:10 gears and I ran the formula and was at 14%. This was a 7000 lbs. 4x with big meats and running on flat ground. I asked the tech guys at our local supplier and they said thats normal. I wasn't happy with the numbers and asked about other ways to reduce slip and was told it wasn't really possible to change the final ratio. I was reccomended to change to 3:54 gears (about 13.5%) less than 4:10. Now for the sake of arguments I will say that I own a General Auto Repair shop, but I farm out most tranny work, I can rebuild early trans, but for the real tech know how I ask. I know that every mechanic has a theory or idea about something and rarely do two agree. That makes it kinda fun. Not being an expert on auto trans I have to take the word of other experts. The formula I use has worked great. How do you determine the actual slip in real time driving taking in to consideration that most vehicles run other than factory size tires rendering the speedo inacurate? Rick

 

There are two ways to correct the speedo error. The hardest is to measure the diameter of both tires (current and original), subtracting the difference,dividing by the original diameter and multipling that to the speedo reading. One mistake I made was trusting a tire chart for diameter; LT tires aren't bad but car tire sizes vary widely between companies so measure yourself. Easier is to run on the interstate or a state highway with mile markers for 10 miles at 60 mph with your oversize tires. The difference from 10 minutes and divided by 10 will tell your speedo error in mph. Since you now can get the true rpm for a real 60 mph, you have the 4 of the 5 unknowns and can solve for that mystical factor.

 

I found that the speedo gear which is the same for many ford applications, come in 5% increments. One tooth = 1/2 mph, or 5 tenths. They are color coded but the dealers have no way of looking them up other than make, year and model, with which "factory tires". I talked the local ford parts guy into finding the gear with the correct number of teeth for me and after a couple trys dialed it in per the speedo check posts on the freeway. I did a little math and found that if you are spinning 2500rpm with a 1:1 final than add 15% for slip your rpm increases only 375rpm. A 5% slip is only 125rpm. Im curious where the 5% info comes from.