Mark Tutor, general manager, America Online Racing
Second Gear...Faster, Faster
We race a '79 Ford Courier pickup with a 2.3L Ford four-cylinder. It has a four-speed standard transmission with a rearend gear ratio of 3.76:1.
The problem is that Second gear is high enough to run, but we are at max rpm with the engine about three quarters of the way down the straight. We currently run Second gear, but Third gear is too low to keep our revs up to where we need them.
We run 14-inch tires and have thought about going to 15-inch tires to get more height, but we are not sure if this would help us enough.
Our other obvious option seems to be a gear change. We are unsure whether it would be better for us to run Second or Third gear. What is your opinion, and what ratio is our best option for that gear? We run on a 31/48-mile dirt track with approximately 5-degree banking.
We also need to know what would be a good spring rate for each wheel of the truck. Our truck weighs about 2,000 pounds.
Some further information would help us answer your questions more accurately. So I will give you a broad-brush response that may cover what you are doing.
If at all possible, it would be better to run a higher gear. If this is not possible, your choices should be backward through the transmission gear selection. The reasoning for this is simple. When you are in a higher gear, you are delivering the engine torque straight through the transmission. The fewer number of times you have to reduce the gear ratio, the more torque you have made available to rear wheels.
I do not know the tire circumference you are currently using or the tire circumference you are contemplating using. I made a guess that your current final gear ratio is in the mid-6.00 range, probably 6.56:1. If you increase your rear tire size by 1 inch in circumference, you are making a gear ratio change of about 0.10. This change at this gear ratio range should yield an rpm decrease of about 100 rpm.
If you go from 14-inch tires to 15-inch tires, you are talking about a circumference change that, most likely, will be in the 3.25-inch range. This should give you about a 325-rpm decrease. I think this will be pretty close for a starting point.
Remember that when you make a final gear ratio change of 0.10 (6.56 to 6.45) you are going to see an rpm decrease of about 100 rpm. When you make a tire-circumference change of 1 inch, you will see a change of about 100 rpm. Now as you move through the gear ratio range, the amount of change will increase with higher ratios and decrease with lower ratios. In the range you are running, these rule-of-thumb tips should be pretty accurate.
There are some things to remember. When you change the ratio of the transmission, you must multiply the ratio change in the transmission by the rearend gear. For example, if you have a 3.50 rearend gear and a 1.23 Third gear in your transmission, your final drive is 4.305 (1.23 x 3.50). You change Third gear to 1.35, and you have a final drive of 4.725. The change in transmission ratios is very dramatic on the final-drive ratio due to the multiplication factor.
Now please understand that I made some very broad assumptions. There are many other variables that come into play. These variables include the rpm range at which the engine makes power.
If you have a gear that is so high or so low that you are out of the rpm range of the engine, a very small gear change can yield little or no results. Conversely, if you are close to the gear ratio, a very small change can produce dramatic results. Even the air pressure in the tire affects the final drive. If you've ever seen a Top Fuel or Funny Car leave the starting line, you can understand what I am talking about.
I hope I have helped you.
Mark Tutor General Manager
America Online Racing, Mooresville, NC