1967 Rambler American - Gear Ratio

The Reality
The Rambler weighs 2,771 pounds, so we ran the numbers and came up with 461 for an engine-horsepower number. Adding a 180-pound driver and and 10 gallons of fuel at 7 pounds per gallon, the race weight is more like 3,021, so the horsepower required is closer to 483. We plucked a 360 from a Jeep Grand Cherokee, and with some Edelbrock heads, a flat-tappet Lunati cam, and 10.5:1 compression, we made 480 hp at 6,000 rpm in the Jan. '06 issue, before we dropped it in the car. We solved the equation using 6,000 rpm for the ideal trap rpm knowing that with converter slippage the final rpm would be about 6,300. We plugged in the the 26-inch-tall tire, divided it by 130, and got a rear ratio of 3.57.

The Rambler's design restricted the tire size that will fit under the rear without either installing minitubs or relocating the springs. With the 15x7 rear rims, the tires are within 11/48 inch of the springs, and we had to trim the inner fender to get the tires to stop rubbing. The biggest tire we could get in there was a 235/60R15 that is only 26 inches tall. A larger tire, like a 275/60R15, has an overall diameter of 30 inches and would allow us to run a 4.10 ratio. Since gear ratios multiply torque, the higher numerical ratio would help the Rambler run a quicker 60-foot time. More tire allows more gear, both because the greater footprint would help traction and the increased height would still keep the trap rpm where we want it.

We have the weight and we have the horsepower, but for the first three runs, we used the stock 2.71 rear gear. The result was a baseline of 12.75 at 112 mph, which is exactly what Comp's Desktop Drag said it would be and about 2 seconds slower that the pure power-to-weight math wanted us to believe. The run pointed out that the small tire might be enough if we don't hit it too hard on the line, and we are definitely going to need a better gear ratio if we want to go fast. Knowing this, we are going to try a 3.55 gear with the small 235 tire before we return to the racetrack and see if we can go fast without cutting the car for minitubs.

PARTS LIST
DESCRIPTION	PN	PRICE
'67 Rambler American	N/A	$2,000.00
Engine total from Jan. '06	N/A	7,296.11
ADDITIONAL PARTS
TCI Torque Command 904	601100	1,009.00
TCI 10-inch StreetFighter	751600	432.88
Transmission dipstick	22160	74.02
Royal Purple transmission fluid, 5	N/A	8.95 / quart
Exhaust Labor	N/A	100.00
FlowMaster U-Fit dual kit	15936	183.39
Flex-a-fit radiator	52180R	899.95
Powermaster Ultratorque starter	9415	229.95
Stockton Wheels	15x5 and 15x7	440.16
M/T ET Street and VW front tires	235/60-15 and 165x15	411.90
PAINT AND BODY
Total from Dec. '06		645.40
TOTAL		$13,767.51*

*Low 12s on the motor in a reliable car that isn't a Chevy. Send rants to CarCraft@primedia.come.t.Corrected 12.36 at 115.36Uncorrected 12.75 at 112 mph

We were inspired by small-tire cars running in the 9s using nitrous and ignition timers instead of brutal wheels-up launches. We were also inspired when we found that by using 15x7-inch wheels with 5-inch backspacing, the 235/60R15 M/T ET Street radials tucked under the fender with only inner fender-lip modifications. Both wheels are from Stockton Wheel; the front skinnies are 15x5 with VW 165R-15 tires.
We were inspired by small-tire cars running in the 9s using nitrous and ignition timers in
We had Jeff's Smith's buddy Tim Moore weld in an exhaust bung for the Innovate O2 sensor. We've become addicted to the ease of tuning with the little handheld to get the car close to race ready. Then we only have to tweak the air/fuel ratio at the track.
We had Jeff's Smith's buddy Tim Moore weld in an exhaust bung for the Innovate O2 sensor.

For the baseline run, we used the stock fuel system, gas tank, and factory Jeep mechanical fuel pump. We haven't used the Innovate to tune the car, but we did use it as a safety measure in case the car was lean. At the top of the track, we were seeing 12.5:1, which is on the rich side. In theory, the best ratio is between 12.8 and 13.5 for best power. In the real world, you tune for mph.
For the baseline run, we used the stock fuel system, gas tank, and factory Jeep mechanical