Gear ratio question

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shobes

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I took my car too the track and just couldn't believe the RPM's I was spinning by the traps, seems way crazy for what was supposed to be a 3.91 gear set in my Duster. 340 V8, built up mediumish, 27560R15 tires(28"), A-833 4 speed which is obviously 1-1 final drive. So at 3500 rpm I'm doing a true 64mph,( I know my speedo isn't right but I've watched enough police radar signs to know it's off 4mph.) Plugging this info into a formula I come up with it being a 4.56 rear gearset, just looking for someone else to confirm this for me. Thanks.
 
shobes said:
I took my car too the track and just couldn't believe the RPM's I was spinning by the traps, seems way crazy for what was supposed to be a 3.91 gear set in my Duster. 340 V8, built up mediumish, 27560R15 tires(28"), A-833 4 speed which is obviously 1-1 final drive. So at 3500 rpm I'm doing a true 64mph,( I know my speedo isn't right but I've watched enough police radar signs to know it's off 4mph.) Plugging this info into a formula I come up with it being a 4.56 rear gearset, just looking for someone else to confirm this for me. Thanks.
Click to expand...


What was the trap speed? Did you actually measure the tires?
 
Well, this is not rocket science. There ARE online calculators, but---

easiest way is to beg or borrow a GPS

Measure the tires all the way around, then do some math

Figuring the TRUE rear ratio is easy, first---do you know if you have a limited slip rear? If not jack up ONE rear tire, front tires blocked, put the car in neutral Can you turn one tire? If so, turn the tire TWO turns, and count the driveshaft

If you cannot turn the single tire, you have limited slip. So now, jack up BOTH rear tires, and turn one of 'em JUST ONE turn, and count the driveshaft


According to this:

http://www.race-cars.net/calculators/mph_calculator.html

3500RPM, 28" tire, 3.91 rear is 75 mph

According to that same calculator, you would indeed have a 4.56 gear

According to this:

http://www.gordon-glasgow.org/tirecalc.html

your tire size is indeed 28" (27.99)
 
The rear end is for sure limited slip. Tire size was based on a chart from another site listing the factory measurements on BF Goodrich TA tires.
Trap speed was 102 mph.
I knew the gearing had seemed low from when I first bought the car, but I didn't give it a ton of thought until I had it at the track and barely had enough RPM'S to make it thru. 5500-5700 at traps, the motor will turn 6000 RPM but that's up there.
Perhaps as a double check in spring we'll take the cover off when we change differential fluid and count the two sets of teeth.

Anyhow, thanks for the input just wanted to make sure I was looking at the formula's correct. I'm new to the forum, new to actually owning an A body having wanted one for near 30 years and finally taking the plunge. Great forum, I'll enjoy getting my feet wet and posting more going forward.
 
5500-5700 ain't enuff RPM to indicate a 4.56 gear.
 
StrokerScamp said:
5500-5700 ain't enuff RPM to indicate a 4.56 gear.
Click to expand...

Actually that is EXACTLY what the numbers indicate

A guess of 5700RPM, 28" tire, and 4.56 is a speed of 104 mph

Pretty close for guessin' top trap RPM I'd say
 
5500-5700 is the range, i have to be honest and admit it was my first time racing at the drag strip and i was getting used to it all. the rpm range right at and just past the traps was 550-5700, so right at the traps it could have been 5500 I didn't shut it down right away.

I'm pretty sure they must be 4.56 gears, but when i goto my storage place soon i'll turn the rear tires and count that versus a turn on the drive-shaft.
 
Really? Sounded kinda low to me. I stand corrected.
 
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