Rear end gear ratio

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Jax

Jax

FABO Gold Member
FABO Gold Member
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Currently running a 4.11 rear end gear.
My trap speed is 127 mph.
A999 with 1:1 3rd gear.
29 inch slicks.
My rpm at the finish line is 7200 rpm.

Would like to lower my rpm to around 6500. What rear end should I use?

I tried different calculators, but when I plug in my numbers, the current gear ratio is way different that what I have.
Hence the reason I am asking for some guidance to figure out what rear end ratio to purchase
 
This is simple math what am I missing?
6500/7200 x 4.11=3.71s

But I gotta tell ya I think you TC is slipping pretty bad..
With a 29" tire, your Zeroslip rpm should be
(127 x 1056x 4.11)/ x 29 x 3.1416) = 6050
If the tach is trustworthy, your TC is slipping
7200/6050=19%
That, friend, I think you can take to the bank.
Thru the traps, a good TC might slip 8%? maybe less at your power level, Ima guessing. In any case 19% is huge. Imagine how fast you car will be with a tight TC and maybe 4.30s

With 4.30s the math says your rpm @127 should be 6330 at zero-slip, say 6840 at 8% slip..
But if you have enough power to go 133, then back to 4.11s and 133=6330 @zero slip, say 6840 at 8% slip
But if your cam is done early, these rpms might be out to lunch. I mean to carry the Revs to 6840 thru the traps still making power, is gonna take a wicked combo.
I'd be willing to bet a wooden nickle that your ET does not match your speed.
 
Last edited:
Well, know that you mention this. You made me think.
I have autometer tach, and an external digital shift light. I have been using the feature of Max rpm on the shift light as my max rpm. But I think that RPM was maybe recorded from a burnout instead of down the traps.
Will have to do a reset on it, and record my end traps again
 
Thanks for the reply man, it is good to be on this forum and have persons willing to help like you.

I was ready to order a set of 3.73 gears to use when a spray 100 shot of NOS, thinking my rpm was that high....LOL
 
I agree with Aj, time to do a lap and look at finish line rpm. Its likely less than you think, or your converter is garbage. What is your 60 ft? What e.t.? 127 mph should be around mid tens, could be as good as 10.40s.

For what its worth, i love my vdo replay (not recall) tach.
 
I am running 10.30. And my 60ft is 1.35
 
Jax said:
Currently running a 4.11 rear end gear.
My trap speed is 127 mph.
A999 with 1:1 3rd gear.
29 inch slicks.
My rpm at the finish line is 7200 rpm.

Would like to lower my rpm to around 6500. What rear end should I use?

I tried different calculators, but when I plug in my numbers, the current gear ratio is way different that what I have.
Hence the reason I am asking for some guidance to figure out what rear end ratio to purchase
Click to expand...
3.91. Hey I just posted one. lol
 
Just throwin this out there.......you don't have "1:1" in any gear with an automatic. Ain't happenin. But I get what you're sayin.
 
Yeah but, can you put this kind of power thru a loc-up clutch? IDK; I'ma guessing no, or not for long.........
For argument's sake lets say the tach is right.
What if the TC was ok at a more normal slip, and the rest was in the loc-up clutch?
So you buy/install a new TC and for the first couple of runs everything seems rosy; but after a couple more runs, the tach starts to climb again.
IDK; do guys race in lock-up? When do you engage it?
I'm not a racer, don't run automatics, and we're not in the Racer's Forum, so please, cut me a lil slack, ok?
 
It would be custom made. I never ran one in my hot rods but seen other racers at the track with them. They do work if you can afford them.
 
Definitely something off because that shows a trap speed of 150mph or so.
 
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