RPM vs. MPH, does this seem right?

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Jun 9, 2005 | 10:19 AM

zupmanZ28

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RPM vs. MPH, does this seem right?

I just ran an ATAP log, and upon reviewing it, when looking at mph versus engine rpm, something doesn't seem to be adding up to me. Mind you i'm not accounting for converter slippage here, but it seems like the numbers are off by a lot.

Its a 4l60e (stock), with a PT4000, and 3.73's. 26.1" tire (MT drag radial).

Heres what i think is screwy:

RPM......MPH......GEAR
5649___47_______2
5830___56_______2
5936___60_______2<-----
6056___64_______2
5797___78_______3
5804___81_______3
5850___87_______3
5910___92_______3
5974___96_______3
6002___98_______3

I ran the mph calculation for the line of data with the arrow next to it (above), and it tells me that with a 26" tire, 2nd gear (1.62:1), and 3.73's, the mph should be 76. Which is a difference of 16 mph.

I then ran the mph calculation for the last line of data above, and it tells me that with a 26" tire, 3rd gear (1:1 ratio), and 3.73's, the mph should be 124.9 mph. I understand that i haven't calculated for converter slip, but 27mph? That seems like an awful lot to me. The converter was unlocked btw.

Anybody have any thoughts on this? I'm not positive what the second gear trans ratio is, i think that it is 1.62:1. Can anyone confirm that?

Anyone have any thoughts as to whats going on? is this normal?

Eric

Jun 9, 2005 | 10:27 AM

SteveC

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From: Poway, Ca. where GOD and the sun always shines

Here you go, just plug in the numbers:

http://www.f-body.org/gears/

SteveC

Jun 9, 2005 | 10:47 AM

zupmanZ28

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Quote:

Originally Posted by SteveC

Here you go, just plug in the numbers:

http://www.f-body.org/gears/

SteveC

Your site confirms my calculations, except it seems that the second gear ratio is 1.75, not 1.62. Which means in second gear its only off 10mph, not 16.

Any thoughts on whats causing this?

this table is 1st, 2nd and 3rd gear respectively, from left to right

Jun 9, 2005 | 11:17 AM

12secSS

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Those gear calculators are mainly for manual transmission equipped vehicles, those value are way too optomistic for an A4 with an unlocked converter. Your readings (and I will have to double check) sound about right. I would log more trans PIDs to see if your transmission is slipping. Again, do NOT go off of those online gear calculators ... you will just be setting your self up to draw a wrong conclusion.

EDIT: Here are the true ratios for a 4L60E
1st - 3.06
2nd - 1.63
3rd - 1.00
4th - 0.70

Jun 9, 2005 | 11:31 AM

zupmanZ28

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Quote:

Originally Posted by Flame Throwing SS

Thats exactly what i was wondering. I guess i'm still used to my old t-56 car. Those calculations were nearly right on for it. I'm gonna go run some more logs, and see if anything else catches my eye.

Either way, looks like i have found my problem. the car ran 11.4's @119 with a 3.42 gear, but since i've had the 3.73 rear in, i've not been able to run better than 11.58@116. Seems as though i'm running out of gear. Guess i'm going back to the 3.42's.

Any more input is greatly appreciated

Eric

Jun 9, 2005 | 12:11 PM

cablebandit

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out of gear? I locked my converter and was running 140mph at the end of the 1/4 with 3.73 @ 7300 rpms. That gear oughta be gravy for you!!!

Jun 9, 2005 | 12:16 PM

zupmanZ28

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Quote:

Originally Posted by cablebandit

out of gear? I locked my converter and was running 140mph at the end of the 1/4 with 3.73 @ 7300 rpms. That gear oughta be gravy for you!!!

i have no urge or want, nor should i have a need to lock the converter. Plus, i've heard about very bad things happening when locking a Yank convert at WOT. My shift points are no where near 7300 rpm either, more like 6300.

Eric

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