390 gears how fast
Thread Starter
Race Director
Joined: Mar 2005
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From: Southampton NJ
Intermediate
Joined: Jun 2006
Posts: 26
Likes: 2
From: San Diego
I had 4.10s and found them TOO Deep, made 1st gear useless at WOT w/ excessive wheelspin. On OEM tires at 30PSI and with 386RWHP 3.73s hook up! With either gears I found no appreciable difference in freeway gas mileage.
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Team Owner
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From: salem OR
Team Owner
Joined: Feb 2001
Posts: 27,253
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From: Short Hills, NJ
I vote no on the 3.90s...though if it was still back when ya had 3.15s and you were trying to decide between 3.73s and 3.90s then I might say go 3.90s because they work and it's not a 100 miles a day daily driver. Ellis has 3.90s and they're fine for weekend use and driving to/from the track. But for now I wouldn't get rid of those 3.73s, it really is the PERFECT gear for a street driven A4 car. 
Also keep in mind that your final gearing is actually slightly higher than 3.73 as it is because your tires are 26.13" (295x35x18 BFGs) tall as opposed to 26.85" (stock) as well so that helps, in theory you might have a 3.80 rear in there.
A lower overall diameter tire such as the 275x40x17s or 255x50x16s (both somewhere between 25.7" and 26.1") would give ya even more gear so to speak.
You might also consider a higher stall converter as well.
Also keep in mind that your final gearing is actually slightly higher than 3.73 as it is because your tires are 26.13" (295x35x18 BFGs) tall as opposed to 26.85" (stock) as well so that helps, in theory you might have a 3.80 rear in there.
A lower overall diameter tire such as the 275x40x17s or 255x50x16s (both somewhere between 25.7" and 26.1") would give ya even more gear so to speak.You might also consider a higher stall converter as well.
Safety Car
Joined: Nov 2007
Posts: 4,537
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From: Bethesda MD
gains from 3.73s to 3.90s are going to be small. In the F-body world, A4s switching from 3.73s to 4.10s saw less than a .1 increase, frequently just .05. The only time I saw guys get more was when they went to a bigger tire as part of the switch and got a better hole shot out of it.
I agree with LS1LT1. If your stall is less than a 4000, that could be an area for big improvement.
I agree with LS1LT1. If your stall is less than a 4000, that could be an area for big improvement.
Team Owner
Joined: Jun 2001
Posts: 26,714
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From: Houston, TX
I admit to not being an expert on automatics, but I still am willing on venturing an opinion. If you already have a high stall converter, I don't see the lower gears doing much good. You are already getting a ton of torque to the rear tires via the torque converter, and your high stall is putting you in the meat of the powerband the instant you take your foot off the brake and leave the line. So what is lower gears going to add?
Team Owner
Joined: Feb 2001
Posts: 27,253
Likes: 134
From: Short Hills, NJ
I admit to not being an expert on automatics, but I still am willing on venturing an opinion. If you already have a high stall converter, I don't see the lower gears doing much good. You are already getting a ton of torque to the rear tires via the torque converter, and your high stall is putting you in the meat of the powerband the instant you take your foot off the brake and leave the line. So what is lower gears going to add?
The increased gearing will mean even more if he were to ever go with a cam or heads and cam but again only if the tires hold.
The one area where it could get questionable is with the traps and what gear the car will be crossing the line in (3 or OD?), this also will depend on tire heights chosen.
Le Mans Master
Joined: Aug 2004
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From: Monroe Township New Jersey
St. Jude Donor '05-'06-'07-'08-'09-'10-'11-'12-'13-'14-'15-'16
St. Jude donor in memory of jpee '14
I vote no on the 3.90s...though if it was still back when ya had 3.15s and you were trying to decide between 3.73s and 3.90s then I might say go 3.90s because they work and it's not a 100 miles a day daily driver. Ellis has 3.90s and they're fine for weekend use and driving to/from the track. But for now I wouldn't get rid of those 3.73s, it really is the PERFECT gear for a street driven A4 car.
Also keep in mind that your final gearing is actually slightly higher than 3.73 as it is because your tires are 26.13" (295x35x18 BFGs) tall as opposed to 26.85" (stock) as well so that helps, in theory you might have a 3.80 rear in there. A lower overall diameter tire such as the 275x40x17s or 255x50x16s (both somewhere between 25.7" and 26.1") would give ya even more gear so to speak.
You might also consider a higher stall converter as well.
Also keep in mind that your final gearing is actually slightly higher than 3.73 as it is because your tires are 26.13" (295x35x18 BFGs) tall as opposed to 26.85" (stock) as well so that helps, in theory you might have a 3.80 rear in there.
A lower overall diameter tire such as the 275x40x17s or 255x50x16s (both somewhere between 25.7" and 26.1") would give ya even more gear so to speak.You might also consider a higher stall converter as well.
If you are only looking for that extra .15...... as Marc has already stated ....... "A lower overall diameter tire such as the 275x40x17s or 255x50x16s (both somewhere between 25.7" and 26.1") would give ya even more gear so to speak".
A quick, dirty & cheap way to do it
Melting Slicks
Joined: Apr 2006
Posts: 2,197
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From: Phoenix az
i dont give a damn about top speed, ill never drive 150
0-150 really fast
Tech Contributor
Joined: Jul 2002
Posts: 14,950
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From: St. Louis, MO
St. Jude Donor '08
I'd go the shorter tire route. M/T 17 drag radials will give you more or less the effect of 3.90's at a MUCH lower cost and they hook like crazy.
IMHO 3.73 to 3.90 is too small a difference to warrant the high price of a diff change. You could spend the money on a cam instead and get a much greater gain. And don't forget the sound of the cam.
IMHO 3.73 to 3.90 is too small a difference to warrant the high price of a diff change. You could spend the money on a cam instead and get a much greater gain. And don't forget the sound of the cam.