View Full Version : gear experts... SOLVED

08-21-2005, 10:41 AM
Ok guys, I'm gonna be changing from 2.73 rear gears to 3.55 in my 20th. Which speedo gear will I need to get the speedo to read correctly?? :crazy: Thanks, Herb :hick:

08-21-2005, 01:44 PM
i hope this works!

08-21-2005, 01:48 PM
if you cant quite read the part# or what have you,just ask and i will let you know for sure.

08-21-2005, 02:10 PM
Thanks!! :D

08-22-2005, 08:04 PM
Yeah, I'm going from 3.08's to 3.55's(originally had a 2.73 7.5" rear). Maybe it's my piss poor vision but I can't see it. If you could post that part # it would be greatly appreciated.


08-22-2005, 09:00 PM
I don't know if you got a confirmed answer to your question, so I thought I'd try to help. I can't tell you exactly what tooth-count gear you need, but I can tell you how to figure it out. You need a gear with 1.3 times the number of teeth as what you had stock. Just for a round number, if your stock gear had 10 teeth, you'd need a 13-tooth. I got that by dividing 3.55 by 2.73 (3.55/2.73=1.3). If that number comes out to some weird fraction, just go with the closest available. Sometimes you can change your gearing so drastically (4.10's or whatever) that there's not a speedo gear that will correct the change, unless you actually crack open the tranny and change the drive gear also. At one time I was going to drop 4.10's in my F150 that came with 3.55, and my stock speedo gear was a 20-tooth. I needed a 23-tooth to correct, which everybody said did not exist. But then, I never changed the gearing, thank goodness (gas prices!!). You sure you want to make this change? That's 30% lower gearing, or basically 30% more RPM than you're used to. So your 2000 RPM at 70 (I'm just guessing) will turn to 2600. She'll really snap off the line, though. You know you can also effectively raise or lower your gearing by messing with tire diameter, don't you? You probably weren't looking for an answer this long, but good luck with your search.



08-22-2005, 09:31 PM
i dont know what tranny you have for sure,probably an aod.chart says for that most ford rear drive automatics have 7 or 8 teeth drive gear on tranny output shaft.so for 3.55 gears it says you need a 20 tooth orange driven gear with part#C8SZ-B.hope that helps you out.

08-22-2005, 10:38 PM
fordman3, yea I'm sure. The car is real lazy with the 2.73s, and works hard to accelerate, might even pick up a couple mpgs with the 3.55s...lol. ;) Seems like your F-150 gear and 30thbirds chart agree on a 20 tooth gear. :D

09-08-2005, 04:14 PM
i am currently changin over my factory 3.27 gears in my super coupe to a 4.10 how much time can i expect to lose in the 1/8 mile if it ran a 9.60 with 3.27?

09-08-2005, 09:59 PM
By lose time do you mean cover the eigth or quarter in less time? The 4.10's will actually help you cover the 1/8th or 1/4 mile in less time. All things being equal I'll venture a guess of 3-4 tenths.

10-09-2005, 01:15 PM
Just an update, the 20 tooth speedo gear is not the right one for 3.55 gears with aod.:nono: Changed the speedo gear and speedo is still 10 mph fast, looks like I need a 23 tooth gear.:disappoin

10-10-2005, 06:26 AM
hey let me know becuase i need to do it also:) how much off is it normal 15mph?

10-10-2005, 08:54 AM
Well with the original 2.73 gears, it was dead on. When I swapped in the TC axle with 3.55 gears it was at least 20 mph off or more. (couldn't really tell, the digital speedo was buried to go 70 mph.) I bought a 20 tooth speedo gear, that's the one the FRPP catalog called for, and now it reads 80 ,ph at 70 mph actual speed. I'm gonna try a 23 tooth speedo gear as soons as it gets here.:wtf: :hick:

10-10-2005, 03:31 PM
o i dont have a digital so im doing like 50 55 in a 35 right now and people are passing me. so ill just wait till you get yours figured out then i will

10-10-2005, 08:08 PM
I've got 3.73 gears and i have a 21 tooth purple spedo gear. It's 6-7 mph off. Don't ahve a 23 tooth gear cause I don't feel like crawling under the car to change the gear every year or so (supposidly the 23 tooth gear is very fragile).

10-26-2005, 10:45 PM
Ok got it fixed now!! Used a 23 tooth speedo gear, and the speedo is dead on now, whoo hoo.:D :banana: :banana: