dagmando

I do not have the ability to jack the car up and spin the wheels to count driveshaft rotation but I am started to suspect I have non-stock gearing in my new-to-me 89 LX 5.0 Convertible 5-speed.I verifed thegear is stripped on the maroon "wine color" 16 tooth stock speedo gear so it is bouncing all over.I am currently using a Gamin NUVI for my speedo and want to fix the gear but need a bit if info: I read that 15mph in first is 1000rpm for stock and 2000rpm for 3.73. I do 1900rpm at 15 mph but I am only running 1500rpm at 55mph in 5thand could not see how it could be lower. Any advice on what gearing you think I have is much appreciated so I can fix it right the first time.

Thank you in advance.

91ford302

sounds 3.55 to me.

85GTsleeper

3.55 or 3.73

raven_ghostwolf

there are 2 different gear sets for the T-5 one set uses a 2:95 first gear and the other is a 3:35.
Both use different gear ratios for 1 - 3 and 5th all 4th gears in every manual transmission I can think of is 1:1:get a RPM to speed in 4th is most accurate way to try and figure your rear gear size. but even in that case your over all tire diameter can cause a small to large error in speedometer accuracy. I created a spreadsheet on my computer where I could put in tire diameter, rear gear size, and each gear ratio 1 - 6th gear to give me a computed speed for engine rpm (ok i'm a geek).
using the 2 T5 1st gear ratio's plus 3:55 & 3:73 gear sets and 25.5 inches diameter tire(average for stock size for mustangs 225/60 - 15, I run 245/45 -17 same diameter +/- .25 inches) here is what I came up with.

2:95 first gear 1900rpm (most likely gear set for an 89)
3:55 - 13.76 MPH <--- most likely what you have
3:73 - 13.10 MPH

3:35 first gear 1900rpm
3:55 - 12.12 MPH
3:73 - 11.53 MPH

raven_ghostwolf

Additional info for T5 gear ratios if you want to try and figure it out your self

T5 gear sets
1st 2nd 3rd 4th 5th R
3.35 1.99 1.33 1.00 0.68 3.15
2.95 1.94 1.34 1.00 0.63 2.76

using your 5th gear rpm to speed 1500rpm at 55mph in 5th

3:55 / 0.63 - 50.87mph
3:55 / 0.68 - 47.13mph

3:73 / 0.63 - 48.42mph
3:73 / 0.68 - 44.86mph

one thing I found on a T5 site was according to them you could actually have the 3:55 gear set T5, the 2:95 started getting used in 90.
with what you said here is something interesting I ran up in my spread sheet.

with 3:35 first gear and 0.68 5th and a 3:08 (a factory optional rear gear in some years)!
1st gear @ 1900rpm = 13.97MPH
5th gear @ 1500rpm = 54.32MPH

check the tag on your T5 and hit this site to ID your transmission
http://www.allfordmustangs.com/Detailed/349.shtml

86GT 93LX

2.73..... autos came with 3.73s maybe 3.55 but im pretty sure stock gears are 2.73 on all gt t5 cars till 93 stating from 85 but anyway i am at the same as u and i know i have 2.73s with posi so it cant be 3.73s

86 5.0L

sorry but thats wrong

I havent checked to see what rpms am at at 56mphs, but it sounds like 3.08s to me. 5-Spds came with either 2.73s or 3.08, Autos got 3.27s, doesnt matter if its LX(5.0) or GT,

EDIT:

86GT 93LX

yeah i wasnt sure, but my 93 auto has 3.73s.
i knew his couldnt be 3.73s, i dont think any t5 came with 3.73s stock??? do u know"? i was told that they did it on the a4ld 4 banger cars cus of the extra gear (4) overdrive and to help the car on take off.
i do about 1500-1600 at 60mph with 2.73s

86 5.0L

ALL 85(86?)-93 T5 equiped 5.0s came with 2.73s, 3.08s were optional

ALL 85(86?)-93 AOD equipped 5.0s came with 3.27s

86GT 93LX

yeah cus the a4ld has an extra gear compared to the aod so that makes sense so only 4 bangers with the a4ld are going to have 3.73 stock gears and their not posi anyway, so rather useless unless used with a turbo 4 motor and t5! then might be fun for a one legger,lol