Gear mystery...

[mustangmontage]

Hey guys, first post here.

Name's Garrett and im having quite the time finding out what is going on in my rear end.

Car's specs;

98 GT.
T-45 pulled from a 97 cobra convertible.
Axle code (ME) "3.27 I believe".
Factory 17" wheel option.
No knowledge from original owner of gears being installed.

On the interstate, at 70 MPH, I spin 2700 RPM in 5th gear. I am not fantastic at math, but something doesn't add up here. I have searched about different possible ratio's in the T-45 but it appears the final drive is all the same. Any idea on whats happening here?

Thanks in advance guys!

 

[1bad9d5]

Welcome. In my opinion there couldn't be anyway possible your ratio is 3.27 and your running that high of a RPM at that speed in 5th. I have 3.73s and run 2700 at 80.. Have you confirmed that when it reads 70 your actually doing 70? That is all I can think of right now than again im not perfect at this either lol

 

[mustangmontage]

I have not yet, just got out of back surgery. Can't drive for another week or so. Great idea though! I will definitely try that as soon as I can.

 

[daltonduree]

410s brah.

 

[1bad9d5]

Hope you recover well man. Yea its just a thought because when I first bought my 95 gt the speedometer was off reading 10 miles off on the highway, so check it out, if indeed your doing the right speed I have to say your gears aren't stock.

 

[mustangmontage]

If the speedometer ends up being accurate, that would put me at right around a 3.90 I think. Yea, back surgery at 19 years old isn't exactly fun, but that's what football does. lol

 

[1bad9d5]

I still think they would be higher than 4.10s 2700 at 70 in fifth is pretty high man and yea I had spinal fusion surgery and got a plate in my neck when I was 19 I'm 22 now so I know the process

Sent from my EVO using Tapatalk

 

[mustangmontage]

daltonduree I think you are on the money.

 

[chris91]

Jack up the rear of your car and put it on stands. Have one person turn the rear wheel ten complete turns then while you are counting how many times the driveshaft turns.

It turns 35=3.55 41=4.11 ET

 

[MustangChris]

do what Chris91 said.

 

[RichV]

Yep, Chris91 FTW.

Before you start make a chalk line on the wheel and the pinion to make it easier. Trans in neutral and you're set. If you take the diff cover off, the pinion gear is stamped with ratio as well. But I'm not sure how easy it is to see it with the diff in place.

 

[lutter94]

Assuming both engine speed and vehicle speed are accurate, here's the math. 5th gear ratio is 0.67:1

Crankshaft speed = 2700 rev/min
Driveshaft speed = 2700 rev/min / 0.67 = 4029.8 rev/min

If you have 245/45R17 tires installed that means you have (245mm x (0.45 x 2)) / 25.4 mm/inch = 8.68" of sidewall
Add that to 17" tires = 25.68" or 2.14 feet
Circumference of the tire is pi x d = 3.14 x 2.14' = 6.723 feet/rev

70 miles/hour x 1 hour/60 min x 5280 feet/mile = 6160 feet/min
So axle speed is 6160 feet/min / 6.723 feet/rev = 916.25 rev/min

Driveshaft speed/Axle Speed = Rear End Ratio
4029.8 rev/min / 916.25 rev/min = 4.40:1

Throw a gps in the car to see how fast you are really going if you think its inaccurate......

 

[KillNThrill24]

Chris is dead on. I'm assuming 4.10 tho based on speed to rpm ratio

Sent from my SCH-I605

 

[mavisky]

My guess is that your speedo is off about 10mph and you're probably sitting on a set of 3.73's. Much more common than the 4.10's and that sounds like about what my speedo was set at before I cracked open the case and matched the gears and VSS on my T5 to the rear diff.

 

[mustangmontage]

Found out my problem.previous owner swapped tailshafts on the tranny. Meaning I have a 21 tooth speedo gear on 7 tooth drive gear. I need an 18 tooth gear to correct the problem. Thanks for all the help though guys! Appreciate it!

 

[lutter94]

So if thats the case, that would mean you are going 81.6 mph rather than 70 mph. If this is correct, I redid my math, and it comes out to 3.7698:1 gear ratio, I would chock it up to 3.73's......still better count your driveshaft rotations in comparison to your tire.....