Do I have gears?

[slomach03]

This is a weird question. And if it has been covered in any other threads already, I'm sorry. I purchased my automatic 03 Mach from a small dealership who did not know of any previous mods done by the previous owner aside from a Flowmaster catback. When I went to a JLT RAI I also found out there was a K&N drop in filter. On with the story. When I go on the high way or interstate my gas mileage drops off drastically much more so then in town driving. Also my RPMs seem on the high side for just 75-80mph cruising, usually north of 2800RPMs even in OD. This is my first mustang and my first experience with the 3:55s. Is what I'm describing perfectly normal or is it possible I have steeper gears and the dealership didn't know? Again sorry if this has been covered previously. Thanks for the help.

[Vndcatr]

i cant speak for an Auto but with my 4.30s at about 80mph in 5th gear i turn about 2800+ rpms with a mach 3650...

[311-420]

2800rpm at 75mph? That doesn't sound like 3.55s..should be about 2000rpm at 70mph if theyre stock. If your speedometer is calibrated correctly, it sounds more like 4.30-4.56 to me. Someone with a geared auto will chime in.

[ab_mach1]

Sure seems like they aren't the stock gears based on the RPMs you mentioned.

[slomach03]

Thanks for the input guys. I've had suspicions about them for a while. I guess there's no way to know for sure aside from how high the cruise RPMs are

[Vndcatr]

Originally Posted by slomach03

Thanks for the input guys. I've had suspicions about them for a while. I guess there's no way to know for sure aside from how high the cruise RPMs are

there is a way, i cant remember.. like count the tire revolutions or something ? some old school way

[311-420]

Originally Posted by Vndcatr

there is a way, i cant remember.. like count the tire revolutions or something ? some old school way

Jack the rear of the car up, rotate a wheel and count how many times the driveshaft rotates per revolution of the wheel

[slomach03]

Sounds like a plan. How many revolutions am I looking for for stock or otherwise? Thanks

[311-420]

Originally Posted by slomach03

Sounds like a plan. How many revolutions am I looking for for stock or otherwise? Thanks

3.55 would be roughly 3 1/2 rotations

[slomach03]

Sounds good. I'll take a look and see what I find.

[sutyak]

Play around with the Tremec gear calculator: http://tremec.com/calculadora.php
A stock 245 tire is about 25.7" tall. Gear ratios (I believe) are:
2.84
1.55
1.00
0.70 (OD)

Given what you say, I'd lean towards it having 4.10's.

Of course, this calculator is literal, so if your computer hasn't been updated to the proper gear ratio, then the mph would be wrong. Might need a GPS app on your phone to verify the mph.

[Mach828]

These guys got it, but I've always used the gear/rpm calculator on drag times. Sounds like you have 4.10s if you are on stock tires at 75 hitting 2,800rpms.

http://www.dragtimes.com/gear-ratio-...tton=Calculate

[slomach03]

Forgot to mention I do have 255/40 tires vs the stock 245/45

[slomach03]

Went ahead and spun the rear tire. I first chocked the front tires and put the car in neutral then jacked up the rear, put a mark on the drivers side rear tire and the floor then spun it and watched the drive shaft. The driveshaft completed two full turns and then a little more and then the wheel completed a full turn. So basically I got a little over two full turns of the driveshaft. This seems weird. Any guesses?

[311-420]

Maybe your diff clutches are worn and the other wheel isn't turning at the same speed. Try to get a buddy to turn one wheel while you turn the other and see if that works.

[4a7191a]

If it's 410's or 390's I'd for sure keep them. Gears make these cars so much better.

[SLO-RIDE]

The reply saying to rotate the rear tires and count the driveshaft revolutions works, or if you are not sure if you want to change and go to another ratio check out this website below. Pick the trans. you have, tire size, gears, and click.......you have the RPM at any given speed. There is a lot of good info on the site..... Hope it helps.

http://www.paladinmicro.com/pmicro.p...nGearCalc.htm#

[slomach03]

I'll get someone to help me with the rotations and if I do have higher gears I'll be keeping them for sure. Thanks for all the help guys

[slomach03]

Update: I downloaded two different speedometer apps and checked them against my cars speedo. They both held accurate within +/- 1mph. I do have 255/40 tires which according to American Muscle would mean my speedo uncorrected is off 2.5% so at 60mph I would be doing 58.5 and at 80 I would be doing 78mph. I went out on the highway and checked it out. On my speedo at 80mph (which was the same as the apps on my phone) I was turning 2700rpms with cruise control on in overdrive. If those tires are uncorrected then I would likely be doing 2800 at 80mph. What do you guys think?

[Mach828]

Sounds more like 3.73s then

http://www.dragtimes.com/gear-ratio-...tton=Calculate

[slomach03]

Originally Posted by Mach828

Sounds more like 3.73s then

http://www.dragtimes.com/gear-ratio-...tton=Calculate

Exactly what I was thinking. Too high for stock and a little too low for 4.10s so it's 3.73 or a rare 3.90

[03Shadow Grey]

Still strange the tire/drive shaft rotating did not pan out.

[slomach03]

Originally Posted by 03Shadow Grey

Still strange the tire/drive shaft rotating did not pan out.

That puzzles me too I literally got 2.25 turns out of it but I plan to try it again once the weather clears up

[311-420]

We had the information here all along. I knew there was a thread with it...

Post #7 http://www.mach1registry.org/forums/...ear+mph+thread

[slomach03]

Yeah looks like it. I'm gonna say I have 3.73 gears and call it a day