Speedo correction

[grrman13b]

I've got a 91 LX 5.0, automatic. I did the rear axle test where you mark the tire and the drive shaft and count the revolutions to get gear size. I've calculated I have 3.73's. Now... I've also put in a 3.73 speedo correction gear in the trany with no luck of a good speedo reading. The way I see it, it's off by around 10 mph or so. I bought the correction gear about a year ago, I'm pretty sure it's a 21 tooth purple gear. It had a orange gear in it to begin with with still the same results. Someone please tell me an accurate size to fix this problem!

 

[blown5.0]

1983-98 auto and 89 and under 5 speeds use a 7 tooth drive gear so the correct 3.73 speedo gear is a 21 tooth red gear.

90 and up 5 speeds stepped up to the 8 tooth drive gear and need the 23 tooth white speedo gear for 3.73's

so you have the correct speedo gear. are you sure you have 3.73's in it? you are correct about the wheel and driveshaft counting. I'd do it again and make sure both rear tires are off the ground and the ebrake is off, then mark both, and hand spin the drive shaft counting the turns until the right rear tire has gone one rev, be carefull as if the 2 rear tires dont both turn the same way and same revs the diff inside my slip and give a false reading. another thing to check is pull out the speedo assembly and look at the drive gear inside the tranny and make sure its the yellow 7 tooth. you may have a different year tranny in it with a different drive gear. also what size tires do you have, that will effect the speedo a little too.

 

[trbkrb]

Blown5.0, where did you get your info? Everything I've seen says both the auto and 5 speeds have the 8 tooth gear for 90 up model stock transmissions. See Steeda's website:

http://www.steeda.com/store/-catalog/speedometer.htm

For the 3.73 gears, the 21 tooth purple gear is correct for the 7 tooth 89 and earlier trannys. The closest gear for the 8 tooth trannys is the 23 tooth white gear, but it will still be off. The 23 tooth is really closest for the 3.55 gears, but it is the largest one made.

Since you are only 10 MPH off, I wonder if you really have 3.55 gears, as it would seem like you would be off by more if you really have 3.73 gears. The orange gear is stock with 3.08 gears and has 20 teeth, the purple has 21 for 3.27 gears, so that is not much of a difference.

And tire size might have some affect on the speedo, but usually only 1-2 MPH at the max on a Mustang since the total tire diameter does not change by much at all. See
this link for tire size calculations:

http://www.miata.net/garage/tirecalc.html

Bottom line---- try the 23 tooth white gear and see how close it gets. Since the 21 tooth is still that much off, that makes me believe you have the 8 tooth drive gear.

Good luck!

 

[blown5.0]

got my info from the texas mustang parts catolog.

even the steeda site says 90-95 oem t-5 have the 8 tooth drive gear for the 5speeds.

basically look in the tranny to see which color drive gear you got. if your speedo reads high by 10mph at 55mph with a 21 tooth then a 23 tooth would lower the speedo reading.

 

[trbkrb]

1983-98 auto and 89 and under 5 speeds use a 7 tooth drive gear so the correct 3.73 speedo gear is a 21 tooth red gear.

You are right.... it must be a typo in your first post I quoted. Just wanted to make sure I was giving the right info!

Thomas

 

[Michael Yount]

More info:

http://www.geocities.com/mgnedell/notchback/tech/speedo.html

I've got a lot of experience with mine on correcting speedos without benefit of knowing what's supposed to work - as the 5.0L/T-5 never came in my car in the first place. If tire sizes change, that adds another variable to the mix. It's not complex, but you do have to pay attention to what you're doing. You need accurate measurements of what your speedo is reading, and what your actual speed is -- using Interstate mile markers seems to be the best way. Whatever the ratio (% difference) of the speeds is - that's the same percentage that you need to adjust your speedo gear ratio by. More teeth on the drive gear (6 to 7) speeds up the speedo. More teeth on the driven gear (18 to 19) slows the speedo down. If you have to change the drive gear, on the T-5 - the tranny has to come out. But it's pretty easy to swap once you've got the tranny on the ground.

Hopefully you'll find the info you need in the site above, but if it's still a mystery, pm me and I'll do my best to walk through it with you. My speedo was a full 25% fast - it read 75 mph when I was going 60. The driven/drive gear were 7 tooth/18 tooth - which is a .389 ratio. Since I needed to slow the speedo down, I knew that I needed less drive gear teeth or more driven gear teeth or both. So, .389/1.25= .3111 was the new ratio I'd need. 6/19 = 3.16 which was pretty close. Upon installing the 6/19 my speedo clicked off 49.4 miles on the odometer over 50 miles by the Interstate mile markers - which means it was accurate to about 1.2%. Based on timing intervals between mile markers while the cruise was set on 60 mph, the speedo was similarly accurate.