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Discussion Starter · #1 ·
i found out my speedo was off by about 11 mph at 75, it reads 86 according to the speedo. any idea what gear that would be? i would assume it's 3.73's since its not really far off, the tires on it are a little smaller but only a tenth of an inch so that wouldnt cause this big of a jump.
 

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86/75 = 1.1467. 3.73/3.27 = 1.1407. On the surface it looks like you may have 3.73s. But, there have been several people on the forum who have thought that (including myself) that later found out the speedometer was wrong.

The only way to make sure is to chock the front tires, put the car in neutral, jack the rear wheels off the ground, place a mark on the driveshaft and a mark on the wheel, turn the wheel one complete turn and count the number of times the driveshaft goes around. 3 1/4 times is stock 3.27 gears, 3 1/2 times is 3.55 gears, 3 3/4 times is 3.73 gears and a little over 4 times is 4.10 gears.
 

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There are also little tags that you can put on when you put the differential cover back on- stock the car comes with some, and although I've never seen aftermarket ones, you may want to just take a peek back there and see whats up.
 

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86/75 = 1.1467. 3.73/3.27 = 1.1407. On the surface it looks like you may have 3.73s. But, there have been several people on the forum who have thought that (including myself) that later found out the speedometer was wrong.

The only way to make sure is to chock the front tires, put the car in neutral, jack the rear wheels off the ground, place a mark on the driveshaft and a mark on the wheel, turn the wheel one complete turn and count the number of times the driveshaft goes around. 3 1/4 times is stock 3.27 gears, 3 1/2 times is 3.55 gears, 3 3/4 times is 3.73 gears and a little over 4 times is 4.10 gears.
+1
 

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Discussion Starter · #5 ·
cool thanks, ill try that when i get a chance. the car is quicker than a stock gt, one was messing with me the other day so that is why i assumed. ill check and make sure though
 

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If you have a GPS that will tell you your actual speed, find an open stretch of road where you can stay at a constant RPM/Speed in 4th gear and see what your speed is at a certain RPM (say 2000).

Then use this calculator

Rear-Axle Gear Calculator

This is what I did to find out that I had 3.73 and not just a wack speedo. Though, when I do my brakes soon, I will verify using the driveshaft method.
 

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If you have a GPS that will tell you your actual speed, find an open stretch of road where you can stay at a constant RPM/Speed in 4th gear and see what your speed is at a certain RPM (say 2000).
Just keep in mind that that only works if your tach is correct. If someone has installed aftermarket gauge faces, the needle may not have been reinstalled correctly.

The easiest method to know for absolute certain what gears you have is the method laid out by Eagle2000GT above.
 

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Discussion Starter · #8 ·
according to my gps at 40 i have 3.73's, gauges are stock so tach should be correct
 

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If your gauges are stock and you are running stock size tires then at 40 mph in four with 3.73s your tach should read approximately 2000 rpms. In second gear at 40 mph it would be around 4000 rpms.

I could be that the previous owner used a hand held to correct the speedometer and then restored the stock tune when he sold the car.
 

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Discussion Starter · #10 ·
well i bought the car from a friend of mine, he happens to be "car dumb" so if that was the case it was the guy before him who did it.
 

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Do any of you guys know what my RPM's should be with stock gears at 70 mph with an automatic? My Mustang is about 200 RPM's higher than my F-150 that has the same gears. I just assumed it was because the tire sizes are different.....:dunno
 

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Using the link below for an AR70W and 3.27s: 2000 rpm x 70/64.8 = 2160 rpms. If you have 3.73 gears: 2000 x 70/56.8 = 2465 rpms.

PaladinMicro

You can use the tire size calculator below to get the diameter and revolutions per mile for any tire size. When you put the tires size in the table above multiply by .97 to change the tire diameter from unloaded to loaded diameter (see footnote 1).

http://www.1010tires.com/tiresizecalculator.asp
 

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Using the link below for an AR70W and 3.27s: 2000 rpm x 70/64.8 = 2160 rpms. If you have 3.73 gears: 2000 x 70/56.8 = 2465 rpms.

PaladinMicro

You can use the tire size calculator below to get the diameter and revolutions per mile for any tire size. When you put the tires size in the table above multiply by .97 to change the tire diameter from unloaded to loaded diameter (see footnote 1).

Tire Size Calculator - tire & wheel plus sizing


Yep...seems like the Mustang is at about 2100 and the F-150 is a tad below 2000...just a snitch though. Thanks a lot for the info. Didn't mean to butt into the thread....:)
 

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If you feel like taking the differential cover off you can see "3.27" or "3.55" or "3.73" or "4.10" stamped on the pinion in front of the ring gear. That way you know for sure what you have. If you have something other than Ford gears I'm not sure if the ratio will be stamped on it or not.
 
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