I just bought a `92 F250 which I was told had the 4.10 gears. It has a 460 with a C6 tranny and turns 2500 RPMs at 60 mph. Tire size is 235x85x16, or, 31.7" tall. I read the axle code today on the door jamb, and it is 39, which indicates 3.55 gears. According to the ratio calculator at Randy's Ring and Pinion web site, it has a 3.73 ratio. I don't know what to believe. I was told it had the 4.10s by the dealer that sold it to me, which wouldn't surprise me if they lied. But, I can't figure out why the door tag would be wrong.

 

>I just bought a `92 F250 which I was told had the 4.10
>gears. It has a 460 with a C6 tranny and turns 2500 RPMs at
>60 mph. Tire size is 235x85x16, or, 31.7" tall. I read the
>axle code today on the door jamb, and it is 39, which
>indicates 3.55 gears. According to the ratio calculator at
>Randy's Ring and Pinion web site, it has a 3.73 ratio. I
>don't know what to believe. I was told it had the 4.10s by
>the dealer that sold it to me, which wouldn't surprise me if
>they lied. But, I can't figure out why the door tag would
>be wrong.



Are you sure your tach is accurate? the tach could be off enough to make the 3.55 look like 3.73.....

 

I'm going off of the factory tach. I have a Supermeter tach I can install to check things out.

 

A couple points:

Your tach is almost certainly accurate. There really isn't a whole lot to go wrong with it.

Did you *measure* the diameter of your tires? If you calculated it using the tire size, I'll guarantee it's wrong. Between tread wear and the weight of the truck, you could be off by two or three inches. Measure it this way: Park the truck on a hard surface like concrete or asphalt. Measure the distance from the ground to the center of the hub. Double it and you have your diameter.

I made the same mistake a few weeks ago, and I thought I had 3.55's in the rear! As soon as I adjusted the tire size in the math, they looked a lot more like 3.08's.

 

I checked the axle tag and it indicates 3.55 gears, I think.
tag starts off reading 55 and then some more numbers that didn't make sense, I'll have to check those tonight.

 

I bought a truck with 33x12.50x15 BFG tires on 12x15 rims. Worn down, the tires on the truck measure barely over 30 inches tall. Go figure.

 

My vote is for the tach being off. I once checked a Sun tach with test equipment. It was close at 5000 rpm but not at lower rpm's. I don't remember the exact value, but may have had an error of 500 rpm at 2500.

 

>I bought a truck with 33x12.50x15 BFG tires on 12x15 rims.
>Worn down, the tires on the truck measure barely over 30
>inches tall. Go figure.

Tires that are marked in inches are rarely the size that the sidewall would indicate.

Most brand new 33's measure only 32 something and it also epends on what size rim it is on.

Brian
:-)

 

I measured my tires and they are only 30" tall. I also found out that my speedo was off by 3 mph. So, at 60 mph(actual as timed by a watch and using mile markers), my motor is turning just below 2600 rpms, which indicates 3.73 gears. I have no idea why the speedo is off, and why my tires are so much shorter than what they should be.

 

You know what you can do, is put the rear axle on stands. Put a chalk mark on the driveshaft next to the axle, right before the u-joint. Then, turn a tire so it rotates the axle. Turn the tire 1 revolution, and the driveshaft will turn 3.55 times, 3.73 times, or 4.10 times. If if turns 3 and 1/2 times, obviously it's a 3.55. If it turns over 4 times, it's a 4.10. If it turns in the middle, it's a 3.73.

Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster American Thunder Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.

Then theres:
99' Mustang GT 4.6L
88' F-250 Heavyduty 4x4 351/c6
95' Mercury Cougar 4.6L V-8
80' E-350 300/6 with a
3 spd column shifter. Weighs around 7,000lbs w. 65mph top speed, who wants to race me!

 

And don't forget that a C6 torque convertor is non-lockup and so doesn't quite turn at 1:1.

 

The for sure method would be to pull the cover and check. The ratio to tire charts are not always accurate. Also as others have stated most tires are not their true size and the torque converter does have an effect on true R.P.M. and final ratio.