OK guys, If I set my cruise control at 60 MPH even, I am right at 1900RPM Is that normal for what I think are 3.55s? I think my tires are larger than stock but I will have to get the measurements tomorrow during the day. Something makes me want to say 265s.

1991 F250 7.3 IDI E4OD, assumed to have 3.55s according to the factory specs.

Does anyone concur or differ from my findings?

 

Shootin from the hip here, but that sounds about right.

 

to get the correct readings, you put it in 3rd gear (not o.d.) go 60mph, keep check of your rpm's then measure your tire height (exact) then check this chart

 

No sounds like you have 4.10's. my truck with 4:10's turns 1900rpm at 60 and my older 91 with 3:55's would run 70mph at 2000rpm. if your tires where larger in diameter you would turn less rpm at higher speeds. 265's are very close to the same height as 235-85-16 the stock size

 

there should be a tag on your axle houseing, if its still there and you can read it it will have the gear ratio on it.

 

265's are just the tread width. the second number is your "aspect ratio" meaning percentage of your tread width. 265/75r16 265 is the width of the tread, 75 is 75% of that tread is the height of your sidewall. you could have anywhere from 265/35 thats basically a low pro rubber band tire to a 265/85 thats a tall wall tire.

 

and if that fails to reveal, if youre ambitious you can pull the diff cover, and on the ring gear there will be 2 numbers stamped like 41-11 or something like that, divide one into the other for the ratio. One is the teeth on the ring gear, the other is the teeth on the pinion gear.

 

The tires say 235 85 R16. What number is the tire height? By my testing I am at about 2700 RPM at 60MPH when I shut off O/D.

 

235/85/16 = 31.7x9.3/16 Tire height 31.7, width 9.3 16in wheel. Going off the chart it looks like you are right between 3.91 and 4.11, my best guess 4.10's If your tires are extremely worn clear down to a 28.5 inch tire which i am not sure is possible you could be running 3.55's

 

The tires are new. Are 235 an 85 measurements in mm?

Would a worn IP/injectors or low tire pressure cause higher RPMs? There is also the possibility of my rear brakes dragging due to someone not adjusting the star wheel.

I took a picture of the tag on the differential. I will upload it when I get home.

 

My tires say 80 PSI on them, so I assume I need to put 80 PSI in them to be at max mpg and gearing. I think they have somewhere in the 50s right now. Shame on me.

 

Expanding on what Oreo said (do a search for tire size to get more info):
235/85r16
235 is the tread width in mm (millimeters)
85 is the aspect ratio as a percentage. Aspect ratio is the ratio between the tread width and the sidewall height. This means the sidewall is 85% of the tread width -> 235*.85= 200 mm sidewall height.
r16 is the rim size in inches. 16 means 16 inch rims.

Its a confusing system.

235/85r16 is almost the same height as 265/75r16, but the 265's are 30mm (just over an inch) wider treads.

Also, the 80 PSI on the tire is the MAX pressure, 50 might be ok.
You only need to go to 80 PSI if you're fully loaded. If you're running light, 80 PSI will cause the middle of the tread to bulge out more, and wear out faster than the edges. If you're running too low a pressure, the middle will push in, and the sides of the tread will wear faster.

A lot of people talk about a 'chalk' test to check tire pressure, I've never done it, but do a search. I really couldn't say what pressure you should run, and it will change with from one tire to another, and front versus rear. Also, running 80 PSI while empty will make the tires stiffer, so its going to be a little bumpier ride.

 

Here is the tag on my differential cover, assuming it is accurate.




I didn't think 80 PSI was what I wanted. I think the 50 or so may be a little too low, though. The tires have a slight bulge right now. I will investigate this chalk line idea.

 

I see 3.55

Your gearing is 3.55 according to that tag. The head of the bolt kind of hides the 3 and you can clearly see the ". 55"

 

I do agree with the tag showing 3.55 however i don't agree with the MPH and RPM numbers to be 3.55. the only reason is because i have a truck with each gearing. 3.55 at 70mph is 1900-2000 and the 4.10 at 60mph is 1900-2000rpm. The ip and injectors should not change the rpm just your mpg if they are worn