Hi, I just bought a 1997 f250 4x4, ex-cab, HEAVY DUTY, 5-speed.
Ok, this is the issue. My axle code is C5. the C5 code always has been the 4.1 rear end ratio. But, my truck is turning 2500 rpm at 70 mph.
the truck has 255/85-16 tires. doing the math, (5280/8.65 x 4.56 x.76 /60 x 70 = 2472) indicates that I have a 4.56 rear end. I had a buddy drive beside me on the phone and my speedometer was accurate!
does the C5 code on a 97 HD indicate 4.56???? I'd like to know cause for just being my daily driver, 2500 at 70 is way to low, especially with diesel prices the way they are!!!
1988 F-350 srw, Crew Cab, 2WD, 7.3L, Rebuilt C6, 4.10 Rear, Manual GP Switch, No Soup Bowl. Getting a 3.08 Rear Soon.
Not sure about your code or your math (no disrespect, I make mistakes all the time), my truck has 4.10 gearing and spins just below 2500 @ 70mph every morning. On the math part, I could not follow the "/8.65". Where does that come from? I assume the overdrive is .76.
BTW, I get better mileage running 65 or under, but I can't stand to go that slow. Getting right at 18.7mpg running 70mph.
Bone stock 94 1/2 DI turbo 4x2 F250, Rancho steering stabilizer to eliminate front end shake, sensor safe clamp mod, filler neck mod
Gun Control: The ability to consistently hit what you are aiming at.
Graduate, UA, Mech. Eng.
I have a 1984 Chevy M1008 Pick-up running 4.56's and 235 85 16 tires. I can not even hit 70! The governor kicks in at about 3600 and I hit that at about 69 MPH. Your RPM sounds about right for 4.10's. I have 3.55's in my 96 and 265 75 16 tires and I run about 1800 at 70.
I have been considering tires sizes and gear options so I happen to have thrown together a little spreadsheet...
Based on 4:10 gear, the ZF-5 OD ratio of .77, and 255/85-16 turning 629 revs per mile, the theoretic rpm at 70 mph is 2,316 rpm.
Reality when compared to the figures my spreadsheet generates has shown that if I add about 3% to the claculated rpm, I come real close to what the tach shows. Your calculated 2,316 ends up at 2,385. So if you are on the low side of 2,500 on your tach, you are probably calibrated about right.
For reference, if I plug in 4.56's with your OD and add the 3% factor I come up with 2,653 rpm...?
Last thoughts, the PSOM is supposed to be adjusted when you change tire size, though you say you have confirmed your speedo's accuracy.
Finally, you could jack up the rear end and (since the "C" in C% represents a locking dif you shouldn't need to block the opposite wheel) rotate the tire one revolution and see whether the driveshaft rotates 4 time (for a 4.1) or closer to 4.5 times (for a 4.56).
97 F350 XLT, 4x4, Toreador Red, CC, PSD, MLE0, ZF5, 3.55 l/s, LUK clutch,Velvet Rides, ISSPRO gauges, v/cover vented to tailpipe, DPPI downpipe - modded to 4" right below firewall seam, open element intake using Duralite filter, coolant filter mounted between washer fluid bottle and jack, gutted EBPV, 99+ I/C installed using early 99 Y-pipe and custom pipes, 4-pos chip, BigBopper Seat Mod, Bilsteins, 130cc injectors, homegrown 4" exhaust, uses 36" Magnaflow muffler (Model #12773 body is a full 30" long - nice and quiet!) Custom Fuel System using Carter electric pump, Aeromotive regulator and frame-mounted filters...
2003 Tahoe MS27TT (27' Lite travel trailer), Prodigy Brake Controller.
1991 5.0 LX, runs 12.57 @ 109 mph
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