As for the axle ratio, yes that could easily have been changed to just about any ratio available for the 10.25" axle. The ratios that I know of that are available for that axle are 3.55, 3.73, 4.10, 4.30, 4.56, 4.88 and 5.13.
Does your truck have the ZF 5-speed manual, C6 3-speed auto or E4OD 4-speed electronic overdrive auto transmission? Also, what is the tire size printed on the tire side wall?
Gear ratio, road speed and engine rpm calculations are simple for the 5-speed manual. It can be very confusing for the E4OD auto, due to so much locking and unlocking of the torque converter and shifting back and forth between 3rd and 4th gears which that transmission does frequently, at least mine does.
The 5-speed as a 1:1 4th gear and .71:1 overdrive 5th gear. The C6 has a straight 1:1 3rd gear, no overdrive. The E4OD has a 1:1 3rd gear and a .71:1 overdrive 4th gear.
Since the stock tire size for the F350DRW pickup of that year is 215/85-16, I'll use that size for the gearing/speed/rpm calculations. These tires are about 30.5" diameter.
I'll do the calculation in direct 1:1 gear, for simplicity. You can drive the truck in 1:1 gear and hold a constant speed to compare to the calculated numbers. Lock out overdrive by pushing the overdrive lock-out button on the end of the shift lever, if you have the E4OD auto.
The formula to find axle ratio from a known RPM, MPH and tire diameter, with the transmission in direct 1:1 gear and either a manual trans, or an auto trans with torque converter locked is-
(RPM x TireDiameter) / (MPH x 336) = Axle Ratio
Substitute 355 for 336 if the transmission is a C6 auto, or if the E4OD's torque converter is unlocked.
So if the truck is spinning 2500 rpm at 65 mph in direct gear with 30.5" tires with a manual trans, or auto with the converter locked, the axle ratio would be....
(2500 x 30.5) / (65 x 336) = 3.49
3.55 is the closest available gear ratio for that axle, so I would label the above example as a 3.55 axle ratio.
The formula can be turned around to solve for engine rpm instead. In this example, I'll use 4.10 as the axle ratio.
(MPH x AxleRatio x 336) / TireDiameter = RPM
(65 x 4.10 x 336) / 30.5 = 2936 rpm
So if the truck has 4.10 gears, it should be spinning 2900-3000 rpm at 65 mph in direct gear, with converter locked.
In your case, you state the truck is spinning about 2800 rpm at 70 mph, but I'm not sure what gear the tranny is in or what kind of tranny it is.
Let's say it's an E4OD auto in 4th gear with the converter locked.
(2800 x 30.5) / (70 x 336) = AxleRatio x .71
85400 / 23520 = AxleRatio x .71
3.63 / .71 = 5.11
In the above calculation, the truck appears to have 5.13 gearing in the axle.
JIMNLIN wrote: Spring packs on our older trucks had more springs but they were thinner and shorter vs newer gen truck 70" long wider/thicker springs and fewer springs.
I would drop by your local Ford truck salvage and look at a DRW rear suspension in the same era
I had a 97 f350 like the one he bought but mine had a gas 460, it had about 1/2 the springs that the one my son just bought.
That is right for a '97 F350 with stock springs. Your son's truck's springs are not stock.
To get a truly definitive answer on the axle ratio, you can pop off the rear axle's differential cover, drain the oil and count the number gear teeth on the ring gear and divide by the number of gear teeth on the pinion gear. Sometimes the ring gear is also stamped with the ratio on the face of the gear.
On a used truck, it is good to change out the axle oil when you get it anyways, since there's no telling when was the last time it was changed, or if the truck has been operated in deep water and has water contaminated oil. I would pop the cover and count the teeth, and then fill it with a good synthetic gear oil. I use Valvoline synthetic 85/145 in mine.
If the truck has a factory limited slip differential, you'll want to go to the Ford dealer and buy the little bottle of friction modifier to add to the gear oil so the limited slip clutches don't stick.
I think it has the 4:10 rearend, we are gonna change the gear oil in the rearend and all fluids and filters. It seems to run very good and it has 230,000 on it. only problem so far is someone cut all of the wires to the trailer plugs in the bed and at the reciever. but we are getting them labled.Thanks for all the info.
SoCalDesertRider wrote: To get a truly definitive answer on the axle ratio, you can pop off the rear axle's differential cover, drain the oil and count the number gear teeth on the ring gear and divide by the number of gear teeth on the pinion gear. Sometimes the ring gear is also stamped with the ratio on the face of the gear.
On a used truck, it is good to change out the axle oil when you get it anyways, since there's no telling when was the last time it was changed, or if the truck has been operated in deep water and has water contaminated oil. I would pop the cover and count the teeth, and then fill it with a good synthetic gear oil. I use Valvoline synthetic 85/145 in mine.
If the truck has a factory limited slip differential, you'll want to go to the Ford dealer and buy the little bottle of friction modifier to add to the gear oil so the limited slip clutches don't stick.
If it's an E4OD automatic (there was no C6 in 1997), it sounds almost like the trans is not going into overdrive.
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