Someone told me to look at the metal tag on the front differential and it would tell me the gear ratio in my truck. I know the rear end was rebuilt but the shop couldn't tell me what was in it. They didn't write it down so they couldn't obtain that information. Since they didn't touch the front, someone said the tag on it should tell me.

I cleaned off the only metal tag I could find and it showed me this.

ED D7TA ATA
603808 5 4 09

Does anyone know what these mean?

My truck is a '77 F150 4x4.

 

The axle was made in 1976, is a Dana 44 ratio and I suspect yours has a 4.09 gear ratio. Axle codes are also found on the door tag.

 

The front axle will always be exactly the same or a fraction different than the rear.

ie: Rear 4.10:1
Front 4.09:1

 

The reason I'm asking is because we are thinking of having them changed because the 400 engine is streaming at 55 MPH. We lost our first engine after driving it to Yellowstone at 70 MPH on the freeway in 95+ degree weather with the air conditioner on. The drive from Olympia, Washington to the border of Idaho (6 hour drive) the engine was so hot that the engine would not idle and we lost all oil pressure at a idle. The truck was eating oil like a fruit drink and the engine only had 105,000 miles on it.

Everyone that test drives this truck is shocked at the revs the engine is doing at anything over 40 MPH.

Would going with a 16" rim and bigger tires help?

 

Yes. But not a whole lot.

I have a '74 F250 highboy with a 4.09 front Dana 44 and a 4.10 rear Dana 60.

With 33" tires, it still revs high at highway speeds, so you'll need some big tires to get the revs down.

But that's no reason a 70MPH trip should have ruined the motor, even for 6 hours.

You might want to look at an overdrive tranny, or swap the gears to something higher, like a 3.73 or whatever can be fitted in there, but you'll have to change both front and rear, or never use 4x4.

 

The # on your tag 603808-5 is a Dana Bill of Materials number. If the differential hasn't been changed, any supplier of Dana rearend parts should be able to cross reference this. Also be advised, if you order any parts for this axle, they, your supplier, will need this #. If you can't get or haven't already gotten info you requested, write me back and I'll look it up at work.

 

I believe the calculation for RPM's is something like this:

RPM= ((Miles Per Hour)*(Final Drive Ratio)*(Tire Revolutions Per Mile))/60

You can get revolutions per mile from the tire manufacturer's web site, final drive ratio is your top gear transmission ratio * your axle ratio.

So for my '74 highboy with an NP435 running 4.10's and 245/75-16's (aprox 31" tall) at 65 MPH the calculation looks like this:

RPM=((65)*(4.10)*(680))/60

RPM=181220/60

RPM=3020

The 360 in my truck will run all day at this speed so long as you keep it cool and feed it enough fuel. Engine life (longterm) and fuel economy obviously suffer, that is the benefit of an overdrive transmission.

Anyway, hopefully this will help you calculate the effects of larger tires and/or an overdrive on lowering your highway RPMs. I'm with krewat, however, in that prolonged cruising at 70 mph should not have destroyed an otherwise healthy motor (though it might destroy your hearing!).

If you have an automatic, you may want to check it for slippage, & proper shifting, and I would check the cooling system and look for other potential problem areas.

 

Here's a little bit easier formula to use for calculating RPM (you dont have to know the tire's revs/mile, just the tire diameter in inches, it's not quite as accurate, but it will get you in the ballpark.

RPM = (MPH X GearRatio X 336) / TireDiameter (in inches).

Add 200-300 rpms for torque converter slip if you have an auto tranny without a lock-up torque converter. With a 400 engine, the auto trans would likely be a C6, so no lock-up converter in that one. This formula assumes the transmission is in 3rd, or direct 1:1 gear. If you want to determine your engine rpms in different transmission geers, multiply your axle gear ratio by your transmission gear ratio and substitute that number for GearRatio. The C6 ratios are: 2.47 (First), 1.55 (Second), 1.00 (Third)

You can plug in different values for axle ratio or tire size into the formula to see the effect of different size tires or axle gears on engine rpm's. With a non-overdrive trans and a gas engine, you want your engine rpm's around 2500-2800 at the speed you want to cruise at. That provides a good balance between power and mileage.

If you have metric size tires, here is a formula for converting the metric size into inches:

TireDiameter(in inches) = ((2 X SectionWidth X AspectRatio) / 2540) + WheelDiameter

Metric tire size example -- LT235/85-R16, where 235 is the SectionWidth (in millimeters), 85 is the AspectRatio (a percentile), 16 is the wheel diameter (in inches), R means Radial, LT means LightTruck. To find metric tire width in inches, divide the SectionWidth by 25.4. There are ~25.4 millimeters per inch.

I agree with the others that running 70mph with 4.10 gears should not have blown up your motor, although it is not the best thing to do for it. By your description it sounds like your engine was using oil (which was surely aggravated by the high engine rpms), and ran low on oil, causing poor lubrication, overheating and, over such a long time, finally mechanical failure. I imagine it probably got low on water too with all the overheating.

I run 4.56 gears with 33" tires and no overdrive (C6 trans) and can run 70 but don't do it much because it sucks up too much gas. I keep the speed below 60 and everything is fine.