I've got a 90 F150 302 5-speed 4x4. The sticker has the code for 3.55 gears. I have 31x10.5r15 tires. I'd don't know the final drive ratio of the transmission in 5th gear. MyF150 has the highest payload of the F150 series...shares it with the lowest F250....but I'd guess I have the mazda 5 speed...not the ZF. How do I identify?

I'm running 2000rpm in overdrive at 55mph and 2500 @ 70mph. Does this seem right? That seems aweful high for a truck. can't imagine what a 4.10 would be running.....My father in laws chevy does 1500rpm at 70mph.

If I had the transmission gear ratio I could calculate it out myself. Maybe someone swapped out the 3.55 for something else.

Thoughts

 

You can look at the rear axle tag for what orignally came in the axle. You can mark the driveshaft, turn a tire by hand, and count the rotations of the driveshaft to find out what gears are actually in it. Say the driveshaft turns 3 and 1/2 times to 1 turn of the tire...it's a 3.55...

Your truck probably has the M5OD, not a ZF, and it will have a pretty low hauling/towing capacity.

Heres a forumla for calculating the RPM...

MPH x GEAR RATIO x 336 / TIRE SIZE * OD RATIO

My truck...

70mph x my 4.10 gears x 336 / 35 * ZF's .76 = 2100

 

The overdrive ratio in a M5OD is .80:1. A ZF is I think .76:1.

If you have an F-150 with a 302, you have a M5OD unless someone swapped it.

Your RPMs actually sound low. At 70 in a truck, you should be pushing 3000 rpm. A truck is made for hauling and working, not mileage and comfort. That would explain the Chevy's behavior!

 

I turn about 2400 in overdrive at 70 in my truck (302, 235/75/15 tires). 3.55 rear and an unknown overdrive ratio. Just doing some rough calculating, I come up with around .88 or .89:1.

 

Hmm...3000 rpm? To me that sounds high...not for a truck w/o OD but for one w/ OD. The trucks I have run about 2200-2400ish around 70...

 

define limited towing/hauling...

The vin indicates it has the 6000-7000 payload..highest for F150 and lowest for F250.

Should I look into getting bigger tires?

 

I think you're confusing 'payload' with 'Gross Vehicle Weight Rating'. GVWR is not in the VIN, but on the sticker on the driver's door jamb. For an F150, it's probably somewhere around 5900 (which is about what my '90 F150 is). Payload is the GVWR minus the weight of the truck, or, the maximum amount it can carry. My F150 has about a 1500 lb payload, but it's 2wd, yours is probably a little less with that t-case underneath.

Jason

 

you are correct...I meant to use the other word.

 

Hmm, I turn about 1800RPM at 70MPH in my F-150. Gotta love the M5OD + 2.73 rear. Of course it does get 20mpg while doing it.

 

well you can go to 4lo.com they have a section in there about gear ratios you just put in the info and leave blank the one you want to know.

 

Yeah, I really should have said 2500-3000. That was a little high. My '89 does 3k at 70, though, and I consider it more of a work truck than my '91. Personal bias, I guess.

 

I have a 92 F150 with the M5OD/M5R2 and 4.10 rear axle and I am doing 2000 RPM at 55 MPH or about 2200 RPM @ 60 MPH when in fifth gear. But I have 235/75 x 15 tires on it.

And it will get 20 + MPG--if the speed is kept below 60 MPH. And it has a 5.0 V8 and 4x4!! It seems that these trucks get the best gas mileage when the engine is near it's torque peak--which in the case of the 5.0--is 2400 RPM. I just cant go 70 MPH and expect to get 20 MPH--I have to drive much slower because of the 4.10s in back.

I would suspect that you have somehing other than 3.55 gears ---probably 4.10s or something lower.

And my truck weighs 4350 with a full tank of gas. And the GVW is 6100 pounds. Hence my payload is 1750 pounds.

 

1. I now always make sure to check the speedo gear color (where it applies). for a little while I had a 3.55 axle (swap) with a 3.08 speedo-gear. (91' F150). I was wrong for assuming they had the right gear in their transmission so I just kept the gear they were using.

2. I've always had good luck using this calculator to backward calculate gear ratios. Of course use 1:1 for the transfer case ratio.
http://www.4lo.com/calc/gearratio.htm

3. Does the "spin the wheel and count rotations" method work on open diffs for other people? If I spin only one wheel, I have never got this method to work..unless BOTH wheels where manually spun at the same time. I realize it shouldn't matter, but it seems like it does on the three trucks where I've tried it. If it weren't for the fact that it works when I move both wheels I'd suspect that I didn't know what I was doing.

 

yep, the 4lo.com calculator seems to be indicating someone swapped out the factory 3.55 for a 4.11.

 

Your 89 doesn't have OD though, so that would make sense. The trucks with OD run about 500 RPM lower...give or take. A truck w/ OD won't be running 3000 rpms on the highway, unless theres an unusual setup.

The 302's tq peak is more like 3000 rpm. Make sure your speedo is accurate, and also remember that our tachs are not totally accurate.

When my F-150 had it's 302/M5 in it, otherwise the same as it is now (recently swapped motor/trans), it got a best of 16 mpg hwy. Thats with 35" tires, the 6" lift, and weighing in at 5500lbs, average weight of 5200ish empty. Its heavy for an F-150 but it did pretty decent

There are 100 gas mileage threads in this forum that one could look through to get others MPG's...

If you have an M5OD, which you probably do, it's rated for towing about 3000-3500lbs. You could tow more, but not much more. You're severely limited by the lack of granny 1st gear. You'll be burning up your clutch trying to take off, and taking off on a hill would be very interesting.

The GVWR on my F-150 I think is 6600 lbs. There is a light duty F-250, which is usually a 5.0, M5OD, semi-floating axle'd F-250, with a GVWR near that but probably a little higher. The regular "heavy duty" F-250s, running 4.9Ls or 351s, use a ZF trans and full floating axle, with GVWR around 8600 lbs.