Hi guys, I've got a '88 7.3 non turbo 4x4 w/C-6; this thing wails going down the highway; must have low gearing. My question:
Since we only use this truck about a total of 2-3 months of the year (but most of it on the highway), what's the best and cheapest way to decrease those RPM's on the highway? I've pondered going to skinny but the tallest tires I can fit (it's a stock 4X4 F-250) as it's about ready for a new set of shoes; can you swap front/rear end gears with the differentials in the truck and can that be done by an amateur? Last but not least, I looked into an overdrive unit, but those seem to be too expensive for our limited use; this truck is not a fancy looker, but basic, reliable transportation.
Thank you in advance for any advice!
Jim

 

I have a 1988 C6 F-350 4x4 7.3L w/3.55's to change the actual gears is relatively easy. Takes with the right equipment about an hour on the rear axle. Front axle is a little more tricky and involves pulling both front hubs and wheel assemblies. But if it is and old rig probably needs a good going through anyway. Trouble is you have a limited choice of gears to actually install and that vintage was 3.55 and 4.10 which by the way you describe is what your running. By putting 3.55's in 65mph will turn right around 2600. I ran 315's on my truck and it made a slight improvement. I guess it really makes a difference how much money you really want to spend on an old truck.

 

Could also be a locked up fan clutch.

 

Winger, I think we're turning at least 2500 rpm's at barely 55 mph; most likely we've got the 4.10's?? The 3.55's sound like a big improvement; do you have any idea how much gears cost per axle & is there an actual difference in gears for the front & rear axles? I think that I'll go for the tall tires as we need new ones anyways. Are the 315's gonna fit on a stock F-250 4x4 w/ext. cab?
Thanks,
Jim

 

Yeah my brother in laws truck was a screamer had 4.10's. We switched it out with a little elbow grease. But his was a 4x4 350 with the D-60 axle. Yours is either a D-44 TTB or a D-50 TTB. Those front drive assemblies usually we rip right out and put them in the scrapper. So, I have never taken a 44 or a 50 apart that was a TTB. The D-50 hubs will fit on a D-60 so we keep those.

Price on gears well I am not sure to be honest. I have had so many donors that price has never come about. You will need to change the ring and pinion gear they have to match. Both front and back. The pinion bearing is pressed on and the ring gear needs to be torqued. But in the grand scope a good shop manual and some patience it is not a hard job.

 

My truck also had 4.10s when I got it and I swapped to 3.08s. The gear change was not hard at all and I had no prior experience with R&P swaps. I got my set new off the web (sterlings are not avalable from factory with 3.08s) and it cost about $USD225 and worked out to $CAD400 delivered to my door. That was almost 40k miles ago, so it has definately paid for itself by now.

A gear change is the best bang for your buck IMO.

 

Hey David, did have a 4x4? It sounds like from the price and your description that just the rear was changed?? From previous posts, it sounds like the front axle it tougher to deal with; does the 3.08 bog the truck down? We don't have the turbo model, so with only 170 or so hp, I'm wondering if that gearing would work ok for our truck? BTW, I think Winger said he runs 315's tires, would those fit on a stock F250 4x4?
Thanks guys for your help,
Jim

 

My truck is not a 4x4, but I am hoping to convert it some day. So the short answer is yes, my swap was only on the rear. I'd be BSing you if I told you I knew exactly what is invoved in swapping the front gears, but i think its not too much more trouble than the rear.

The 3.08 gears made the truck handle much better for my purposes, and the only time I noticed a lack of rear axle torque is at speeds below 10 mph and with a medium sized trailer. But since I have an auto trans, and the torque converter is able to make up for most of the difference. I would not recomend swapping to 3.08 gears if you plan on towing more than 3500lbs consistently, because starting from a standstill can be troubling if you are going up a steep hill (but it will still do it).
After 20 mph, the truck is limited by only its Hp, and it is a pleasure to hold the fast lane with it. Top speed is a little over 90 mph, and about 90% of my driving happens at or above 70mph, so 3.08s were a good match for me. As for accelleration, the 6.9 has so much bottom end torque, that I can still hold pace with many newer trucks on the road, and oddly enough, accelleration is best below 40mph. If the truck is empty I can't say I lost much in the way of 0-60 performance.

I should warn you that the dana 50 and 60 (both reverse types) are not avalable in a 3.08/3.07 ratio even through aftermarket, but the d44 (I'm not sure what your truck has) is. But having said all that, I dont think you have to go all the way to 3.08s, 3.54s will already make a big difference, and those can be found used for less than what I paid for my gears.

If you had a manual tranny I would have suggested a ZF swap, but since you have a C6 it looks like you and I are in the same boat. I'll be putting an E4OD in mine shortly, but thats far from an affordable proposition.

 

I run 285/75-16 tires on my 4x4, I can get them to rub the front springs in a hard turn off road.
315 tires will rub the front springs when you turn the wheels all the way to the lock.

 

Hey guys, what about running 315's but staying skinny on the width? Will those still interfere with springs, etc?
I guess I'll have to figure out what kind of axles we're running when I get back to Alaska (where the truck is) to find out my gear ratio replacement options, but since the truck needs shoes, I'm gonna get the tallest tires I can on there without having to lift it or anything; BTW; is there a cheap way to lift these trucks about 2" to allow tall tires? The radical gearing in this truck keeps us from taking it on long road trips to Anchorage, Homer, etc.
Thanks,
Jim

 

yes it could by increasing the length across th diameter

 

I have done many gear changes on 8.8 's and they are very easy. The rear Diff. in your 250 if similar. The price on gears will vary greatly but if you do it go with ford gears the difference in money will be worth it. Especially if you tow anything. You will probly be able to get away with only having to buy a .001 dial caliper and a flex mount. If you go with cheaper gears you will want to check the pinion depth which requires another tool! Also 315 is the width of the tire (315mm). If you want to decrease your rpms with a tire then you need to increase the hieght or overall diameter. Example, if you have 265/60 tire you would want to go to a 265/75. This will decrease the rpm. If you have a 75 tire wall now you will not be able to go any larger.

 

285/75-16 tires decode like this

285 MM tread width
75 is the aspect ratio which means the sidewall height from the tread to the wheel is 75% of the tread width
16" is the wheel diameter

So,
There are 25.4 MM in one inch
So a 285 tire is about 11.2" wide. (285/25.4=11.22)
The sidewall is 11.2 * .75 = 8.4" tall
So (8.4 * 2) + 16 = 32.8" tall tire

A 235/85-16 tire works out like this.
235/25.4= 9.25"
9.25*.85=7.8
So (7.8*2)+16=31.6" tall

A 265/75-16 tire is almost exactly the same diameter as a 235/85-16 tire, but the tread is wider.

Probably the best tire size to increase milage would be a 255/85-16 tire if you can find them.
The only manufacturer I can get them from here is Toyo, and they are pricey.
I don't think the fuel savings would be enough to justify the tire cost for a set of four tires.

A 315 tire would be 12.4" wide

 

Hey guys, thanks for the tire tips! I had my math/configuring wrong. Sounds like a gear change makes the most sense & cost effective compared to anything else.
Thanks,
Jim

 

Gear Venders . 94 F-350 7.3 IDI Turbo with 35X12.5X16.5R 70mph = 1600rpm