I have a 2003 F250 4x4 extended cab with short bed and six speed manual transmission. I don't do any towing and drive a lot on the highway and was wondering if there is a different 5th gear that can be purchased to lower the RPM's about 500 from the existing 5th gear. Mike

 

Looking at this problem realistically, if you are worried about a few dollars of fuel cost, then changing gears in the transmission would probably be too costly. The OD gear in the S6-650 is already 0.72:1 so you're in HD automatic transmission territory anyway.

Since the mainshaft is at a set distance from the countergear, just replacing the OD gear with one of another diameter isn't an option. You would also have to replace the OD countergear. The lowest ratio that I could find in replacement part catalogs is the 0.72:1 ratio gear that you already have so a custom gear set would have to be made for you. Now for a custom design, and casting (carburized steel) for the new countergear and OD gear and the labor involved, you'd be looking at a lot of money. Probably around $3000 at least.

The second solution is a front and rear axle gear change. It is the cheapest but the worst solution. That would cost around $1200. With 33" tires assuming you have a 4.10 rear axle ratio, in OD gear at 60 MPH, you'd be at 1860 RPM. [(MPH/60) x 4.10 x 0.72 x 630 Rev per Mile = Engine RPM]
Changing both front and rear axle ratios to:
3.73 would drop you 168 rpm to 1692 rpm
3.55 would drop you 250 rpm to 1610 rpm
3.08 would drop you 462 rpm to 1397 rpm
2.73, which isn't even available on a 10.5" axle, would drop you 622 rpm to 1238 rpm.
Plus you have to consider that anything below 3.73 will make your truck a real dog to drive. You'll have a terrible acceleration.

The third solution is the overdrive unit. That would do it and do it well enough but at a cost of $2800 plus about $600 installation for a $3400 total cost.

The fourth solution is to purchase a lighter and smaller vehicle. Again, not cost effective and it comes with a huge penalty of not being able to drive what you really want.

What's the difference between a Ranger and an F250 as a yearly fuel cost difference? According to government figures, a 2003 4wd 4.0L automatic Ford Ranger costs $1693 in fuel per year. I'll estimate fuel cost for your truck (F250 4WD, 5.4L 6-speed, 4.10) at around $2168. (Using the same gov't 15,000 miles at 45% city, 55% Hwy at $1.92 per gallon formula used on the Ranger and your truck getting 11 city and 16 hwy which is what I got on my 1999 F250 4WD 5-speed) That's a $475 difference.

Re-gearing your transmission would probably cost around $3000 dollars so it would take you 6.32 years to recupe the loss.

Changing gears to 3.08 would cost around $1200 so it would take you 2.53 years to recupe the cost plus you'd be driving a giant and tepid pile of dung around town.

Installing an add-on overdrive unit would cost $3400 and would take 7.2 years to recuperate.

We won't even go into the loss incurred by trading vehicles. You almost always loose.
The best solution is to keep driving what you have. Come on, it's not that bad! It's better than a lot of vehicles out there. The Saturn V rocket got about 5 inches to the gallon and look at all the trips they used it on. (And it wasn't even 4WD and only half as stylish and comfortable as an F250.)

 

Damn, that was a GREAT post!!

 

Made me smile too. I like the mileage of the Saturn V. 5 inches to the gallon! Is that at liftoff or travelling 15000 mph? LOL.

 

Great post man!

 

Good job remember it is never a good job to chace money with money!

 

How about just going larger diameter tires next time you need new tires? 35" tires should let you run about 6% lower rpm - and if you're buying new tires anyway it won't cost much. And if you don't like the performance you can switch back at the next tire change. Would 35" tires require a lift for clearance?

 

I have to ask the obvious question... Does 6th gear work?

 

Those are all good posts. The reason I asked about the 5th gear swapout is I had an "84 Volkswagon GTI and swapped 5th gears out to get lower RPM's. The gear was an aftermarket item and didn't cost that much, I thought maybe someone (or even Ford)made a simular gear for highway crusing. The gear the F-250 has in it right now is obviously made for heavy hauling. I've thought about larger diameter tires too, I sure there's a tradeoff in weight and rolling resistance you have to factor in.