Does anyone happen to know the stock gear ratio on a 91 F250? I am swapping tires to 33" and am wondering if I should swap the gearing?

 

I believe you could have a few different ratios. Try posting the axle code from the door jam sticker and someone here should be able to decode it for you.

 

It said axle was 39? Is that what your looking for?

 

Code 39 is a 3.55 ratio with standard open differential. Ratio choices in F250 & 350 trucks of those years were 3.55 and 4.10. Differential choices were open front and rear or limited slip rear and open front.

Depending on what engine you have and what you do with your truck, you may be disappointed with 33" tires and 3.55 gears. I know I was disappointed with that combination in my F350 CrewCab 4x4 with 351 engine. I swapped to 4.56's and noticed a very significant increase in pulling power as well as an increase in gas mileage under most circumstances. With a 460 or diesel engine, you may be able to live with the 3.55 gears and 33" tires, unless you use the truck for towing. If you're towing or hauling a camper, like I do, you will want a lower (numerically higher) ratio.

 

I have a 460. With a E40D and 456 gears what is your estimate on gas mileage, and how much am I looking at the gears costing? Will I notice quiet a differnce in torque?

 

PS do you know what type of axle they came with?

 

Your '91 F250 should have a 10.25" Ford/Sterling full-float axle. Since you have a 460, you will likely be happy with 4.10's. I went to 4.56 because my engine is small and my truck is heavy, especiallly when loaded. Since you have overdrive, your mileage should be acceptable at highway speeds and with the new gearing it should likely go up some when loaded heavily. Yes, you should notice a difference in pulling power (torque) with the lower (numerically higher) ratio. May not notice it all that much when you're empty since you already have plenty of torque from that big block, but when loaded you will feel the difference. With the big block and 33's, there really isn't much need to go lower than 4.10's unless you really work the truck pulling heavy loads, or are planning to run larger tires than the 33's.

On the cost, if your truck is 4wd, you will need to change the gears on both front and rear axles, which doubles the cost of a gear change. My gear change was $1500 for both axles plus $300 for a locking differential I had put in the rear while they were changing the gears. I used Dana Spicer gears in the front Dana 60 axle and Precision Gears in the rear. These gears were not cheap, about $275 per set. There are cheaper gears out there, but I wanted the best quality my money would buy and wasn't afraid to pay for it. Superior Gears are a little cheaper and still good quality. I considered going with those in the rear axle and would have saved about $90 that way. I inquired about Richmond gears but found that they do not make a gearset for the 10.25" Ford axle. I had my gear change done at a local shop that specializes in axle and driveline work. The Ford dealer is usually more expensive than an axle shop. 4wd shops and 'speed' shops usually do gearchanges too.