I got a 1990 ford f 250 3/4 ton truck w/aod tranny. turning 245-80-16 tires. I don't know what the stock rear axle ratio is. I wanted to swap the gears out in order to get a little better gas mileage. Any one who might be able to tell me what the rear ratio is and maybe a good suggestion to swap the gears to? Thanks in advance for your suggestions.
If you already have an AOD, I don't see much point in going to a higher rear gear. Your RPM on the highway couldn't be too high, even if you had a 4.10 gear. Now if you had a 4.56 or something, maybe I could see it...I haven't heard of a 80 aspect tire...You sure thats not a 245-85-16?? maybe they make 80's, and I just haven't seen em...You can tell your rear gear from the VIN data, or there should be stamped data on the axle also. Maybe a tag on the rear, if a limited slip. That should tell the ratio. As a last resort, you could use a tach, and then calculate it. In calculating on the puter, if you really have that size tire, they are appx 31.4 inchs dia. Just slightly smaller than the rated 31.7 of a 235/85/16. Lets say you have a 4.10 rear, which is fairly standard on many f-250's. It's unlikely it has anything lower unless it was a special case work truck. The ratio of the AOD in OD, is .67. If you had the above, at 70 mph, you are only turning appx 2057 rpm or so. Any lower than that, and you would have trouble making it up med grades, etc. I really doubt it would help the MPG much. In fact, many say changing the gear ratios of the trucks with 300 sixes, has a minor effect overall on MPG, even if you make a pretty big change in RPM. Just torque mainly...You still are gonna have that gas pedal mashed down, low geared or not....I don't know what engine you have...If I had an OD, I would want to keep a fairly low rear gear like the 4.10 or 3.73. Get the best of both worlds. Plenty of torque in lower or your 1:1 gear, and decent MPG with the .67 OD kicked in. My 68 f-250 has a 4.10 rear. But I don't have an OD. You think you rev high....:/ I wish I had an OD...I may get one some day...But I kinda want a heavy duty 5 speed truck manual, if I'm going to ditch the T-18, which is nearly bulletproof. They ain't cheap though...MK
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