Can anyone tell me what I might see for a fuel mileage change when I swap in 4.10 gears into my '92 F150 I-6 automatic?

I am currently running 3.08 gears, according to the door sticker, and I have 31x10.5x15 tires.

Currently I get 14 mpg city/ 17mpg hwy.

 

When I had the 3.08s in my 300, I got 19 highway/ 14 around town. I put in 3.73s and changed from 29" to 30" tires and now I get 16 everywhere. 4.10 is a good rear end for a V-8 powered truck, but IMHO it's too big a gear for a 300. You're going to be pulling about 2160 RPM @ 65 in OD with an automatic and over 2450 with a manual tranny. Since your engine's torque peak is at or below 2000, that really seems excessive. Go for the 3.73s instead, they'll work perfectly with those tires.

 

I would have to agree, StrangerRanger's recommendation is spot-on.

 

I would think more like around 9-10mpg...ugh..I couldnt afford it out here in CA!

 

OK,

Now I am the one lost about this.
I was thinking the lower number 3:08) gear gave you better fuel economy & a gutless truck and the higher numbers (4:10) gave you crappy mileage but a truck that is more stout.

OK, so tell me what would happen with my truck.

I currently have the 302 with AOD running 3:08
I have thought about putting a trac-loc in with a 3:55 gear.

 

Thanks guys for the input on 3.73's. That is the way I'll go then. Nice to hear I might actually improve mileage.

Parks911- it has to do with not lugging the engine I believe. There is alot more to it I am sure.

 

The thinking is this: with a small engine geared too high (numerically low) it has to spend so much time at wide open throttle to get the truck moving that you end up sucking up alot of gas. A bigger engine, or a lower gear, will mean the engine isn't working as hard and ends up getting better mileage, and more power too. Basically what you want to do is match the engine size and gearing to the wieght of the truck. A healthy mis-match in iether direction can send fuel mileage down the drain.

That is what happened when I drove my 6500 lb crew cab with only a 351 and 3.55 gears with 33" tires (effective ratio about 3.30). Around town the thing would guzzle gas because I would have to floor it to get it moving from a dead stop if there was any kind of hill. Out on the highway, as long as the road was flat, it got great mileage, but pulling a hill it was pedal on the floor, driving in the right lane, watching the gas gauge creap left. This was even more so when loaded. Now with 4.56 gears I get better mileage and more power, as long as I don't drive too fast (no OD).

 

What would you recommend for new gear in my truck?

It is a 88 F-150 with a stock 302 AOD 4X4.

Right now it has the 3:08 in the rear. At least that is what the tags states.
I'm also going to put a locker in the rear end when & if I change gears.

I want to get normal MPG & not to lose any power.

 

What size tires are you running?

 

you quick!!

I have the stock 15" tires.

Not for sure of the actual dimension. I don't have access to the truck right now to look.

 

I'm guessing stock tires are about 29 inches tall then... I would think 3.55 would get you a little more power and mileage, since you have overdrive. If you're towing you could go with 3.73's.

 

3:55 was my original guess but the rear ends and transmisions are all greek to me.

My camper only gets towed twice a year. 70 miles to set it up & then 70 miles to get it back home in the fall. But that really shouldn't matter.

 

Yeah I wouldn't go any higher (numerically lower) than 3.55. If you do alot of offroading and not much fast freeway driving, 3.73 would work there too. I mention that because you said you wanted a locking diff. By the way, I put a LockRight in the back axle of the F350 and I love it. It's a whole different truck now between the locker and the low gears.

 

I'm looking at the powertrax locker that Summit has. My off-roading time is going to be limited. Probally just to get to some of my old camping spots out in the middle of nowhere.

The last time I went serious off-roading had some expensive results that I'm working on now. Pics are in my gallery.

This is what they are advertising....

 

Wow, yeah I remember you posted those pics of your truck a long time ago. That's too bad that happened. Glad you're getting it fixed!

I'm not sure which Powertrax locker Summitt sells. Powertrax also makes the NoSlip which is supposed to be a little less harsh than the LockRight but costs about $150 more. It's the same basic design as the LockRight but it has some springs in it that soften the engagement. I saw one at the drive line shop a couple weeks ago. The LockRight is alright for me though, I can live with the clacking. If you drive in alot of snow and ice and dont do much mud or sand and as you said only do mild 4wheeling, you might want a limited slip instead of a locker. I have limited slips in front and rear in the Bronco and they help alot.