I was looking at my door Jamb today, and I noticed that my Rear axle ratio is 3.55. I found a website that told me how to get my gear ratio and I found it should be 3.85, due to an upgrade in tires I got a couple of months ago. i went from 29' tires to 31.5' tires, also I've noticed my gas mileage hasn't been the best lately, could that be why I'm getting around 10-12 to the gallon? And how do you fix rear axle gears to make them 3.85? THANKS I don't know what to do but I'm sick of my MPG's. Thanks

 

I had a 3.55 and went to 4.10 after increasing tire size to 31.6" and I'm glad I did! A lot better power through the gears now. City mpg improved and highway mpg went down slightly, about 1 mpg. If you have a 4x4 you'll need to re-gear the front diff as well. Re-gear means changing the ring and pinion.

 

aaron, buddy, your mileage probably hasn't changed that much.

Your speedo and odometer are reading incorrectly now that your tire size has changed - by as much as 8-9% if my calcs are correct.

Frankly I take MPG claims with a grain of salt unless they can be substantiated with a GPS or radar. The ford odometer is not accurate, and any change in tire size makes the odometer reading utterly useless.

I would hesitate to spend $1600 ($800 for a 2wd) on a re-gear to save $4 a week on gas... First, get some expert help to recalibrate your speedometer/odometer (speedo gear replacement for older truck, programmer for newer truck). Once this is done you will know your 'real' mileage. With this, determine if you need to regear. Personally, I would NOT regear with 3.55 and 31.5 tires. I have 3.73 and 33" tires and the truck is fine - I did have to recalibrate the speedo with a programmer to get it to read correctly.

give us some more info on your truck - year, 4x4 or 2x4, engine size, etc.

 

What does the door tag, what is the axel code? going from stock 29 inch tire to 31.5 is no where near enough to justify a gear change. What year is the truck? after 98 you can get an edge programer and recalibrate the speedo and that will make you feel better. Wont really change the actual mileage. I have 32 inch tire with 355's and average about 15 with the tuner reprogrammed and all..

 

I can squeeze 14 out of my 2wd with 35's and 3.55's. Like the guys have said, you need to make sure your speedometer is calibrated properly before getting an accurate mpg calculation.

 

go 4.10's so it'll be worth your effort



remind yourself you drive a truck, if MPG's was your priority you wouldn't have a truck and you sure wouldn't be adding larger tires

 

Yeah guys thanks for the help, it's a 1998 f-150 4.6L V-8. Rear wheel drive

 

With a 98, all you have to do is replace the VSS gear to compensate for the different tire size.

Compare the speedo reading to a GPS, how far off is it? Stock, when the GPS says 60, the speedo will probably be showing about 62.

A very common reason for an early 4.6 to lose gas mileage is clogged EGR ports - eventually it will trip the CEL and give you a P0401 code.

 

I run a 32.75" tire with 3.55 gears, and I put gas in it when it needs it! Truthfully, though, I did some calcs when I first got the big wheels, and compensating for the diameter difference error I was pulling 16mpg average.

I wouldn't worry about re-gearing in your case. Like it was said before, why spend all that coin to save a couple bucks a week on gas? Just fill her when she needs it (giggity) and keep on truckin'.

 

I am wondering if I should re-gear myself. I think my truck still has the original 3.73's in it but I am running 35" tires... I just can't seem to find anyone to do the work. I don't want to just take it to Ford. I want a gearing expert to do it but searching on the net in my area has brought no luck. I guess I just don't know what I am looking for exactly or who does this type of work.

 

03King, most off-road shops that outfit Jeeps have someone who can change gears. Another place to look is a shop that repairs OTR (semi) trucks. Make some calls and ask a lot of questions.

 

heck i wouldnt be above doing it myself, but i dont have the tools to do it.