I have a 97 f150 4x4, 4.6 engine. I originally had the 16" alum rims with p23570R16 tires and averaged about 15mpg. I recently bought a set of Ford factory 17" rims with new P26570R17 tires. My gas mileage went down to 10mpg. I changed the speedogear, which was only off by 1 tooth. W H Y the drastic change in mileage? I put the 16" set back on and getting the 15mpg again, but I want to run the 17's. Please help!!!!

 

You have effectively changed the gearing with the larger rim/tire and the engine is working harder on every acceleration.

Get on a 10 speed bike.
Take off in first gear.
Take off in tenth gear.

Which takeoff, done over time will cause you to drink more Gatorade?

 

That doesn't seem to make sense! I have a 97 4x4 4.6L, which had the 17" rims and I have always gotten around 18 mpg with it. I can understand if you go bigger than that.

What gears are in the rear end of your truck? Maybe the Off Road Package is geared differently, if so, you will need to change gears to get it back.

The more I think about it, I think the ORP does have different gears! Amazing how things make more sense after you type them out!

Joe

 

The tire size is your entire problem. You went from a 29" tall tire to a 31" tall tire. That 2" makes a HELL of a difference. If you want to run 17s either live with the gas mileage or get some new gears. Depending on what you have now (probably 3.55 gears) then 3.73 will get you back close to stock but not quite. Now if you have 3.73s now some 4.10s will get you pretty close to stock when running the 31s. Whole thing to keep in mind you're going to be looking at somewheres around $800-900 for a gear swap. If it was me I'd just put up with the mileage, I mean come on, its a truck.

 

About a month ago, Iswitched to the 17" wheels with 265/70/17 Dunlop radial rover at's. Before, I had the 16" wheels with 235/70/16 tires. I also had to drop the speedo gear two teeth to bring things back to the right scale. I have a "97 f150 4.6 4X4 with 3.55 gears. I tow two snowmobiles frequently and found that ,before I made the change, that I was looking for gas at about 280 miles. (I drove about 65) Just got back from another trip: Now I still have a few gallons left at about 320 miles, towing at 70!! I worked this out to be about 12.5 mpg. To me, this is a fantastic improvement and well worth the cost/fun of the switch. I suppose I'll pay more around town though. By the way the only other mod I've made is a K&N filter. Next I,m going to try some Flowmasters and maybe a Flex-a lite fan. Hope this helps. Has anybody else had this same experience??

 

Questions:
City or Highway Driving ?
Terrain, flats or hilly ?

 

I called Ford and they said I have the 355gears. I do not know how you were getting 18mpg. My question is if you originally had the 17" what are you running now. It sounds like you moved higher. Also, what does the inside drivers door tag show for wheel size as original eqpt on your truck? I do supposedly have the ORP, but I also bought the truck used. As far as changing gears, I would have to change both front and rear axle, twice the cost and not worth it. Thanks for your reply fellow Buckeye.

 

While my truck isn't 4x4, I have a '99 SC Lariat, 4.6, 3.55ls, auto with the stock 16" 255/70/16's and I get 16-20mpg depending on how har I drive it. Is there that much size diff in the 17's???

 

The difference in size is 2"; 29" height to a 31" height. Doesn't seem like much but watching the fuel guage go down confirms it.

 

Majority flat city driving.

 

I can understand loosing gas mileage with a larger tire when city driving because of stop and go. However I would think that highway milieage would improve, due to more travel per revolution. Can anyone comfirm or deny my theory?
CurtO

 

Do both rims have the same tires?

 

I'm sorry but I can't see losing 5MPG just by increasing the tires 2". This is not a completely far comparison but I had a 94 Ranger with a 2.4L, 5spd. I had about 26.5 " tires on their stock and went to about 29" tires and saw almost no fall off in gas mileage. I did notice that I was giving it a little(very little) more gas in first gear to start up but that would be the only gear affected and can't see loosing 5MPG with this swap, it just makes no sense to me. I can't see going from 3.55 to 3.73/3.73 to 4.10 making that big of a difference to offset the cost of regearing.

 

Your theory would be true hgiven many factors. One being, what speed are you traveling and two being, what is the terrain? If you are going around 75 then the truck may require more downshifting to maintain speed up a slight incline. The revs of the tire directly reduse the engine revs thus causing to require more downshifting resulting in a lose in MPG. Get it, I hope so cuz now my head hurts form all this thinking.

 

I've got a '98 4x4 ORP 4.6L and run P265/70/17 Dunlop Radial Rover ATs. I get 16.5 MPG doing a 70-30 mix of highway/city driving, running 93 octane. I have 3.55 LS gears. If you bought it used, are you sure the last guy didn't swap out 2.73s or something?
This is very strange to lose 5 MPG over such a swap.

Magnum