Larger Tires=Lower RPM= Better fuel economy???

[Fluffy]

I was talking to Garrett(RRBGTT's Son) yesterday and he was telling me, that in his experience(Also backed up by our mechanic) that he saw better fuel economy running 35's on his 7.3 4WD than smaller tires. I have the stock size tires on my 7.3 4WD and am thinking about going up to the largest that will fit with no modification to the truck.

It makes sense to me, at 70mph, I'm running at 2,100 rpms, he said with 35's he would be at 1,850rpms or so...

Obviously, there is some sweet spot to tire size, when the tires get to large for the motor (EX, toyota pickup on 44's getting 5mpg.)

Being lazy here, can someone tell me the largest size tire I can put on my 99' F250 4WD without a lift kit or rubbing the wheel wells?

Thanks!

[CaptTater]

Depends. If they're hard non aggressive tires and you don't over accelerate sure. LOW RPM doesn't equal economy though. If so it wouldn't drop economy towing a boat. There was test in one of the mags I get showing how heavy large diameter aluminum wheels for low profile tires reduced economy dramatically in cars. So imagine swinging a much heavier and larger tire to accelerate.

[Fluffy]

Sure, but like I said there has to be a sweet spot. I think the 7.3 has the torque to spin a larger than stock wheel, so much that it will benefit.

[Fluffy]

Agreed on lower rpms alone dont equal better economy.

[Pescatoral Pursuit]

Tread (pun intended) very carefully. I thought the same thing so when time came to replace the factory 255/65/17s on my '06 F150, I used 265/70/17s AND LOST A LITTLE OVER 1 MPG ON AVG.

MPG is as much a product of manifold vacuum as it is displacement/ rpm/ etc. If you lug the motor and lower the vacuum, you'll lose gas mileage.

Also had to have the transmission rebuilt prematurely at 105k, which I blame on the larger tires overworking it because I drive like granny going to church on Sun morning.

You will also lose low end responsiveness. Unless you do mostly highway driving, or, you have a high rear end gear ratio (you usually get one of about 3 options, mine was the lowest available,) I would stay away.

Now you could go a little wider, but now you may run into clearance issues or premature front end wear. Hope this helps.

[gandrfab]

Also if you don't go off road'n put street tires on it.

[Mango Tango]

Tires, like the prop on your boat are the final reduction gear in the drive train. Adjusting its size (or pitch) to affect RPM can have a beneficial effect on efficiency if the engine has the torque to spin it without overworking the motor. As long as you don't negate increases in efficiency with aggressive/high roll resistance tires then the 7.3 would probably deliver slightly increased mpg. YMMV

[Stugots]

What you will save in fuel, you will pay in additional maint. But it's like anything else you want to play you have to pay.

[barryd]

I may be wrong but I would think that if you change the tire height, you would have to calibrate the speed/odometer also.

[Stugots]

2 sizes up from stock is okay after that yes.