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

FluffyFluffy Posts: 1,761 Officer
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!
[SIGPIC][/SIGPIC]

Replies

  • CaptTaterCaptTater Posts: 20,096 Officer
    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.
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  • FluffyFluffy Posts: 1,761 Officer
    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.
    [SIGPIC][/SIGPIC]
  • FluffyFluffy Posts: 1,761 Officer
    Agreed on lower rpms alone dont equal better economy.
    [SIGPIC][/SIGPIC]
  • Pescatoral PursuitPescatoral Pursuit Posts: 5,065 Officer
    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.
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  • gandrfabgandrfab Posts: 19,975 AG
    Also if you don't go off road'n put street tires on it.
    :BUNNY gestapo

  • Mango TangoMango Tango Posts: 2,013 Officer
    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
  • StugotsStugots Posts: 570 Officer
    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.
  • barrydbarryd Posts: 297 Deckhand
    I may be wrong but I would think that if you change the tire height, you would have to calibrate the speed/odometer also.
    Barry
  • StugotsStugots Posts: 570 Officer
    2 sizes up from stock is okay after that yes.
  • FishshrimpsnorkelFishshrimpsnorkel Posts: 1,350 Officer
    Ditto on what's been said. Also, remember that the speedometer and odometer will need re-calibration. You can use a GPS for a speedometer.
  • hooknlinehooknline Posts: 5,278 Officer
    You need to recalibrate the speedo, and every inch you go up is another inch of air that goes under the truck instead of over. And the undersides aren't the most aerodynamic things. So that's more drag.
    Overall it's a farce.
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  • gandrfabgandrfab Posts: 19,975 AG
    barryd wrote: »
    I may be wrong but I would think that if you change the tire height, you would have to calibrate the speed/odometer also.

    I think Ice cold air does that.

    http://www.icecoldair.com/
    :BUNNY gestapo

  • rrbgttrrbgtt Posts: 6,753 Admiral
    How would Garrett know???? He won't leave factory size tires on anything long enough to check the mileage to compare.......:grin
    From hell's heart I stab at thee; for hate's sake I spit my last breath at thee. Ye damned bobcat!
  • FluffyFluffy Posts: 1,761 Officer
    Thanks for the info so far... Garrett told me that Walter(the mechanic) is his source. Walter is also the guy claiming to get 23 mpg with a lifted 4wd 7.3.
    [SIGPIC][/SIGPIC]
  • rrbgttrrbgtt Posts: 6,753 Admiral
    Thanks for the info so far... Garrett told me that Walter(the mechanic) is his source. Walter is also the guy claiming to get 23 mpg with a lifted 4wd 7.3.

    He probably never exagerates.....:grin
    From hell's heart I stab at thee; for hate's sake I spit my last breath at thee. Ye damned bobcat!
  • sonnysonny Posts: 283 Officer
    I think it also has to do with you gearing if your 7.3 has 3.73 gearing it probly wouldnt help it might hert it but if you have4.11 or lower gearing then I could see it helping the bigger tire size you go the higher it makes your gearing

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