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Old 02-06-2011, 02:38 PM   #1 (permalink)
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Larger Tires to Increase Gear Ratio

I'm considering larger diameter tires for my Grand Prix next time I need to buy them. Reason would be to reduce my engine RPM at cruising speed (I have plenty of power and want to get closer to optimum BSFC on the freeway when I need to maintain a constant speed instead of P&G.

My speedometer will not be accurate. I'm ok with that.

But if my diameter goes up 2" then my ride height goes up 1". Will the height increase cancel out some/most/all/more than all of the ratio improvement? Maybe it just means I need that underpan mod as well!

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Old 02-06-2011, 03:11 PM   #2 (permalink)
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It depends on the vehicle. If the gearing is way too low, then yes, it will typically help. However, if it's not geared all that badly to start with, then the gearing change may not overcome the aero penalty.

What do you turn as far as RPM at 60mph?
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Old 02-06-2011, 03:46 PM   #3 (permalink)
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You must also take into consideration a few other things...one is increased mass of the tires, suspension travel clearance, alignment, steering radius.
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Old 02-06-2011, 06:23 PM   #4 (permalink)
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You also need to consider how much room is in the fenderwells. Rememeber the worst case is with the suspension fully compressed and the sterring fully turned. Any rubbing could cause a very bad situation.

Plus, you need to be aware that tires can vary widely in rolling resistance, so any change you might make could be completely overwhelmed by the effect a different tire has.
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Old 02-06-2011, 10:24 PM   #5 (permalink)
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you should have no problem with fitting one or two sizes larger, unless you have lowered the car and then how much.mine is lowered about one inch and i still run one size larger with no problems.
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Old 02-06-2011, 11:41 PM   #6 (permalink)
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I think it's hard to know in what cases it'll work and in what cases it won't. For example, my Tempos like the gear-up a slightly taller tire gives, while my F150 won't stand for it.
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Old 02-07-2011, 12:03 AM   #7 (permalink)
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Pickup truck guys try this all the time and invariably it backfires and they get WORSE MPG, even after speedo error is corrected for.

Wheel/tire assemblies are flywheels. The rotational moment of inertia goes up with the square of radius.

Everytime you accelerate away from a stop (even if you accelerate very gently) you have to pour energy into those flywheels in order to spin them up. Guess where that energy comes from?

This is why vehicles with bigger wheel/tire diameter tend to eat brakes.

If you could drive without ever starting or stopping, accelerating or decelerating bigger tires would help, but that ain't the real world.

Maybe we need a sticky to address this evergreen fallacy.


Empirical data shows thar gearing works and bigger tires are counterproductive.
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Old 02-07-2011, 12:20 AM   #8 (permalink)
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Ooo, I wish I could do a good test on that right now.

I do believe my Tempos, '59 Bel Air, Corvair, Bug, and Bus all had improved fe because of taller tires, but I have no fuel logs or decent testing for that.
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Old 02-07-2011, 12:54 PM   #9 (permalink)
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Thanks for the posts everyone; it seems this one is not as clear cut as I'd assumed.

My reason for changing was to get closer to BFSC at highway speeds, but maybe that slight improvement would not compensate for all the other considerations. The GP runs about 2500 rpm at 70 mph

Big Dave, I understand the flywheel concept and agree that getting to speed would take more energy; conservation of momentum means that, unless I'm braking, I'll also coast further due to the additional energy in my wheels/tires.

It sounds like the verdict is out on whether or not this approach improves MPG....
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Old 02-07-2011, 02:02 PM   #10 (permalink)
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Big Dave -

Quote:
Originally Posted by Big Dave View Post
Pickup truck guys try this all the time and invariably it backfires and they get WORSE MPG, even after speedo error is corrected for.

...
Can you tell me the typical before/after tire size change you are talking about? Can you point to a non-ecomodder thread on this?

CarloSW2

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