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Old 06-08-2008, 08:35 PM   #11 (permalink)
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When y'all say "bigger" you mean "taller" right?

A taller rear tire on a rwd 2wd truck or rwd car will slightly change the effective rear gear ratio and speedometer. Generally it makes for better mpg. Especially if you have a mechanically injected vehicle.

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Old 06-09-2008, 12:42 AM   #12 (permalink)
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yep we are talking taller
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Old 06-09-2008, 06:51 PM   #13 (permalink)
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JV-Tuga asked:

“Have those hundreds of pickup drivers adjusted for the offset odometer reading of the distance travelled?”

Big Dave answers:

Yes. Reprogramming the speedometer on Ford (I mostly talk to Ford guys) pickups is easy. There is a tone ring in the rear axle and all you have to do is tell the computer what size tires you have. Some guys (computer geeks) do it themselves, for others like me its $75 at the dealer. If they have not reprogrammed the speedometer, I ignore them.

JV-Tuga asked:

“Also, outside of stop and go traffic, wouldn't those larger flywheels be beneficial, like on the highway?”

Big Dave answers:

If all you did was drive the Interstates, that might be true, but nobody drives like that. Even light hot-shotters and driveaways drive enough surface streets to destroy any advantage the large diameter tires give them in MPG. These subsets do tend to use larger diameter tires to gain load capacity.
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Old 06-09-2008, 08:06 PM   #14 (permalink)
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I'm driving a '98 Forester sometimes, and I'm in the same boat - the 2.5 flat-4 makes plenty of go, but the tranny downshifts as if it's bolted to the 2.0. It doesn't let the engine dig into that store of power at all. Even up modest grades, I have to take off the cruise control and feather the throttle to keep it from downshifting.

I notice in the photo that your roofrack doesn't have its crossbars on.

Lead or follow. Either is fine.
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Old 07-11-2008, 10:47 PM   #15 (permalink)
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instead of starting a new thread, i'm going to ask in this one, since its related.

is there a formula for calculating my miles traveled back to stock after going to larger tires? i have gone from 29" to 31" tires on my explorer. well, thats the listed size, not actual. i still have a stock tire/rim i can bolt back on to get a static load radius, to go with whatever the static load radius is for the 31's.

technically, all my fuel log entires (i have a bigger excel file at home than here) are off because of the tires, but i've been using it as more of a comparison between driving styles. now i want to know what my actual best mileage is.

yeah, i know i just need to get the speedometer re-programmed, but its not really in the budget at the moment (just got my transmission rebuilt). does the scanguage correct for tire sizes? or will it be off as well? i've been saving for one of those anyway, my wife's car has a lot of that stuff built in already, and i'm really starting to like it.
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Old 07-12-2008, 02:18 AM   #16 (permalink)
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Tony -

I use a GPS to sporadically check my one-size-larger-than-stock tires :


Here's one calculator on the Internet :

(Miata) Tire size calculator

The Scangauge has a "speed" setting for compensating for tire size differences.


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Old 07-12-2008, 10:45 AM   #17 (permalink)
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same model cars with different tyres

I have 2 Citroen Ax10RE's one with 145/65/13's the other with 135/80/13's. The manual recommends 135/70/13's. The Ax with the 145's is much better to drive powerwise, remembering this is a gutless 45bhp vehicle that cant stand load, ie passengers or hills. The AX with the 135's feels too tall geared. The diameter is greater by 35mm and the circumference is 6% longer, so its like I'm always driving up a slight rise... The 135 AX is higher of the ground too by 17.5mm. I havnt done any mpg around this as I'm a newby without instruments and havnt driven them enough. My reckoning is that the viability of tyre diameter modifications would be model dependant. The main consideration being loading ability of the vehicle in question. If its a powerful donkey with lots of power that climbs hills without any loss of speed at constant throttle I'd say yes.

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