Re: changed to R/T size tires, now fuel mileage dropped?

From: Andy Levy (andylevy@bigfoot.com)
Date: Wed Mar 28 2001 - 23:43:11 EST


If you put on a larger tire, the truck will "think" it has gone less
distance than it actually has. Thus, your calculated mileage will
drop. You're looking at approx. a 3.6 percent difference in tire
diameter, so calculated mileage will drop by a similar amount, assuming
the tire has the same roling resistance and your aerodynamics haven't
changed.

Back in January, I replaced my original Wrangler RT/S 31s with BFG 31s.
My speedometer (and odometer) now run about 4.316030267125wer than previous -
the BFG is a taller tire (not all 31s are created equal, plus these
don't have 28K of wear on them). As a result my calculated gas mileage,
miles driven/gallons to fill tank, has dropped by about that same
amount. It may have dropped a tiny bit due to pooer aerodynamics from
being higher as well, but probably a negligible amount.

In terms of gearing, a taller tire should actually increase your gas
mileage, because you're going further per revolution (similar effect as
with going to a higher gear ratio (lower number)). Folks who put big
tires on their trucks often have to move to a lower ratio (higher
number) to keep similar performance. Example: Bernd moved up to 4.10s
when he put 32s on his truck. Partly for the extra kick 4.10s give,
partly because he had to make up what he lost with the larger tires.

Similarly, as Patrick noted, shorter tires will translate to a lower
gear ratio, decreasing actual gas mileage - unless you compensate by
re-gearing your differential (or tranny).

If I'm smoking crack here, someone please smack me.

Patrick and Kelly Engram wrote:
>
> "might sound weird, but when I went from 255/60/15 to 255/55/17 RT rims
> and
> tires, I noticed that I can't go over 20mpg on my overhead anymore. I
> used to get 21 doing 60mph or less on the hiway, I can barely get 19.5.
> This
> could be due to the difference in tire size has thrown off the
> calculations,
> but I think the RT has the same overall diameter as the stock tires,
> 255/65 (mine were 255/60)
> Dunno, could be wrong. I am sure there are others than can answer."
>
> -Interesting thing to note. The stock 255/60R15's should have an
> overall diameter of approximately
> 687mm. The R/T size 255/55R17 stands about 712.3 mm tall, roughly 2 1/2
> cm., or almost exactly 1 inch
> taller. Who's the math wizz on the list? If you have a taller tire
> that at the same rate of revolution as the smaller
> size, will travel further in distance due to an increased
> circumference. The fuel mileage on the computer
> would read lower, but the distance traveled would be further than what
> the computer thinks, due to it taking
> its reading off the speed sensor on the transmission.
> So, is the gas mileage affected? Only if the replacement tire is
> shorter in diameter, not taller in diameter. Being shorter will raise
> the engine rpm at any given speed, increasing fuel consumption.
> Patrick

-- 
-andy

http://home.twcny.rr.com/andylevy/ --- andylevy@bigfoot.com ------------------------------------------------------------- modesty, n.: Being comfortable that others will discover your greatness -------------------------------------------------------------



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