For an easy comparison fir the speed he posted in the original thread. If
you use 60mph as the base the difference ends up being 2.13mph less showing
on the odo that what he is actually doing. That would make the computer mess
up the gas mileage and show it as poorer when it really is not. Looks like
he needs to get his computer calibrated for the r/t tires.
Jeff Durling
'01 4x2 QC SLT+
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
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