Tire size affecting gear ratio

[SHOOT1SAM]

Can someone explain how changing tire sizes affects the rear end gear ratio? I've been told that taller tires will essentially give me a higher gear ratio, which, as I understand it, would make towing somewhat easier. If that is accurate, at what size tire difference does this begin to take effect? Also, if the above is accurate, wouldn't that give a negative effect on overall fuel economy for someone who doesn't tow too often?



Conversely, again if the above is accurate, couldn't someone with 4. 10's reduce their tire size and gain in economy?



Thanks in advance.

Sam

 

[AggieJustin]

Changing to a taller tire gives you a numerically lower rear end ratio which makes towing harder. Since the larger tires have a larger circumference, it takes less turns of the driveshaft to make one full revolution of the tire, which can sometimes lead to better fuel economy.



There is a writeup in the last issue of the TDR magazine (pgs. 16-17) with formulas to calculate how your choice of tires affects your final drive ratio and also your actual MPH at any RPM.

 

[Mick2500]

Sam,

Here is a good place to learn about tire size and how it affects your RPM/Towing.



http://pw1.netcom.com/~sgalaba/tiresize.htm#ustires



Mick

 

[SHOOT1SAM]

AggieJustin & Mick2500: Thanks for the replies.



If I follow correctly Aggie, I was half right. The taller tires give better fuel economy but adversely affect the truck's towing capabilities. So if I were to swap the 265's on my truck with my wife's 225's when I tow, it should help as far as towing, but not fuel wise? (Please understand that you are dealing with probably the most un-mechanically inclined member of the TDR)



Mick, the tire size calculator is very helpful. On trying to asses the effects of the tire sizes on the rear end gear ratio however, it asks for the "transmission gear ratio". How would I figure this out on an automatic transmission?



Thanks again guys,

Sam

 

[Mick2500]

Sam,

I can't help you there. If you know the current

overdrive percentage (mabye using Fritz's web site

http://www.ecpe.vt.edu/~dfritz/ ) you could do a better comparison. I don't know much about the Auto trannies.





Mick

 

[AggieJustin]

Transmission Gear Ratio

Sam-



Here are the numbers from the TDR magazine for the transmission overdrive ratio:



Post '94 - . 74 five-speed and six-speed

. 69 automatic 47RH, 47 RE



Pre '94 - . 77 five-speed

. 69 automatic 518

1. 0 automatic 727



As for fuel economy vs towing power... larger tires allow you to go faster with less engine speed. But there is a limit where the fuel savings due to lower engine speed are offset by the amount of power required to turn the larger tires.



Smaller tires will effective lower your effective gear ratio (higher numerically... isn't this swapping of terms great?? ) which will provide better towing capabilities. But this comes at the expense of running higher RPMs to maintain highway speeds.



JRM