Tire diameter questions

[Shomeyrwit]

I know there are trade offs but if your using a car almost exclusively for highway driving how do you determine what size tire will put you in the sweet spot for a given rpm torque and speed?

I have played with tire sizes a lot on a variety of cars and trucks and have always kept track of the mpg even if it didnt matter but never actually tried to do the math on when a tire (or gear ratio) is to tall.

Thanks in advance

[cfg83]

Shomeyrwit


EcoModding Lurker

[EDIT] A typo-bad post on my part. See real response below.

CarloSW2

[Shomeyrwit]

it's true :|

[cfg83]

Shomeyrwit -

If I were you I would fiddle around with this website :

Fatboy Raceworks Graphical Transmission Gear Ratio Calculator

You can input gears and tire sizes. By changing the value of the RPM Redline to the RPM you want to drive at, the program will output the "Max MPH" for that RPM.

In terms of finding your drivetrain's sweet spot, I would read this thread on BSFC (Brake Specific Fuel Consumption) and start searching the interweb :

http://ecomodder.com/forum/showthrea...-got-1466.html

CarloSW2

[Shomeyrwit]

Sweet, definatly the type of thing I was looking for

[CapriRacer]

Originally Posted by cfg83

........
Fatboy Raceworks Graphical Transmission Gear Ratio Calculator

..........

A Word of Caution:

Unfortunately, that calculator makes the same mistake almost all of them make - they don't consider that the rolling diameter of a tire is about 97% of the measured diameter. That means the speeds calculated are going to be off 3%.

However, since this error also occurs when a comparison between 2 different tires is done, the resulting error is small enough to ignore.

So BE CAREFUL how you use this information.

[cfg83]

CapriRacer -

Thanks for the correction. I will keep that word of caution in mind.

...cfg...
10/03/13

[moonmonkey]

shomeurwit ,im looking at doing the same thing, as i cant find any other way to change my gear ratio on a toyota echo, im going to buy a cheap set of tires and rims off of craigs list if i can find them,,im going from a 175-65-14 to a 185-65-15, its .44 wider and 1.5 " taller (if i calculated right) ,at a steady speed on the highway,,,where i spend 60+% of my driving now, it should show improvement, . i understand that in city stop and go, getting the bigger mass up to speed could hurt fe. leverage and moments and other enginerring speak.once the larger mass is up to speed would'nt the larger mass have a slight flywheel effect?never seen that discussed. also larger contact patch as a result of larger diameter and slight width increase will increase rolling resistance,.i will soon know if it helps or hurts my specific case. i dont know if you can truly know if it will help or hurt until you try it.,,,so i am.

[mcrews]

I have done a simpler/similar exercice for the last 4 years.
02 Infiniti Q45 comes with 245/45/18. I have used 255/45/18 on the stock alloy rims.
The auto tran has overdrive and the rear raqtio is 2.76 stock.
I consistantly got 25-27mpg over the 23hwy epa avging 65mph
I am running about 1780 rpms at 60mph.
I have sacrificed some off the line with the wider/taller tires but the car did 0-60 in 5.9 sec already! I do about 65% highway and I like the 'fuller' tire. gives me a smooth ride even at 40psi.
The determining factor for me was the front strut lower base is 1/4 inch from the edge of the tire! I could not run any other size up with out going to air suspension.....and I'm not gonna VIP the ride. Also I did not want to dramaticly chang the ratios and relationship of the suspension parts. I began running this larger size at 60k. I am at 192,000 and am replacing the shocks and struts as normal maitanance this month.

Warning: Increase in size increases wieght. I have been told that the static weight increase, say 8lbs is multiplied by 4 when rolling. This DRAMATICLY increases the demands on the brakes and associated parts like the rotor. So I would start with fresh rotors and pads.

Good luck!!

[Shomeyrwit]

Be careful about wider, sometimes it cant be helped but it will have the opposite effect, especially in the front where the width also affects wind resistance more, more rubber on the ground equals more rolling resistance. The extreme of this in my experience was going from a 185 70 13 (I believe) on my 88 Honda civic to a 205 60 14 free fom my brothers del sol, I lost 6 mpg on the highway dropping from 42 to 36 (of course the handling was hugely improved)
Going the other way I had a 92 Crown Vic (police pkg) and going from the stock 225/70 15 to a 235/70 in front and a 255/70 in the rear brought my hwy mpg from 22/23 to 26/27 with an all time best of 30mpg but brought my city mpg from 17/18 down to 14/15 which at the time didnt matter much but once I started college kinda sucked.
I probably would have done better with 235's front and rear but that 16yr old car nut in my head said ya gotta have big tires,,,, and I listened to him a lot more back then