Do tires affect gear ratio?

[eze01]

Does gear ratio change when changing tire/wheel combination? for example manufacture specs out an 18" wheel wheel with 235/40 18 and you change the wheel combo (increase or decrease the tire/wheel combo size) does it have measurable gear ratio change? if so how would one measure? is there a formula or is there web site that shows variance?

[ilko]

Yes. Taller tires will act as if you have taller gears. Here's a gear calculator http://www.grimmjeeper.com/gears.html but it may be more than what you're looking for.

Any online wheel/tire calculator such as this one: http://www.rimsntires.com/specs.jsp will show your speed difference between different sizes wheels/tires.

Hope this helps.

[madkimchi]

are you talking about revs per mile? obviously larger tires have bigger diameter making it less revs per mile. i do not have calculations or how accurate i am but by going 1" bigger in tires (oem 18's to larger 19's or getting skinner tire 40 vs 35) does matter. i think our odometer will be changed by 1-2 mph. you can't really tell by just looking at our odometer but you can tell by comparing our odometer with gps unit.

[beemercer]

yeah it effectively does, a smaller rolling diameter will help you rev quicker but your top speed in each gear will decrease.

[eze01]

So a taller/shorter tire does change gear ratio and it is quantifiable? How much of a profound difference is it and can it lead to gear damage down the road because the manufacture specs out accordingly?

[S4_Artillery]

yeah it does, a smaller rolling diameter will help you rev quicker but your top speed in each gear will decrease.

I haven't seen you post in forever. I thought you left the site. Its good to know your knowledge is still available.

OP. It will not damage your car unless you do some significant staggering.

[deanjohnson]

keep in mind too that your speedo will be off if you change the wheel/tire diameter (assuming that the speedo is based off the drive shaft rotational speed). A smaller wheel/tire combo will read a higher speed versus reality, while a larger wheel/tire combo will read a slower speed versus reality.

[beemercer]

So a taller/shorter tire does change gear ratio and it is quantifiable? How much of a profound difference is it and can it lead to gear damage down the road because the manufacture specs out accordingly?

it should just be the ratio of the rolling diameters.. so a 235/40/17 like I had on my oz's is a 24.4" and a 245/40/18 like I had on my avus's is a 25.7". The 17 should be roughly 5% shorter.

It wont effect anything with the gears/gear ratio etc. It affects the final effective drive ratio (its like another gearing step after the differentials).


You do have to wary about having dissimilar tires sizes, so if you have a bald tire and a tire with 13mm of tread the diameters will be different, and that can strain the diffs. Same with running staggered fitments.

As long as the diameters of all the tires are the same you should have nothing to worry about.

[beemercer]

I haven't seen you post in forever. I thought you left the site. Its good to know your knowledge is still available.

OP. It will not damage your car unless you do some significant staggering.

thanks for the kind words. I still have the A6 4.2 6MT, but possibly not for long. I hang around here and audirevolution.net (moreso there recently) because I like the community and the B6 S4 is still one of my favorite cars out there.

[beemercer]

keep in mind too that your speedo will be off if you change the wheel/tire diameter (assuming that the speedo is based off the drive shaft rotational speed). A smaller wheel/tire combo will read a higher speed versus reality, while a larger wheel/tire combo will read a slower speed versus reality.

good point, this also effects your odometer. A bigger than stock rolling diameter slows the rate miles are accumulated, whereas a smaller than stock rolling diameter accumulates miles faster.

[Tropicgreena4]

tires will not change any gear ratio... a gear ratio is just that, a ratio of the size of the driving gear to the driven gear. tire size can throw your speedometer off, but the gear ratio is preset...

[eze01]

tires will not change any gear ratio... a gear ratio is just that, a ratio of the size of the driving gear to the driven gear. tire size can throw your speedometer off, but the gear ratio is preset...

that's exactly my question. does it change gear ratio (i.e. running taller/shorter tires changing your gear ration from a 4.11 for example to a 4.10 etc) I get the odemeter change because of the change in rotation diameter but didnt understand that it could change actual gear ratio. thanks for the clarification