tires and gear ratios

[backman_66]

I've read a lot about how tires can affect your gear ratio or something? I did a search for gear ratio and didn't find a good answer to my question.. how do different tires affect your gear ratio? or is it just improving acceleration?

and is it the tire width (225 for example) or the sidewall length (45 for example) that actually effects the gear ratio? need some explaining please :/

[KevinK2]

If you are crusing at 50 mph, and suddenly want to go much faster, you down shift your mtx and put your foot to the floor. With an atx, the down shift happens automatically. The engine rpms rise to a higher level where you are higher on the hp curve, and you go much faster than if you stayed in the same cruising gear when you floored it.

Going to tires with a smaller outer diameter has the effect of dropping all your gear ratios, slightly.

Example:

Cruising at 50 in your mazdaspeed turbo 6 speed in 5th gear. Assume it has typical stock Mazda 6 215-50-17 tires. Downshifting to 4th would kick the rpms up about 8%, and 4th would pull harder than 5th could by 8%.

But, if you were running smaller 215-45-16 tires, then 5th would pull just like you had down shifted to 4th with the 17" tires. The 17" tires are 1.84" bigger in outer diameter than the 16's.

The tire diameter is (wheel dia) + 2 x ( ratio) x (width).

17" x 25.4 + 2 X (.5) x (215) = 647mm, = 25.46"

the 16's are 23.62"

doing this gives 8% more go-power due to "tire gearing", raises your rpms 8% vs speed in each gear, and makes your speedo read 8% too high.

[Tire Guy]

Originally posted by backman_66@Mar 15 2005, 06:11 PM
and is it the tire width (225 for example) or the sidewall length (45 for example) that actually effects the gear ratio?* need some explaining please :/

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Generally speaking, it is the overall diameter change that changes gear ratios. The aspect ratio (50, 45, 40 etc), or sidewall height, is simply a percentage of the section width (ie 215, 225, 235, etc). Thus, it is quite possible to have a 50-series tire and a 40-series tire with the same overall diameter. It is the widths that will be different.

Your car was delivered from the factory with transmission ratios, speedometer and odometer calibrations and engine computer calibrations for a tire with an overall diameter of 25.5 inches. It is permissible to decrease overall diameter by 2% without any ill effects to the above mentioned systems.

Going to a tire that is smaller by more than 2% will not only throw off your speedo/odo/engine computer/abs systems, etc., it will increase the wear and tear on your engine and transaxle due to everything spinning faster than before. The better gearing will allow you to be faster off the line, but you will lose top end speed. For autocrossing, it's not a bad idea, but for everyday use, not recommended.

[backman_66]

Very good info, thank you KevinK2 and toyoguy

[GreenNuggs]

Here's a website that will do the calculations for you:

http://www.net-comber.com/tirecalc.html