confused about final gear ratio and tire size

[spederman]

I think i am a little confused. I ended up ordering a set of 3.90 gears. Well my rear tires are no longer stock. They are 295/35/18's. So will that make my final gear ratio feel lower than 3.90's (ie. approaching 4.10)? I'm confused on how all this works Because arent final drive ratio's calculated on stock 17 inch tire size?

 

[hognutz]

you need to find the diamter of the tire. stock is a 245/45-17 it is a diameter of 25.7 inches.

you current tire is 25.9 inches. so you have the equivelent of a 3.86 gear in you car right now compared to a stock tire.

you need a pinon factor of 802 instead of 807 for you speedo to be right on.

now if you went the other way down 24.7 inchs tire then you ratio would go higher to be equivelent to a 4.10 gear.

 

[Chili]

What is important to look at is the circumference of the tire, not wheel size. That said, your new tires are larger than stock, negating to some extent the benefits gears give you, as compared to the stock 245/45/17.

http://www.discounttire.com/dtcs/infoTireMath.dos

For each RPM of the engine, you will travel a farther distance as your tire increases in circumference. When you go from stock gears to 3.90's it decreases the distance you travel per rpm.

 

[spederman]

OK, so since the stock tire size is 245/45/17 (25.7 inch and 785 rev/mile) and i went up to a 295/35/18 (26.1 inch and 772 rev/mile). According to the calculator, its almost negligible in difference. It says my speedo will read 60mph when i am actually going 61. So basically, the final drive ratio will still stay almost right at 3.90. I just wanted to make sure that i didnt have some bizaare tire size to where i was getting 3.90's installed but after the calculations it would be like having 3.27's again, haha.

 

[Chili]

OK, so since the stock tire size is 245/45/17 (25.7 inch and 785 rev/mile) and i went up to a 295/35/18 (26.1 inch and 772 rev/mile). According to the calculator, its almost negligible in difference. It says my speedo will read 60mph when i am actually going 61. So basically, the final drive ratio will still stay almost right at 3.90. I just wanted to make sure that i didnt have some bizaare tire size to where i was getting 3.90's installed but after the calculations it would be like having 3.27's again, haha.

Nah.. They're pretty close, it really shouldn't affect you much..