Like i said this maybe a stupid question. Im just wondering are trucks with larger factory tires still a 3.55 rear gear ratio like my f150 has 3.55 gears with stock 235/75/15 my brother has a f350 with 3.55 gears with 235/85/16 much larger tire so is his effective gear ratio much taller then mine because of his stock tire size?

 

Yes, taller tires effectively give you a taller gear ratio. Your tires are about 29" tall, your brother's tires are about 32" tall. Effectively that would be the same as if your brother had 3.23 gears if he had the same size tires as you. Or another way of looking at it is you effectively have 3.90 gears if you had his tires.

But yes, your brother's truck actually does have a 3.55 ratio, just like yours. The actual axle ratio has nothing to do with the tires. It's the ratio of the number of times the driveshaft turns for every one time the axle turns.

 

Nothing Special, can you please show me how to do the math with that?

 

It's directly proportional to the tire diameter, so if you want to compare effective ratio with two different size tires you multiply by one and divide by the other. To figure out which is which, for instance if you are changing from 28.9" tires to 31.7" tires you are going taller, so the math ought to give you a "taller" (lower numerically) effective gear ratio. To end up with a lower number you need to multiply by the small number and divide by the big one. So with 3.55 gears you take 3.55 * 28.9 / 31.7 = 3.24.

It seems complicated when you try to write it down like this, but the math is really simple, just ratio * old tire size / new tire size.

 

Ok so his extra weight plus more rotational mass and larger tires along with the same engine as my f150 makes it a complete dog lol

 

Even though the effective ratio may be an advantage to the F150 you typically have a smaller engine that makes less torque. That one of the reasons the larger trucks can get away with a taller tire while running the same gear set. Some applications (diesels, big blocks) are betters off keeping the RPMs down in high gear

 

Nothing Special, thank you for he clarification. I am planning on replacing my TTB with 4:10 with 3:55s front and rear D60 and 10.25". I was thinking about going to a slightly larger tire then the stock tire and there was/is some concern about losing too much torque. 460 EFI powered 250, so I am not too concerned, but I do some pulling of heavy loads from time to time. Thanks again!