Gear ratio questions
 

As many here know I am building a K5 blazer. I have the option of using a set of axles with a gear ratio of 3:07 or keep the stock axles of 3:73.

Here is my question, I will be using this truck for a daily driver and only sometimes going off-road. It will have the SM465 with the 205 so changing RPM won't be a problem.

Will the 3:07 gears be better for daily driving?? The max tire size will be 35", probably 33".

I just don't want my motor running out at a high RPM at 75 MPH. I live right off of Interstate 90 in SD and it is a 75 MPH zone here.

I have looked at a few charts and I can't find the ones for 75 MPH and 33" or 35" tires at either ratio. Any help would be GREAT!!!!!!:metal:

Thanks again fellas.

Patriot------:flagw:

Re: Gear ratio questions
 

Re: Gear ratio questions
 

RPM = axle ratio * trans ratio * 12 * 5280 *speed (mph) / 60 / PI / tire dia (inches)

2345 = 3.07 * 1 * 12 * 5280 * 75 / 60 / PI / 33

2211 = 3.07 * 1 * 12 * 5280 * 75 / 60 / PI / 33

Re: Gear ratio questions
 

RPM = MPH x 336 x gear ratio / tire diameter

75 mph w/ 3.07 & 33's = 2344 rpm

75 mph w/ 3.73 & 33's = 2848 rpm

I would go with the 3.73 gears. If you bump up to 35" tires the 3.07 gears will be a dog to drive. They won't be too fun with 33's either.

Re: Gear ratio questions
 

if you are going to go up in tires size you definatly want to use your 3:73 axles. 3:08 gears would be real doggy and you would be working your engine and transmission harder to turn them. 3:08 gears are all right for a 2x4 with stock type tires but they will make your transmission shift up and down all the time in your application

Re: Gear ratio questions
 

with 35s you should atleast have 373s.. be better with 456s. 307s would lug the truck and worsen the MPG. with 33s the 373 should be fine

Re: Gear ratio questions
 

Thanks fellas for your input. Even with a SM465 you guys think I should still keep the 3:73 gears??? Isn't 2200-2400 the optimum RPM for highway driving??

Patriot------:flagw:

Re: Gear ratio questions
 

Hey Patriot, I'm in the same boat. I have the SM465 and switched to a 31" tire. I haven't installed a tach as of yet to determine my RPM's on the freeway but they are somewhat low. I also play shifting games during my daily drive to work. My truck has the stock 3.08 gears and I plan to switch those out for the 3.73 gears.

Here is a link I'm using to determine my desired RPM range. 2400 - 2600 seems to be a good highway RPM.

Gear Ratio Chart

Re: Gear ratio questions
 

Thank you sir. That is great info.:metal: I will stay with the 3:73 gears.

Patriot----:flagw:

Re: Gear ratio questions
 

That explains my gas mileage :) What 4100 rpm at 75 isn't optimal?

5.13 gears x 31.6" tall tires = way too much RPM at freeway speed. Now if I ran 44's they wouldn't be too bad.