Gear ratio questions

[Patriot]

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!!!!!!

Thanks again fellas.

Patriot------

[87 V20 Scottsdale]

http://www.4lo.com/calc/geartable.htm

[deepnsteep]

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

[68 TT]

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.

[pcwillsie]

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

[spudz]

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

[Patriot]

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------

[my90k5]

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

[Patriot]

Originally Posted by my90k5

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

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

Patriot----

[68 TT]

Originally Posted by Patriot

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------

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.