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earnies2 · 04-15-2022, **(Subject: 355 to 308 ratio? )**

I have a 2009 prostar ISX 485, 13 over 355 rears on 22.5 low tires.
If I drive 55 mph the truck gets decent fuel millage 7.5 - 8 if I drive 62-65 millage goes in the toilet to 5 mpg.
If I change both rearends to 308's what can I expect for road speed and millage?

tree98 · 04-15-2022, **(Subject: 355 to 308 ratio? )**

You can expect to beat the liners out of your engine prematurely. Gears that tall will keep you below 1500rpm to often. Why not just drive 55-58 mph and keep your boost as low as possible as often as possible. Get the 7.5-8 mpg you mentioned and pocket all the money you save on fuel.

Kw9006nz · 04-15-2022, **(Subject: 355 to 308 ratio? )**

I went the 3.70 route and not only did I get a significant bump in power from my butt dyno but also netted .5 mpg average across the board. With the price of fuel nowadays 4.78 central nebraska they will pay for themselves in a little over 6 mo

Rawze · 04-16-2022, **(Subject: 355 to 308 ratio? )**

I am starting to think there is some collection of idiots over on social medai, or at some stealers$it somewhere that is trying to convince people to get taller rears?.. where is this complete garbage of 'taller' engine torturing rears specs ideas coming from?.

tree98 · 04-16-2022, **(Subject: 355 to 308 ratio? )**

(04-16-2022 )Rawze Wrote: I am starting to think there is some collection of idiots over on social medai, or at some stealers$it somewhere that is trying to convince people to get taller rears?.. where is this complete garbage of 'taller' engine torturing rears specs ideas coming from?.

Been a rash of it here lately lol.

earnies2 · 04-16-2022, **(Subject: 355 to 308 ratio? )**

(04-15-2022 )tree98 Wrote: You can expect to beat the liners out of your engine prematurely. Gears that tall will keep you below 1500rpm to often. Why not just drive 55-58 mph and keep your boost as low as possible as often as possible. Get the 7.5-8 mpg you mentioned and pocket all the money you save on fuel.

Are the liners and engine differant on a 2018 versus my 2009? I'm basing my facts on all the newer trucks all have tall gears and get way better fuel millage than my 2009.
Please explain.

earnies2 · 04-16-2022, **(Subject: 355 to 308 ratio? )**

(04-15-2022 )Kw9006nz Wrote: I went the 3.70 route and not only did I get a significant bump in power from my butt dyno but also netted .5 mpg average across the board. With the price of fuel nowadays 4.78 central nebraska they will pay for themselves in a little over 6 mo

What was your ratio before the swop and millage?

earnies2 · 04-16-2022, **(Subject: 355 to 308 ratio? )**

(04-16-2022 )Rawze Wrote: I am starting to think there is some collection of idiots over on social medai, or at some stealers$it somewhere that is trying to convince people to get taller rears?.. where is this complete garbage of 'taller' engine torturing rears specs ideas coming from?.

What was your ratio and millage? I know you only ran 55...I just can't seem to discipline myself to run 55 when I'm getting passed by prime and swift in Oklahoma whare the speed limit is 75...

Rawze · 04-16-2022, **(Subject: 355 to 308 ratio? )**

(04-16-2022 )earnies2 Wrote:
(04-16-2022 )Rawze Wrote: I am starting to think there is some collection of idiots over on social medai, or at some stealers$it somewhere that is trying to convince people to get taller rears?.. where is this complete garbage of 'taller' engine torturing rears specs ideas coming from?.
What was your ratio and millage? I know you only ran 55...I just can't seem to discipline myself to run 55 when I'm getting passed by prime and swift in Oklahoma whare the speed limit is 75...

my 1 million mile average for 80,000 freight is 8.3 mpg... i have 3.55 rears and even they are slightly tall.