Is it worth the money to change the gear ratio?

Polish_Drive · Oct 21, 2014

Ratio on my Freightliner M2 106 is 4:11 (manual trans), at 65mpg rpm's go up to 2200. Couple of my friends have M2's, but with CAT engines. They tell me, their rpms are 2200 at 65mpg as well.

To help my engine and my MPG I would need to travel @ 58/59 mph, kind of too slow IMHO. Specially if speed limit is 70.

Respected Mechanic here in Chicago gave me an estimate of $1500 for a ratio change. I'm aiming at 1600 rpm at 70mph.

Also, I'm not sure if changing ratio might be a beginning of future problems.

What do you think?

davekc · Oct 21, 2014

Polish_Drive said:
Ratio on my Freightliner M2 106 is 4:11 (manual trans), at 65mpg rpm's go up to 2200. Couple of my friends have M2's, but with CAT engines. They tell me, their rpms are 2200 at 65mpg as well.

To help my engine and my MPG I would need to travel @ 58/59 mph, kind of too slow IMHO. Specially if speed limit is 70.

Respected Mechanic here in Chicago gave me an estimate of $1500 for a ratio change. I'm aiming at 1600 rpm at 70mph.

Also, I'm not sure if changing ratio might be a beginning of future problems.

What do you think?

Hard to tell without a lot more information. It might work but a lot of things go into the equation. I would do some research via CAT and Freightliner. Horsepower at certain RPM's will be critical to its performance. Could wind up with a higher fuel bill if not done correctly. Especially if you are doing this with a small engine.

Dynamite 1 · Oct 21, 2014

Not exactly the same but i have 2 c9's with auto trans [ 6spd. ] with 4:11's and @ 65 mph. they are turning 1600 rpm. The other c9 has 3:58 and turns 1450 @ 65 mph.

Polish_Drive · Oct 21, 2014

I'm sorry too confuse you all. I have Cummins engine and my buddies have CAT's. My engine is a cumm. 280HP ISB'07 280/2600. Trans is a FULL, FS6406A, manual 6 speed

jamom123 · Oct 21, 2014

I have a m2 106 with a mbe900 260 hp. 6 speed stick shift 4.33 gear ratio. At 65mph i'm running about 1750-1800 rpm's. I'm averaging about 11 mpg.

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sewmun · Oct 22, 2014

Some of the other posters have various rpm because they might have a trans with an overdrive in it.I have a FL70 w/a Cummins,6-speed & it came w/a 4:11 gear 70mph was 2400 rpm to much for me.So I bought a 3:42 gear(whole pig unit) from a truck parts for $500.00 & put it in myself ,my Son helped it weighs almost 300Lbs.And it lowered the rpm by 400.I usually run 63 mph around 1700-1750,70mph is 2000rpm,I found out on a hill in WY that 84 is 2400.
When I run 63 now it gets close to 11.5,it used to get 9.5 before.
It is a lot quieter in the cab too.
I have a ISC 225 HP,660Lbs torque & this gear change slowed it down a lot but I go out to WY & TX with it & climbs all the hills & mountains ok.I suppose off the interstate the higher ones would be a job for it but I never go there so it works for me.

Polish_Drive · Nov 2, 2014

OK,

So my truck ratio is done. I got to say, I am very happy with the outcome. 65mph @ 1700

It did made a $2100 hole in my wallet, but I think it will return soon.

I got to find out exactly what the ratio is now. Ive picked up my truck right before closing and I had no time to speak to the owner.