Transmission swap question

[cheech147]

Was wondering if any Mack gurus can help me out. I'm looking at a 70's Mack with a 300 engine. Thinking of buying it. Problem is it had a 5 speed and the current owner swapped it out for a 13 speed. Seems to me he defeated the purpose. He said he likes shifting. What I'm concerned about is if the 13 speed will work well with that engine since the hp and tq range was compatible with the 5 speed. I drove 5 speed 300's and 350's with no problems. Can anyone enlighten me if this was a good idea or not.

[j_martell]

More gears isn't exactly a bad thing.....but I don't know for sure. I've only ever run "limited torque band" trucks with multi range transmissions....

If it was setup with a 5 speed then it would be a maxi dyne, which might be kinda nice with lots of "options" and tons of power band.

[Joe Ditchkus]

You would not have to split every gear on the topside of trans. The 13 speed gives a much better low gear and overdrive in 13th unless it is a 13 speed direct which is not common.Joe D.

[Don Hancock]

_{I used to drive a Cruiseliner with a 300 + - and a 13 ran good.}

[cheech147]

Thanks for the replies, maybe I'll give it a shot.

[masterwelder]

Sorry I didn't see this sooner. You need to ask to see if the rear end gears were changed when he swaped the transmission. The five-speed had a different final ratio than the 13 does most likely. My rears with the 5 speed were 5:73. When I switched to the 13 speed I dropped to 4:64s and that's still not enough to have a decent top highway speed.

[Vladislav]

Sorry I didn't see this sooner. You need to ask to see if the rear end gears were changed when he swaped the transmission. The five-speed had a different final ratio than the 13 does most likely. My rears with the 5 speed were 5:73. When I switched to the 13 speed I dropped to 4:64s and that's still not enough to have a decent top highway speed.

If not a big secret, what tire size do you have?

Vlad

[Green Dash]

_{I used to drive a Cruiseliner with a 300 + - and a 13 ran good.}

if you are referring to the 315hp 300+ that engine was not a Maxidyne and needed at least a 9 speed behind it. If the above truck came with a 5 speed then it has the Maxidyne 300 with a different torque curve.

[mandt428]

I have R600 with 237 hp transmission. It's time to rerace the trans., I too are suffering with not enough highway speed.

Can I change existing 5th speed gears with overdrive gears?

The transmission is X107, has 2 sticks, 5 main, high and low and reverse in the aux box.

x1070 is an overdrive transmission, same vintage.

Regards Mick Downunder.

[rick harper]

When comparing swapping transmission keep in mind an old Mack five or six speed is a 1 to 1 final ratio in top gear. Most 13 speeds have a faster final gearing(overdrive). Also one usually has to cut or lengthen the driveshaft due to transmissions having different lengths. Bell housing need to be correct also. Here is a formula to compare speed. tire factor x rear axle ratio x final gearing divided into rpms. Tall 24.5 rubber tire factor is 7.93. Tall 22.5 rubber tire factor is 8.32. Lets say you have tall 24.5 rubber a Mack 13 speed and 4.17 ratio. 7.93 x 4.17 x .71=23.48. Lets assume top rpms is 2100. Divide 2100 by 23.48=89.4 mph. If your speedo head is mechanical it can read 5 mph faster than you are actually going. Also worn tires will decrease your speed due to rolling height.

[jeff20878]

I also found a link on the Eaton/Fuller Roadranger web site that does road speed calculations: http://www.roadranger.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf

[ws721]

Not all Mack 6 speeds are direct. Mine has a .62 overdrive. That is a great deal faster than 13 or 18 speeds

[mm826]

In 1988, I put a rto 12513 in my 1976 F model sleeper. It had a 300hp & a 5 speed to start with, & 4.17 rears. The truck ran great. More power & more top end, about 10 mph. Aat that time only the high gear high split was overdrive, .87 I think. Today's 13 & 18 speeds have the high gear both direct & hi split are overdrive. Todays 13 & 18's are RTLO 16713 A or heavier. You have to shorten the driveshaft a little & change the clutch fork knuckle. But it wil be a whole differant truck for the better. And for road work the 72 was alot better than the 62 mph.