45mph w/ 5.13 rear ratio how can I go faster

[mack31]

I have a 1951 L model. With a 707a. And a 5.13 rear ratio. The truck maxes out at 45 mph. 
 

how can I go faster??? 

[tjc transport]

either regear, or add an overdrive unit.

[Joey Mack]

2 SPD. Rear end?? Possibly..

[mrsmackpaul]

Over drive and or bigger tyres

 

Paul 

[steve s.]

The 4 things that affect road speed are engine speed, transmission ratio, rear axle ratio, and tire size. You can't do much about your maximum engine speed. Does your transmission have overdrive? If not you could change to a transmission that does, or you could add an auxiliary trans that has an overdrive ratio. You can probably find a faster gear ratio for your rear axle, or change the whole axle to something with a faster ratio. The larger the diameter of the tire you run, the higher your road speed will be, but the change won't be great.  So, what transmission and which rear axle  do you have? They are probably your easiest places to make an improvement.

[Geoff Weeks]

1st take stock of what you have. tire size, power, transmission even tube size and length of driveshaft. As I found out, some of the old truck were driving their driveshafts into critical speed.

 Tires will only gain you a little. Rear ratio changes can make the most difference, with a small amount of change to the driveline. Adding an aux trans can be another way but you have to replace the driveline and set it up carefully not to overspeed the driveline.

Of course this all assumes enough power to drive the truck faster. I have one truck that will top out at 68 MPH on paper but doesn't have the power to do more than 55. Little gained by making changes if the power isn't there.

[davehummell]

My B would do 50 and 52 down hill not safe on the highways last time I came up from Gerharts it worried me on 81. I installed a 373 air ride rear out of a freightliner and I found a double over triplex I have a very good setup for traveling on highways and it doesn't hurt having the 260 hp Mack engine and jake I installed. 

[Geoff Weeks]

Plot out any changes you are considering on paper first. Note how the truck does now over differing RPM, where it seams to pull the best, and how it does starting (esp under load if you intend to still pull a load). 

 When I re ratioed mine, it took several sheets of graph paper and trying different ratios, looking at engine speed and road speed in each gear, both at speed and at starting. Lots of math, but when I was done, the truck preformed just as the math said it would.  Seat of the pants, guesses often lead to unhappy results.

 Tire Rev/mile can be obtained from the tire mfg catalog, often available on line.

 If you have spoke wheels, you are limited to rims sizes for those spokes, to go bigger requires a hub change and that can be expensive and, on an older truck hard to find. Disk wheels you can jump more sizes, but still some rims (24") can be hard to come by and some tire sizes also.

A simple O/D of .73 should put you around 55 MPH top speed, but check the driveline 1st, you may have to make changes to keep it from critical speed.

 The problem with gearing up in the front only to gear down in the rear, is much of the energy goes into heat in the boxes. In a low powered truck, you could easly end up with something that could go fast on paper but not in the real world. Air resistance increases to the square of speed, double speed and the air resistance goes up 4x. Add to that, the inefficacy of changing the driveshaft speed faster to slow it back down, and all the gains you "thought" you'd get can be eaten up in friction and air resistance.

 All this says is to do the math before you buy the first thing or turn the first wrench. 

[mack31]

Good morning guys 

Thank you for the replies. We have a 5 speed trans. It feels like it has extra power. 
 

I will get the trans model and the tire size today. 
 

[mack31]

This truck does not have a tachometer 

[Freightrain]

Seeing it's a pumper, adding an aux transmission is likely not possible due to the pump in the middle?  Changing to an OD transmission maybe an option?   Depending on what rear it has, finding faster gears is another possibility(like 4.10 or the like).  Or changing out the whole rear for a later model housing with faster gearing.

 

Having enough motor to keep it going is likely not an issue.  Even the limited power of that gasser, it should be able to hold 55 mph okay on limited size hills.

[Geoff Weeks]

Being a pumper, it just about eliminates and aux trans, as the midship pump drive takes the same real estate.  That leaves tires (not much change available, looks to have 10.00 x20's), trans or rear ratio.

 Trans, is 5th up against the dash or down toward the seat? If toward the seat, a O/D trans may be an option, but be aware, that could change the speed to the pump as well. Have to make sure everything plays well together.

 Rear ratio, we would have to know what is there now to know if a different ratio can be fitted or if a whole new rear axle would be required.

[Geoff Weeks]

Is it air braked or juice brakes? If hyd it will be hard to find a new rear fitted out for hyd brakes that will meet all the other requirements.

[mack31]

It’s a 5.13 rear and juice brakes. I just want to be able to go to some shows that require the highway. 

[Geoff Weeks]

The ideal would be a new center drop-out for the rear axle you have now. We would have to know what axle model is in the truck now to know what is possible.

 If you don't know or can't find out, a picture of the front of the axle may help ID it.

 Also where is 5th gear in the trans?

[mack31]

5th is down by the seat 

[Geoff Weeks]

Back of the napkin math;

assuming 10.00 x 20 tires and direct in 5th trans gives  2570 rpm@ 60 mph. 

I have no idea what the Mack engine top speed (gov speed) is, so that would be needed before we go further. A .83 O/D would drop that to 2133 rpm@ 60

 So I am a bit confused by the 45 MPH max, either the tires are smaller, there is some other gear reduction or my assumed tire size is wrong.

[Geoff Weeks]

3 minutes ago, mack31 said:

5th is down by the seat 

Ok that is good news, you should be able to find a 5 speed OD that can be fitted. question is what trans model is in there now? Mack more than other mfg tended to use their own proprietary stuff, so someone with more Mack experience could be of help here.

9.00x 20 tires would raise the rpm @60 to 2657 rpm.

 

[Geoff Weeks]

Only one place on the net did I find any spec's and it looks like 2100 rpm could be the max rpm, which is what I guessed at, as that is what larger engines like the 855 and 3406 max is.

so a .83 or .73 O/D would get you near 55-60.

to go more than that would require more changes.

[mack31]

Axle, rear- RA 45

Trans- TRF 321

Tires- 9.00x20

[mack31]

[steve s.]

You have a 4 speed transmission with fourth gear direct. Changing to a transmission with a fifth gear that is overdrive would be a good way to gain road speed. Changing your tires to 10.00x20 (or radial equivalent) would gain you a bit more. I am not familiar with the CR25 single reduction rear axle, but it is quite possible that another series of differential might fit in your axle housing, giving you more gear ratio options. Start with the 5 speed.

Edited September 29, 2023 by steve s.
spelling

[Geoff Weeks]

1st check the present tire/rim clearance on rear springs and tie rod ends. Make sure there is room to go up in size before buying and finding out you have a rub. Also check the dual spacing you have now, to be sure the wider next size up will not "kiss" going down the road.

You can get 9.00 x 20 radials and 10.00x 20 radials, tubeless would be 10x 22.5 and 11x 22.5 respectively. Tire size will only get you one step in rear gearing, like going from 3.90 to 3.73, not much change, and not enough to get what he is looking for.

A lot of mfg "re badged" units they did not make, with their own model and parts numbers. Unless someone with more Mack experience can tell us more, the next thing would be to photo the trans and rear axle to see if it can be ID'd. I wouldn't be surprised to find a Fuller trans and a Eaton or Timken rear.

 

Edited September 29, 2023 by Geoff Weeks

[Geoff Weeks]

Doing a quick internet search, I am not getting a hit on either of the Mack numbers. My guess is they are not Mack mfg'd but vendor supplied. I have no documentation to support this belief, but it is more based on it being of the age and size truck that there were plenty of suppliers that they could use.

The Mack mfg axles that I know are heavier than that would have, same for transmission, 5 speed is the fewest number of gears that I know Mack made.

 For example: My IHC K-7's have a IHC number for the rear axle, the housing has the IHC logo on it, but the guts are Eaton 2 spds.

 If we can determine what the guts are, we MAY be able to come up something. I qualify that a bit because there may be a limited number of ratios made. 5.13 was a fairly "fast" ratio for the day, and there may not be any faster. Higher road speed was achieved with O/D trans, aux trans and larger tires.

[JoeH]

My napkin math says you need to be around 3.84:1 rear to hit 60mph. That would be your easiest way to get highway speed.