The C 6 and the 4.10 gears are where the MPG is going.

I have to apologize, all my worksheets are set up for MPH not KPH.

Also I calculated the MPG using 17 US gallons, I have you at 20 KPG right now.
The speedo error would have you at 22 KPG if that is where the error is at. (see below)

First what tires are you running now?

My calculations show you should be running 66 MPH with 235/85-16 tires.
That may be because the C6 torque converter does not lock though.

40" tires would put you at 84 MPH with the gears you have now at that 2900 RPM.
40" tires with a 3.55 gear would set you at 97 MPH at the same 2900 RPM.
Same 40" tires would run 62.4 MPH at 2150 with your 4.10 gears.
40" tires would run 62 MPH at 1850 with 3.55 gears.

 

Now is this under the assumption of using just regular 32 inch tires? Or will bigger tires lower it even more? Hows the E40D? I thought they were crap?

 

I believe I'm running 32 inch tires right now....now with getting 40 inch tires, that obviously requires a lift....so how many inches of lift are we looking at here? Sounds like I smell new rubber and a lift kit comming very soon lol. So is it even worth it to bother with the 3.08's? Or just stick with a 3.55 and the bigger tires? I think a turbo would benifit it really greatly as well.

 

I'm wondering....if I wanted to do 62mph, whats a good speed that I should get the engine at so that its not just guzzling fuel? Somewhere around 1500rpm?

 

As Dave would say, a turbo will help as long as you don't use the extra power.

PS, KPH x 0.611 = MPH

My truck has always had 235/85/16 tires.

I wouldn't mess with the stock 235/85/16 tires, they are relatively tall but also fairly narrow, so there is little arodynamic penalty for there height. 40" tires will lower your rpms, but you will increace your overall drag, so the gains will be minimal, if any. Braking performance will also take a slight hit.

The best bang for your buck would be 3.08s if you want a minimal investment for maximum result and still have things simple and reliable.

The E4OD alows me to cruise with the speedo pegged (140) and the tach at 2000 RPM at about 35% throttle. It also has a lower 1st and 2nd gear ratio than the C6, so accelleration is also slightly better for loaded conditions.

The list of upgrades I made before putting it in is long, but a tripple clutch billet TC, HD steel carrier forward and OD carrier gears, higher output pump, and mild shift kit are the main parts. I also run a programable transmision controller that has a few goodies of its own.

 

I tow and haul a lot of weight, so I stayed with my 3.55 gears and run 285/75-16 tires which are 33" tall.
Here in the mountains of WV, that is about all I can stand with my T 19 transmission.
I would have to have a lower fist gear ratio with anything taller in the tire or gear department.

 

1500 RPM @ 100KPH (~61 MPH) is good as long as you're not loaded or climbing a hill. After so much rearanging of my truck, my conclusion is that 2000 RPM is the best compromise for the 6.9. I'm assuming the 7.3 will behave very similar under the same conditions.

Having said that, 1500 RPM @ 100KPH is what my truck does, and thats with an overdrive tranny and 3.08s, so even with taller tires, its not likely to be within reach without a tranny swap.

 

I totally forgot about some considerations when making these kinds of upgrades. I've been reading on a few off road sites that if you put bigger tires on, it hurts the MPG's? And when you put bigger tires on and change the diff, doesn't that make your speedo read wrong?

 

The two main reasons for hurting MPGs from bigger tires, is 1, the usuallt are wider as well as taller, so the wheels themselves are causing more drag, but the other reason is that if you lift the vehicle higher up to fit the tires, the overal frontal crossection of the vehicle also changes, since the truck is now higher.

For offroading, there usually isn't much consideration for highway cruising RPM, and ingeneral, diesels are not used. This means that taller gears are not going to help very much in terms of fuel economy, because the engine may just be ligging more often. So thats why its recomended to change the axle ratio to lower gears after going to taller tires in the case of offroading.

Also, if your relative RPM stays the same afterswapping gears and bigger tires, the speedometer will also stay the same (or at least close).

 

So is there any point in even changing the tires at all? So what kind of a place should I look for if I wanna get 3.08's?

 

First question is no, second question is USA!! (remenber where our dollar is right now)

This is basically what I got, and the US$ price is about what I remember.

http://www.ringpinion.biz/product_in...=3eaa81791e2f6

I have since learned that there are other brands avalable besides Motive Gear, so you might want to shop around a little.

You will need a big breaker bar and cheater pipe to retorque the the pinion nut, and you might need a source for shims to make the gears track right. All I did was shave the crush sleave down slightly (probably not recomended) and the gears worked without any shims. Aftermarket shims are avalabe to replace the crush sleave as well.

 

Gearing and tire size changes affect the speedometer reading on trucks before 1992 since the speedometer is driven from a cable off either the transmission or transfer case.
Since any change in differential gear or tire size changes how far the vehicle travels per driveshaft revolution the speedometer will be off by the amount of change.

Speedometer readings in trucks after 1992 that have the speedometer driven from the VSS in the rear axle are only affected by tire size changes.
They read the axle RPM to get the speedo information.

In your instance with an 89 truck, a 40" tire would make 1 revolution for every 1.25 revolution of a 32" tire.
So to figure out how fast you are going, multiply the 100 KPH by 1.25 and your actual speed would be 125 KPH.
You would also have to multiply the odometer reading by the same amount.

A gear change from a 4.10 to a 3.55 would also introduce a change of 1.155.
So now the 125 KPH speed would have to be multiplied by 1.155, so your indicated speed of 100KPH would actually be 144.3 KPH if you changed both the tire to a 40" tire and the gear from a 4.10 to a 3.55.

You can get new speedometer drive gears to correct the speedometer, but there are limits as to how much correction is possible.
There are also after market speedo drives that have a wider gearing change correction range.

That has to be figured out after you finalize the change you want to make.

32" tire going to a 40" tire, divide the 40" tire by the 32" and come up with 1.25.
4.10 gear going to a 3.55, divide the 4.10 gear by 3.55 and come up with 1.155.

Engine RPM divided by transmission final drive ratio gives you driveshaft RPM.
Driveshaft RPM divided by axle ratio gives you axle RPM.

In your instance 2900 engine RPM divided by 1 for a C 6 final drive ratio gives you 2900 driveshft RPM.
2900 driveshaft RPM divided by 4.10 gives you an axle RPM of 707.3.

Now use a tire diameter of 32" * 3.1414 means the tire travels 100.5 inches per revolution.
5280 feet per mile times 12 inches means 63360 inches per mile.
Divide 63360 by 100.5 means the tire has to revolve 630.44 times to travel 1 mile.

Your axle is turning 707.3 RPM divided by 630.44 means you are traveling 1.1219 miles per minute.
Multiply that by 60 minutes in an hour equals 67.31489 MPH.

What appears to be wrong with this calculation is actually tire deflection at the bottom of the tire from the weight of the truck.
A 32" tire is not actually 32" tall when the truck is sitting on it.
Most tire manufacturers can tell you the revolutions per mile for a specific tire at propper inflation the tire needs to travel one mile.

That should be the axle RPM number you use in your calculations for speed.

And as said before, the C 6 torque converter does not lock so that also introduces an RPM to speed calculation error.

You could have a speedometer with four speed ranges on it that could be calculated for a manual transmission that would be dead on using only the tachometer.
The range you use would be determined by the gear you were in.

 

For speedo gears, the combo I run is 16 tooth driven gear, and I think the dive gear is 8 tooth.

The driven gear is easly to change, but the driven gear is in the extention housing.

 

So I dont know much about changing around the gears in the diff, so you're saying that I have to get this ring and pinion....I have no idea what that is, does or how to change it. What if I just bought the ring and pinion and brought it to a driveline place and got it changed there? How much you figure that would cost, the big problem i have right now is that I don't have the tools to do this stuff.

 

also...that link you gave me, it said in the add that it only fits 92's and up?