Finally Figured Out What Was Slowing Me Down

I've got a truck that is governed at 62 miles per hour. The problem has been that even on flat land, when I have the truck floored, I'm not making up any time when I calculate trip time at 60 miles per hour. I should be making up 1-2 miles per hour.

Then I realized... This is a new truck, and it is running low profile tires. The difference between the height of low profile and normal tires is small, but enough to make a difference in speed. The road rescue tech I spoke to thinks I'm right, that the computer is set with bad data on tire size. I'll be trying to get myself into a Kenworth dealer in the next few days to get them to check/change the tire size in the computer from normal to low profile.

This also means that the truck may actually be able to run efficiently at 55 MPH in California flatlands, since I was really running at about 53 with the size mismatch issue, and it wouldn't stay in 10th.

It will also mean that I'll come into hills at a slightly higher engine RPM, which will help prevent speed-bogging.

It is even conceivable that the jerkiness when backing up might be due to the truck thinking it is trying to move larger tires (which take more torque to start moving.) I've already had some changes made that helped with jerkiness, but the problem is still there when doing uphill backing.

Can't wait to see how the truck's performance changes when this is fixed.

I would suggest calculating at 55mph instead of 60mph. My truck will do 65, but i calculate at 55. This helps give a more flexible haul time, as it helps account for traffic, hills, stop lights, etc.. If I arrive at my consignee early, then that is great, i'll call them and see if I can deliver early, if they say no, then I have a little time to relax before my next load.

I would suggest calculating at 55mph instead of 60mph. My truck will do 65, but i calculate at 55. This helps give a more flexible haul time, as it helps account for traffic, hills, stop lights, etc.. If I arrive at my consignee early, then that is great, i'll call them and see if I can deliver early, if they say no, then I have a little time to relax before my next load.

I calculate at 60, when I am on the road and rolling interstates or major highways, in order to determine where my next stop will be.

I calculate at 50 when I first get a load, to see what my ETA to final will be.

Here is something to consider... The GPS calculates your time by the actual speed limit posted in the areas. So if you are going by the time on your GPS and you seem to be loosing time consider that you can't run the speed limit and your time will be a little different.

Here is something to consider... The GPS calculates your time by the actual speed limit posted in the areas. So if you are going by the time on your GPS and you seem to be loosing time consider that you can't run the speed limit and your time will be a little different.

Nah, I calculate based on my remaining clock time. I never trust a GPS to estimate my arrival. When I get within 2 hours of End of Day, I start looking ahead using Google maps and the GPS for traffic and construction.

My GPS doesn't even have the ability to add my governed speed to the truck's data. Which is utterly baffling and amazingly stupid, since governed speeds directly impact fastest route planning at interstate speeds.

Another thin is like my truck, the speedometer is 3 to 4 mph faster than actual speed.

My truck was like that wouldn't get over 60 mph. Tires were quite worn finally got the shop to replace them with some winter tires. Higher profile with taller lugs. Truck will go 63-64mph now put a smile on my face.

Here is something to consider... The GPS calculates your time by the actual speed limit posted in the areas. So if you are going by the time on your GPS and you seem to be loosing time consider that you can't run the speed limit and your time will be a little different.

Unless you can set "max speed" on your GPS device, such as one I use. I have mine set at 53mph and it's ETA is almost spot-on....except of course when I'm driving thru LA. It has been within 15min +/- of my actual arrival. My GPS has live traffic update.

My truck was like that wouldn't get over 60 mph. Tires were quite worn finally got the shop to replace them with some winter tires. Higher profile with taller lugs. Truck will go 63-64mph now put a smile on my face.

You should verify your actual speed with a GPS device (like a GPS app on your phone). If your truck wouldn't go (show) over 60 on the speedo - and with higher profile tires shows 64 - the truck itself ISN'T MOVING ANY FASTER. So your smile is based on an illusion (sorry there driver).

You are seeing speedometer variation based on tire height. And this can create REAL PROBLEMS if you aren't careful.

You really do want to know what your ACTUAL SPEED IS, with any given tire profile - because the nice POLICEMAN isn't going to want to hear about SPEEDOMETER INACCURACIES DUE TO TIRE HEIGHT, when he clocks you going 5 over in that construction zone.

Speedometers are typically "sensed" at the output of the transmission - and calibrated to go with the final drive ratio (rear end) based on the size/profile of the manufacturer's recommended tire size (on the door sticker). Changing any of these variables (rear end gearing, wheel/tire size) is going to throw that calibration off - sometimes as much as 4-5 MPH given the example above.

Rick