racesport Report post Posted March 4, 2012 I have a 2002 American Coach with a 375hp Cummins ISL. There is a Cummins Road Relay installed. Are the fuel mileage figures it displays reliable? Other than fuel mileage I would like to hear from owners what advantages does this equipment afford to me? Share this post Link to post Share on other sites
hermanmullins Report post Posted March 4, 2012 racesport, Welcome to the Forum. We have a 2002 400ISL, with a Trip Tec. I have been told by Trip Tec that I must have Cummins reprogram the ECM to correct my problem. When I check my Engine Data, which tell current Trans/Engine Temps, total miles and lifetime MPG. My lifetime MPG reads between 58.7 and 96.8 MPG. I would love for that to be true but ***********. Also at the same time I have 'Maint." light on on the dash. There doesn't appear to be any problems with the engine and has never ran better. I have just let it be for now and will check it out at a Cummins Service Center when I can go there on a trip. To answer your question about the fuel mileage shown, it's fairly close. but as with all readings like this I still like 'Miles driven divided by Gallons used= MPG. Share this post Link to post Share on other sites
Guest BillAdams Report post Posted March 5, 2012 Your best judge of fuel mileage will be what Herman describes above. Take the number of miles your drove and divide it my the number of gallons used and you will have an accurate number if you take these readings over the course of 3-4 fill-ups. It gets even better if you just keep a running spreadsheet. The numbers on your coach will likely offer "instant MPG" or similar and these numbers have no value of any kind. Sometimes you will get 100 MPG (downhill with a tail wind) and sometimes you will get 2 MPG (up the hill from Baker toward Las Vegas for 15 miles). If the readout gives you averages over a longer period of time it's quite likely that this will be close but there's nothing like a pencil and paper to get the most accurate information. Share this post Link to post Share on other sites
Koliver Report post Posted March 5, 2012 Use a GPS to verify the odometer. Mine was over 5% off. As if my tires were 5% too small. Saves getting a ticket, but leaves you thinking you are getting great mileage when you are not. Share this post Link to post Share on other sites
desertdeals69 Report post Posted March 6, 2012 I have used the road relay for 11 years and it is very usefull in the operation of the motorhome. I used it to constantly adjust my speed to acheve the best fuel milage at that moment. The warnings it gives for low oil and low pressure etc. is, in my estimation , priceless. As far as the accuracy of speed, the ecm needs to be programed with the correct rear end ratio, tire size, and revs per mile. I get 11 mpg towing a Silverado going across the country, generally running about 60 mph. Share this post Link to post Share on other sites
hermanmullins Report post Posted March 6, 2012 Koliver, I use my GPS for correct MPH. I also use it for MPG. My MH odometer is off also. However my silverado is right on. Share this post Link to post Share on other sites
