I traded my 01 2500HD D/A for an 03 3500 D/A. It has stock tires on it and is other wise stock. The 01 was running 65 mph at about 1800-1900RPM. The 03 is running 2000 or slightly more at 65 mph. It is 2200-2300 at 70. My GPS say the speedometer is indicating 2 to 2.5 mph faster than the actual speed by GPS.


The RPM for 65 and 70 MPH seems to high but not sure why it would be. Is this too high? Would the 3500 be diff than the 2500HD?


Just installed 4" Flow Pro exhaust. What a boost. My juice goes back on as soon as I can get the gages back in.


Tom

70mph @ 2100 rpms

Is the 3500 a dually? I believe the tires are a little shorter on the duallies. That would account for the increase in RPM's at the same speeds.





Edited by: duramaxfan

'02 3500 dually ... 2,000 rpm = 67mph ... would suspect the differences in readings could be due to accuracy (or lack there of) in OEM guages


Jim

2000 RPM @70 mph


'03 2500HD stock tires

70mph---tad over 2100 rpms. How can some guys be turning 100rpms less, maybe gauges? Oh yes, stock tires.

Whats the gearing in the rear end?



3.73? Or 4.10?

2000 @ 70mph

All diesels are 3.73s until you get to the 4500's

My truck has 3:73 and stock tires. My indicated speed is actually about 2 mph higher than actual speed. The speedometer is a little jerky at some speeds. I think it needs to be checkout at dealer.


The 05 3500 has 4:10 and a 23,500 lbs GCWR and a higher trailer towing for reg cab, 16,700, 16,300 ext cab, and 16,100 CC.


Thanks guys.


Tom

I have found a similar puzzle. I have used two different mathematical formulae (I found them on the internet, I'm not smart enough to know them - source: Edelbrock.com - the other was another diesel site) and put them into an Excell spreadsheet.


I too have stock tyres and have not made drive-train mods. However, my tachometre shows about 2200 RPM for 70mph.


The math in the spreadsheet show that at 70 mph and with a rear diff of 3.73 in fifth gear (0.71 reduction) the engine RPM should be 2045 RPM. However for the 4.10 axle ratio the RPM should be 2248.


Something's a little odd here.

Gruffid


Do the charts have the tire size (diameter) included in the calculation?


Tom

Yeah, they have tyre circumference built in. I also added a function to determine tyre circumference based on your tyre size (245/75 R16 = 95.72)


(first number * aspect ratio, convert to inches, multiply by 2 for each side of the wheel, add the wheel diameter, multiply by pi)(245mm*0.75/25.4*2+16*pi = 95.72in)


The spreadsheet also calculates for speed based on RPM, or RPM based on speed. This is to show what you're getting vs. what you want.


In addition it will calculate how large of a tyre you need to accomplish a speed at a given RPM


I "stole" all of the calculations, but broke them apart to different cells so you can manipulate the different variables and see exactly how a certain mod to wheel/tyre/axle ratio will change the RPM or MPH. It will also change the results based on different tyre sizes (ex: 275/65R16 vs. 275/85R16).

Gruffid


My tire size is 215/85R16. Where is this spreadsheet. I looked at the edelbrock site and didn't find any data like you described. I will try to develop a spreadsheet from your data. Thanks.


Tom


PS - Do you work at Bell Helicopter in Ft Worth?

Tom,


The spreadsheet isn't on the Edelbrock website. I had to make it by combining several different formulae. The formulae I took from them was the speed based on axle,gearing,tyre size,RPM and the RPM based on axle,gearing,tyre size,speed. I had to visit some tyre specific sites to get the formula to find the circumference from the tyre size markings.


Here's the spreadsheet:( I hope this works) 2004-11-13_112853_Gear_&_Speeds.zip (http://dieselplace.com/forum/uploads/Gruffid/2004-11-13_112853_Gear_&_Speeds.zip)


Gruffid,


PS - no I work in San Antonio

hey tom


i too have an 04 3500 and my gps says the speedo is 2 - 3 mph slow. so i go by the gps. also i have had it in 3 other vehicals and the they were right on with it. our vette with a digital mph was exact with the gps. as for rpm and mph i believe 65 mph = 2000 rpm for me. original tires and wheels for me.


dogboy

Thanks Gruffid. I will play will it. The reason I asked about where you worked was that there was an engineer at Bell that was from Britain who lived in the San Antonio area. He stayed here during the week. I think he had some land and horses and cows in that area. I assume you are from Britain based on the spelling you use for tires, etc. Thanks again for you help. I will let you know how it comes out.


dogboy, thanks. It seems we both have the same readings. I may take mine in for a speedo check. The needle seems to be erratic at times especially around 50-60 mph. Maybe they can get a better calibration on it.


Tom

Tom,


No problem. I'm actually a Texan by choice, born in Georgia. I lived in England for 6.5 years. So, my spelling habits are a little more Brittish than most.


Good luck with the spreadsheet. Let me know if you have any problems or questions.

Update on the speedo and tach issue. The dealer determined that it is defective. They used the test setup for the ABS to verify speed. Apparently it is a widespread problem from what the tech writer told me. The guage unit is on backorder because of the demand.

Tom