PDA

View Full Version : How to find True RPM's



Bottoni 94TD
01-14-2008, 03:38 PM
I have an all stock 94 K2500 130,000 with 4.10's, auto tranny, stock tire size.

I want to find the true RPM's, right now cruising at 65mph tach says 2500, what should it be?? NAPA alternator probably wrong pulley size?

Can you get the actual rpm off the ECM through the OBD port??If you can how??

The ultimate goal here is to get better fuel economy, I want to cruise 65 to 70mph at about 2000rpm or less without changing the gears or getting an OD set up, or spending too much cash.

I am about due for tires, would bigger tires reduce my RPM to where I looking for. How big of tires will fit at stock height??
Thanks!!
This site is great!!

grancito
01-15-2008, 12:07 AM
Your true tach reading should be 2282 at 65 mph, you can put 265-75x16 tires without rubbing and revs will be 2220 at 65 mph, but speedo and tach will read wrong.

sootjunkie65
01-23-2008, 12:15 PM
Anybody know what it would be in overdrive W/nv4500 and 3:42 rears?
265/75/16 tires

6.5SmokeStack
01-23-2008, 01:15 PM
well if you want better fuel economy, you could try gear vendors also much nicer at cruising speed :)

94K30
01-23-2008, 02:11 PM
I have 265's with 373 rear on mine, and I read about 2000RPM on highway at 60-65.....

grancito
01-23-2008, 10:59 PM
94k30, your RPM is about right but you will be traveling a little faster because of the 265 tires.

94K30
02-21-2008, 04:11 PM
94k30, your RPM is about right but you will be traveling a little faster because of the 265 tires.
Hi Grant.... Actually, I clocked it against the measured miles on the highway, and it was within 1% of correct, I am betting that it darn close for a speedometer/odometer. I went 40 miles, it was only different by 4 tenths of a mile....

grancito
02-22-2008, 12:41 AM
Stock speedos usually over read so the bigger tires put yours about right, mine under reads 2.3% with the same tires, but should have been 3.something under.

oldyeller6.5
03-03-2011, 04:15 PM
Anybody know what it would be in overdrive W/nv4500 and 3:42 rears?
265/75/16 tires
i'm running 2300 at 65, and about 2450 ish at 70.

ss68chevyII
03-30-2011, 04:17 PM
I thought I was running about 65 mph on the interstate but after checking that with my gps on the phone my speedo was fast by 5 mph :eek:... I wondered why I was getting passed so often :rolleyes:. Today I went from stock size tire to a 235/85/16 and checking the gps on the way home on two lane roads it was almost dead nutz between the speedo and the gps. I will know more after my run on the interstate tonight. I hope to see the rpms drop slightly too.


Tommy :)