airmike Posted December 21, 2013 Report Share Posted December 21, 2013 I am curious if someone could tell me approximate rpm of a 99 sts at 70 mph. I don't know what other cars may have the same gear ratio/tire size combination. My trans am ran at about 1600rpm, and my wifes mustang runs about 2000 rpm. They were both 6 speed transmissions, the mustang auto. Guess I would like to know it is running at the correct rpm. Link to comment Share on other sites More sharing options...
Texas Jim Posted December 21, 2013 Report Share Posted December 21, 2013 I "THINK" mine is at 2500 at 70... Tried to find a pic of it, but can't seem to locate one that shows that low of a speed... Link to comment Share on other sites More sharing options...
Cadillac Jim Posted December 21, 2013 Report Share Posted December 21, 2013 Here is an estimate based on computing the numbers based on tire size. Inputs: Tire size is 235/60-16R. Rolling diameter computes to 27.1 inches, or 744.15 revolutions per mile. Futher results can be improved by using actual numbers for revolutions per mile. Revolutions per minute of the tire is (number of revolutions per mile) X (RPM) / (60 sec/min) = 868.2 RPM at the tire. Revolutions per minute at the engine is (revolutions per minute of the tire) X (final drive ratio) X (transmission ratio). Final drive ratio is 3.71. For the SLS and other VIN "Y" cars it is 3.11 instead. For the 1995-1999 Aurora, it is 3.48. Transmission ratio for the 4T80E in 4th gear is 0.68, a pretty nice overdrive ratio. Your computed (estimated!) RPM at 70 MPH is 2190, call it 2200. This seems right because GM traditionally tunes its normally aspirated V8 drivetrains for 2200 RPM at cruise. For a VIN "Y" car with the same tire we would get 1836 RPM. -- Click Here for CaddyInfo page on "How To" Read Your OBD Codes-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data -- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC Yes, I was Jims_97_ETC before I changed cars. Link to comment Share on other sites More sharing options...
JimD Posted December 22, 2013 Report Share Posted December 22, 2013 Unfortunately I do not have a useable picture of my '98 STS display at 70 MPH. Another unfortunate factor is the lack of a digital RPM display option on that vehicle so reading exact RPM is not possible without some type of external display.A minor variable is the tread depth of the tires on the drive wheels.Here is what you can expect with a 1998 to 2004 STS model Seville. Jim Drive your car. Use your cell phone. CHOOSE ONE ! Link to comment Share on other sites More sharing options...
Cadillac Jim Posted December 22, 2013 Report Share Posted December 22, 2013 My calculations for 75 mph come to 2350 RPM, and your tach reads 2500 RPM, 6.3% higher. I believe that the tach and speedometer are both digitally driven from numbers computed by the PCM and converted to PWM signals by the DIM, and thus are as accurate as digital readouts, so I would say that the rolliing diameter of your front tires are 6.3% smaller than what I am using, which I got from The Tire Rack as the OEM tires for the car. I recall my 1997 ETC tires being 225/60-16, not 235/60-16, which computes to 2400 RPM at 75 mph, which as I recall is about right, and much closer to your indicated 2500 RPM. -- Click Here for CaddyInfo page on "How To" Read Your OBD Codes-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data -- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC Yes, I was Jims_97_ETC before I changed cars. Link to comment Share on other sites More sharing options...
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