Whats your RPM?

ctfan · August 27, 2005

I'm curious as to what everyone with 285's is turning for RPM at 70 mph.With the 6.0 and 4.10s, I am runnin about 2300 or 2400. The reason I ask this is because I noticed on a friend of mines 98 Z with 285s and 3.73,and he was runnin like 18 or 1900. Is something screwed up with my truck.

William

99-2door · August 27, 2005

The 285s aren't going to affect your speedo to tach reading.

What really matters on that is the factory gear set and stock tire size. Also what tranny. You have a 4L80e probably with 245 stock tires and he has a 4L60e with 265 stock tires.

The computer doesn't know you changed the tire size, unless you had it recalibrated.

dannyukonxl · August 27, 2005

also your truck is made for towing and his is also , but not to a greater extent, i know that i really like having hte high rpm's while cruising at 80 for the power etc, but on the other hand it eats gas

Daubs · August 29, 2005

Yep, your tach shouldn't have a problem, but your spedo will be calibrated for stock tires from factory. I put 285's on my HD and noticed spedo was off (checed vs my GPS). Got Westers and corrected my problem.

Mattt · August 29, 2005

I'm curious as to what everyone with 285's is turning for RPM at 70 mph.With the 6.0 and 4.10s, I am runnin about 2300 or 2400. The reason I ask this is because I noticed on a friend of mines 98 Z with 285s and 3.73,and he was runnin like 18 or 1900. Is something screwed up with my truck.

William

<{POST_SNAPBACK}>

Nothing is screwed up. Thats they difference from 3.73 to 4.10 gear ratio. Your friends Z71 1500 truck comes with a higher gear ratio(lower #). Higher gear ratios run at higher speeds with lower RPMs. However, you also give up grunt off the bottom with a higher gear ratio. They couldnt put that combo in an HD with a 6.0 because you wouldnt have been able to tow anything then. The 6.0 is borderline sufficient for the HD, they made up for that by putting a lower gear in it. Thats why the 8.1 and DMax usually come with the 3.73 gear.

Take notice what RPM you were running with the stock 245s, it would have been even higher than 23-2400RPM. If you went with a larger tire than 285 you could eventually get the 70MPH RPM down to the 18-1900 range, but it would have to be much larger, Im thinking in the 37" or 38" range tire size.

Everyone with that combo 6.0/4L80/4.10/285 will be turning the same RPMs at 70MPH,,,,,,my truck included.

ctfan · August 30, 2005

Ok thanks guys. My speedometer was recalibrated for the tires so its on. Thanks again guys.

William

Mighty HD · August 30, 2005

I personally would have left the speedo alone.

I have 285's, a Nelson tune programmed for the stock tires. The stock tires are 7.782% larger than stock. That means the speedo goes 7.782% slower than what you are actually going.

Example, you're doing 60mph, approximately you're going 65mph, @ 80mph you're doing around 85-87mph.

8100 Power · August 30, 2005

My 2001 with 8.1/Ally and 373 gears runs around 1800-1900 rpms at 60, my 2004 (same setup) with 4.10s run around 22-2400 rpms, thats alot of diffrence. I would think the 373s would get better fuel econ. since it wasn't running as strong......

Ag Teacher · September 8, 2005

I have 285s on mine. Had my cruise set at 78 (on a turnpike with 75 mph limit) and got clocked going 84. Got off with a warning but I don't remember the rpms.