RPM-Speed Relationship

[02x72]

I put in the spare tach senor I took off a truck at the salvage yard since my old one wasn't working, and had a question on the RPM's. At 60mph its reading at 2700 RPM, which seems really high to me, considering the truck has 3:55 gears, even with the C6. Could the sensor be off and give a bad reading, or is this right? Any thoughts?

 

[The Warden]

First, your signature says you have 3.73's. Did you do an axle swap?

Assuming stock tires, that RPM number's consistent with 4.10 gears according to ringpinion.com's calculator. How certain are you about your gear ratio? And, for that matter, how certain are you about your speedo reading? If you have or can borrow a GPS, using that to determine the accuracy of your speedo may be worth your time. The speedo could have fallen out of calibration, or maybe the speedo gears are incorrect for the differential?

I don't know if the tach sensor can fail in a way that would show higher RPM than what you're actually doing...

I hope this will help a bit...good luck!

 

[icanfixall]

Jack up one rear tire. Mark the driveshaft anf the inside of the tire. Now turn the driveshaft till the tire mark makes on complete tire rotation. Thats your gear ratio... A 3 1/2 turns is 3.55. A 3 3/4 turns are 3.73 and just a little more than 4 turns is a 4.10 gear ratio... Hope this helps you figure out the real gear ration...

 

[02x72]

The truck has 3:55. I must of been thinking about the Chevelle's rear end when I wrote that. My spedo doesn't work, so a GPS is how I have been determining that. The axle tag and sticker on the door jam of the donor truck confirm the 3:55 gears, although I guess its possible for them to be different, just not likely. Tomorrow I'll do that driveshaft trick to make sure they are 3:55's. Thanks guys

 

[The Warden]

A swap is CERTAINLY possible. I'd go ahead with what Gary suggested...or you could pull the differential cover and rotate the ring gear until you find the point where the ratio (should be) is stamped onto the ring gear.

If you've ever tried, what speed does the engine hit the governor at?

 

[02x72]

Ive gotten the truck up to 75 fairly easily, it defiantly had a few mph's left in it. From what Ive read, it would be hard for 4:10's to do that, could be wrong though

 

[rjjp]

4.10's and I'm at the governor at 75 in third.
P.S. My governor is set at 3300 for some reason.

 

[The Warden]

When I still had the T-19 in my truck, she would go up to 75 just fine...and the governor would kick in somewhere between 75 and 80 mph. Granted you'll be spinning slightly higher RPM's with the C6, but if you can do it safely/without getting caught, I would see if she'll get up to 80 if you don't want to jack the rear of the truck and count revolutions. My old van, which had a 6.9l, C6, and 3.54 gears, would peg the speedo easily (although I only did it once or twice)...according to the calculator, her governor would kick in at about 90

Again, I've never tried to troubleshoot a tach problem (neither the truck nor the van had/have a tach), but it's really sounding to me like you have 4.10 gears in the back.

Here's a question...what vehicle and engine speed does she upshift from 2nd to 3rd at, generally?

 

[david85]

What is your tire size?

 

[Silver Burner]

What is your tire size?

With those RPM's and that axle, he's gotta be running 195/50/16's!!

 

[FORDF250HDXLT]

What is your tire size?

215/85/16

 

[02x72]

215/85/16

You got it

As for shifting RPM/Vehicle speed, I havent paid really close attention as to when it does. I will tomorrow and report back

 

[david85]

That would explain it then

with 235/85/16 tires a 3.55 these trucks should top out at 85 MPH without overdrive.

 

[MIDNIGHT RIDER]

>>> MATH 101 <<<

Knowing the diameter of your tires, the rear ratio, and the high-gear transmission ratio which should be 1:1, if you know exact speed, then engine RPM can be calculated easily.

For that matter, any unknown factor of the equation can be found, providing the other factors are known.

With two unknown factors, then a graph line will show if X=? then Y=? .

 

[Brianedwardss]

Alright, I'll take a stab at this one since I have a C6 as well. These formulas are perfect world in nature, so assume real world results will be slightly different.

215/85R16 tires = 30.4" diameter, or 664 revolutions per mile/5,280 feet.

664 tire revolutions x 3.55:1 differential = 2357.2 driveline revolutions per mile.

60 mph is the best place to calculate rpms, because it's when the driveline turns those 2357.2 revolutions in a minutes time, RPM'S...

With a manual trasmission, theres no slip. But a C6 has torque converter 'slip', and is not a true 1:1 final ratio.

At 60 mph, your driveline is turning 2357.2 rpms. Now to get to the engine rpms, time to figure in the torque converter slip. I know from owning two C6's that typical slip at full speed is between 2% & 7%. I owned an '85 that had a 6.5% slip, my '88 has a 4.5% slip ratio. I'm gonna use 5% as middle ground here.

2357.2 divided by 0.95 = 2481.26 engine rpm's at 60 mph. I'll confirm this, since when my truck had 3.55's, my tach showed 2500 rpms at ~60mph (part of the needle touching on both gauges at the numbers mentioned.) Since you're using a gps, speedo-correctness is out of the conversation.

If I plug 4.10 into the formula, I get 2865.68

If I plug 3.73 into the formula, I get 2607.07

Ford only had a 3.55 or a 4.10 option, but either way I'm not getting anything within any margin of error to 2700 rpm. I've seen tach's show slow on rpms, or be double, but never just a 'hair' fast. Not saying it's not possible, just haven't seen it. (If anyone has, speak up). Maybe the GPS was having a moment. I'd find a nice straight open stretch and see if you can hold 60/2700 for an extended period and see if it seems to be what the truck and gps want to tell you.

My truck came with 265/75R16 tires and 4.10's. I plugged that into the this formula and got 2702 RPM'S at 60 mph...