RPM at speed
12-12-2010, 05:13 AM
#1
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RPM at speed
This is my first corvette and I'm curious to see what other owners rpms where at for 60 mph or other hwy cruising speeds. I have verified its the stock rear end for this year. Engine is the 350. So at around 60mph im running at approx 2800 rpm. It sounds like the engine is working pretty hard to maintain that speed. Is this normal?
12-12-2010, 08:17 AM
#2
Le Mans Master
I'd say that's normal. Transmission technology has come a LONG way in the past 30 years. The newer 4, 5 and 6 speed overdrive trannies bring the RPM's down to less than 2000 at highway speeds.
Gary
Gary
12-12-2010, 09:48 AM
#3
Drifting
I see from your profile that you have an '80, but you didn't say whether your trans is manual or auto. I have an '80 with the stock rear end and stock Borg-Warner Super T-10. Final drive ratio is 1:1.
Mine runs about 2500 rpm at 60 mph. A difference in your tires vs. mine could explain the difference. The engine isn't actually working that hard to keep your car at 60 mph at that rpm.
BTW, the high rpm at cruising speeds is one of the two main reasons C3 owners put in Keisler 5-speed conversion kits. The 4th gear is 1:1 like stock, but 5th gear is overdrive (either .64, .67, .68, or .82 depending on which version you choose). With the popular TKO-500 5-speed (.68 OD), your engine would be running in the 1600-1700 range in 5th gear at 60 mph.
The other reason for the 5-speed conversion popularity is that 1st gear is higher. Switching to the TKO-500 is like converting your stock rear ratio from 3.08 to 3.99.
Keisler is one of the Corvette Forum sponsors. Just click on their link on the left to learn more. You'll find the link to their speed analyzer there too.
Mine runs about 2500 rpm at 60 mph. A difference in your tires vs. mine could explain the difference. The engine isn't actually working that hard to keep your car at 60 mph at that rpm.
BTW, the high rpm at cruising speeds is one of the two main reasons C3 owners put in Keisler 5-speed conversion kits. The 4th gear is 1:1 like stock, but 5th gear is overdrive (either .64, .67, .68, or .82 depending on which version you choose). With the popular TKO-500 5-speed (.68 OD), your engine would be running in the 1600-1700 range in 5th gear at 60 mph.
The other reason for the 5-speed conversion popularity is that 1st gear is higher. Switching to the TKO-500 is like converting your stock rear ratio from 3.08 to 3.99.
Keisler is one of the Corvette Forum sponsors. Just click on their link on the left to learn more. You'll find the link to their speed analyzer there too.
12-12-2010, 10:40 AM
#6
Team Owner
It largely depends on your use of the car and where you live....like me for a case in point....
I had a 336 rear and a Muncie in my '72...at freeway speeds in Maryland outside DC you can hit 80 for maybe 30 seconds, then back in the soup again, you maybe keep 60 at 2800 rpm or so....
but when moving to FLORIDA here, we cruise at 80 for hour after hour....too much, even the trip coming down I 95 was 10.5 hours at 4000 rpm.....blast hell out of a engine quick doing that....car had FI on it, and got 24 mpg though....
but I put in the cheapest route for overdrive down here 13 years ago..it was a automatic, 700 was first, but never did right by me, so now it's a 200 4r.....80 at 2500 rpm.....cruise all day...
I had a 336 rear and a Muncie in my '72...at freeway speeds in Maryland outside DC you can hit 80 for maybe 30 seconds, then back in the soup again, you maybe keep 60 at 2800 rpm or so....
but when moving to FLORIDA here, we cruise at 80 for hour after hour....too much, even the trip coming down I 95 was 10.5 hours at 4000 rpm.....blast hell out of a engine quick doing that....car had FI on it, and got 24 mpg though....
but I put in the cheapest route for overdrive down here 13 years ago..it was a automatic, 700 was first, but never did right by me, so now it's a 200 4r.....80 at 2500 rpm.....cruise all day...
12-12-2010, 10:51 AM
#7
If the OP is measuring actual speed with a GPS or stopwatch, that's different.
12-12-2010, 11:08 AM
#8
Team Owner
Not true, Mike. A tachometer reads engine RPM. That translates directly to driveshaft rotations. The rear gear and tire size changes cause the relationship between the driveshaft rotational speed and the vehicle's forward speed to vary. With a larger [than designed] tire size, the car will be going faster than the speedo shows {and vice versa}.
ALL speedometers have to be compensated for tire size changes.
ALL speedometers have to be compensated for tire size changes.
12-12-2010, 11:15 AM
#9
Team Owner
Hmmm. Mike said that "tire size makes no difference in speedo indication". And, to be precise, he is correct. The speedo indication is merely a function of driveshaft rotational speed. Sooooo... the speedo will continue to indicate a speed directly related to the [consistent] driveshaft output, BUT the actual speed of the vehicle will be different, depending on tire size [and rear gear ratio]. I guess, that means that Mike is correct!!
{But tire size changes still cause the speedo to be in error with actual vehicle speed.} Now, is everyone confused?
{But tire size changes still cause the speedo to be in error with actual vehicle speed.} Now, is everyone confused?
12-12-2010, 02:57 PM
#13
Former Vendor
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St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
How about this one to confuse things... All speedo's require 1000 rpms of the tach cable to show 60 mph if they are calibrated correctly. From 1953-1982
It's what makes the cable move and how fast that determines the rest... IE: speedo gears, rear end ratio.
Willcox
It's what makes the cable move and how fast that determines the rest... IE: speedo gears, rear end ratio.
Willcox
