LT1 RPM Question

[Todd Stevenson]

Greetings All,
I have a question. If a 1971 LT1 with M22 trans and 3.70 gears is traveling down the highway at 60 MPH, how many RPM's should the engine be turning at that point, generally speaking. Please let me know your experiences.
Thanks,
Todd
#40534

[Warren F.]

Re: LT1 RPM Question

Todd:

Sounds like you've got a rare ZR1 edition of the LT-1 option! I have a '71 LS6 with M22 & 3:70 gears, I also have original F70x15 Goodyear Wide Tread tires as well. Tach says 3100 rpm's @ 60 mph.

[William C.]

Re: LT1 RPM Question

a 3.70 is very close to 20 mph per 1000 rpm in 4th gear.

[Duke W.]

3.7 x 775 = 2867.5 *NM*

[bruce11495]

Re: LT1 RPM Question

From someone who has an LT1, its' generally half the speedo. 60MPH-3000rpm, 70mph-3500rpm etc.

[Terry M.]

Re: LT1 RPM Question

Bruce,
The ratio of rpm to speed that you mention is typical of 4:11 gears.

[Gary S.]

Re: LT1 RPM Question

You can also estimate your numbers using the following web sites:

http://www.idavette.net/tech/ratioc.htm &

http://www.angelfire.com/fl/procrastination/rear.html

If you know your tire diameter, then fill in the gaps.
Gary

[Duke W.]

Re: LT1 RPM Question

I don't know why you guys want to make this so complicated. Just multiply
the OE tire revs per mile times the axle ratio to determine revs at 60 as
I did in my "NM" response higher up the thread. The OE tire revs per mile
are:

53-64: 760
65-72: 775
73-82: 760 (775 for 255/60R-15)

Speedometer gearing for all tires is the same; 775 rev/mi tires just adds about 2 percent error on the high side.

If you know that your speedometer and tachometer are reasonably accurate you can rearrange the formula and compute axle ratio from speedometer and tachometer readings.

Duke

[bruce11495]

Re: LT1 RPM Question

I've had the car for 20 something years and thats what I remember it as, give or take a couple 100rpm. I know the speedo is dead on after running it on the dyno.........

[Barry Chappell]

Re: LT1 RPM Question

I've owned my '70 LT-1 since new. M21, 3.70. Speedo has always read 2,700 at 60 MPH. Duke's formula is correct. My tach could be off by 100.
Unless GM screwed up my speedo and/or speedo gear, then 2,700-2,800 is correct.
Barry

[Todd Stevenson]

LT1 RPM Question

All,
I thank you for your responses. Based on what I have read I guess mine is OK, it reads 2900 RMP at 60 MPH. It is very interesting, however, how many different techniques there are in figuring this out. Thanks for the education.
Todd

[Todd Stevenson]

LT1 RPM Question

Thanks for the response. The RPM's seemed to be high, but based on the
responses I've received it seems to be right in line. My car originally was
set up as a what appears to be the ZR1 (HD Alum Radiator, F41 suspension,
radio delete and CGY engine code) but I have no build sheet or
protect-o-plate and my research stops in 1987 in GA. I have not been able to
figure out how to get info from GA DMV. Until I find some documentation
proving it is a ZR1 I will treat it as a plain-jane LT1 and enjoy it just
the same
Thanks,
Todd