RPM Running High?
Thread Starter
Pro
Joined: Feb 2005
Posts: 616
Likes: 2
From: Sedalia MO
RPM Running High?
I have a 78 SA Vette. While cruising on the road at 70 mph, my tach reads between 2800-2900 RPM. That seems high to me. Not to mention, I think I am going slower than what the speedo reads. The car currently had 245/60R15 on all four corners, I just ordered two 275/60R15 for the rear in hopes of helping, but is mt current tire size really making it that far off? I see other members with the same setup and have not heard of such problems.
Thread Starter
Pro
Joined: Feb 2005
Posts: 616
Likes: 2
From: Sedalia MO
See, that is what I am thinking, but I find it hard to believe someone would install 3.55s and still keep stock heads and torque converter. I mean, those gears are useless without a higher RPM torque converter.
Team Owner
Joined: Jul 2006
Posts: 22,192
Likes: 4,180
From: Kannapolis NC
You can check your speed with a handheld or automobile GPS. You can check your RPM's with a dwell meter on rpm function selection, or any other electronic RPM measuring device. Your RPM's sound pretty close to right to me.
Melting Slicks
Joined: Sep 2004
Posts: 3,227
Likes: 670
From: Northeast
I have a 78 with 3.70 & 255/60/15 tires and run @3300 rpm at 70mph.
It appears in the specification guide that if you have a California car 3.55s were installed in a auto L48 or above 4000 feet altitude.
Curious, what is your suffix on the engine pad? or the code in the top left hand corner of the emission label on the firewall?
It appears in the specification guide that if you have a California car 3.55s were installed in a auto L48 or above 4000 feet altitude.
Curious, what is your suffix on the engine pad? or the code in the top left hand corner of the emission label on the firewall?
Racer
Joined: Dec 2014
Posts: 374
Likes: 42
From: Coldwater Mi
My '79 L82, 4 spd close ratio, 3.70 rear...almost exactly the rpm you're saying!
I didn't buy my FIRST Vette w/ great gas economy as a priority....so I just get gas when it need is....and keep on keepin' on....runnin' thru the gears & SMILIN'!!
I didn't buy my FIRST Vette w/ great gas economy as a priority....so I just get gas when it need is....and keep on keepin' on....runnin' thru the gears & SMILIN'!!
Team Owner
Joined: Jul 2004
Posts: 40,412
Likes: 792
From: Redondo Beach, California
After 20 plus years of not driving it, and having restored it, I was dishearted to find my 68 was cranking about 3000 rpms at 70 mph. I didn't recall that it cruised with this high of an rpm. Out went the Muncie and in went a THO600 five speed. Now about 2000 rpms at 70 mph. Performance through the bottom 4 gears is unchanged. ZZ4 engine.
Drifting
Joined: Jun 2005
Posts: 1,950
Likes: 498
From: Chino Hills CA
Hey Jim, just find yourself a piece of freeway that is exactly one mile. drive it with your speedo at 60 and time yourself to see how long it takes. Divide 60 by the number of seconds it took you and you'll get your speedometer error (or correction factor). Multiply that by 60 and you'll get how fast you're really going. Say it only took you 45 seconds. Your correction factor is 60/45 or 1.33. Your actual speed was 60 x 1.33 or 80 mph. For more accuracy find a 3 or 5 mile stretch of road.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Pro
Joined: Feb 2005
Posts: 616
Likes: 2
From: Sedalia MO
Sorry itvtook me so long to reply. I have been the interm first shirt for my squadron this last week, and was quite busy. Thank you for all the advice. I ordered a set of 275/60R15 for the rear and ehile I put them on, I will do the drive shaft rotation test to set what I come up with.