Jim78SA

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.

Revi

If your car has a 3.55 or 3.70 rear end in it, that's about right.

Jim78SA

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.

65GGvert

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.

BKarol

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?

Sluefoot

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'!!

68/70Vette

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.

lowbuck72

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.

Jim78SA

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.