Car: 89RS 305 auto, completely stock

Speedo and Tach confirmed accurate (radar and analyzer)
700R4 trans - 0.70 final ratio
Differential has "6HP" code, indicating 2.73

70 mph = 2200 RPM and 80 mph = 2500 RPM

By calculation, speed should be higher for that RPM or RPM should be lower for that speed if 2.73. Am certain the rear end has never been touched, but calculations indicate an axle ratio of around 3.45, which would be a "6HL" axle. Could my axle be mismarked? Is it possible to have this much power loss? If so, where? Torque converter? (seems unlikely)

Did rotation check with wheels lifted and trans in neutral. Driveshaft turns 1.25 for every 1.0 wheel turn which defies any ratio logic. Car pulls really well in lowers gears indicating good power transfer.

This doesn't cause me any particular grief, other than the confusion.

 

original content removed due to the moderators lack of tolerance for the truth.

 

BTW, I'm confident about the intended ratio based on the marked axle code, based on the "GU2" RPO code, and based on ownership since mile 1.
So then, let's presume that 2.73 is actual, absent any evidence to the contrary, how about you share your explanation for the RPM discrepancy?

 

the only thing that can account for the discrepency in speed vs rpm (asssuming the given info is in fact correct), is the size of your tires. what size tires are you running? i believe your tires are too small (not tall enough).

 

Just checked that. The RPO code "QPH" calls for P215/65/R15 and that's what is presently installed. They're nearly new and measure at 26" diameter. One thing that I'm wondering, not being familiar with the inner workings of a differential, is whether there's any possibility of controlled slippage. Working through the numbers from back to front I get:

tire circumference= (((215/25.4)x.65x2+15)x3.1415)/12 = 6.81 ft
80mph/60 x 5280 ft/mi = 7040 ft/min
7040/6.81 = 1034 tire RPM @ 80 mph
1034 x 2.73 x .70 = 1976 engine RPM

2500rpm/1976rpm x 2.73 = 3.45, a 26% error. I'm not sure what voltage source feeds the tach but perhaps it responds differently when the engine is under load vs. no load? If I knew where to find that signal I could check in motion.

 

No, considering you've been working on cars since I was in diapers that's a surprise.
Now back to the questions, No the rear differential can't slip and cause increased rpm, if you want to nit pick that comment let me add this- it can if it's broken but it won't be consistant.
Now looking at my calculations-- rear gear x od ratio x 336 for standard trans or locked torque converter or 355 for unlocked torque converter divided by tire diameter = rpm.
In your case 3.42 x .70 x 336 x 70 divided by 26 = 2166 rpm. This looks like you have a 3.42 ratio.
Turning tires and looking at driveshaft revolutions is not always accurate with an open differential.
This is why I suggest you remove the differential cover and verify the gear ratio. Why guess about mechanical problems that may or may not exist when this one simple check may hold all the answers.

 

True!

I had a similar issue when I first got the firebird. Although I had a 2.73 ratio, my highway RPM @ 75 MPH was around 3k TC unlocked, TH200c transmission. By all the calculations I should have had a 3.23 ratio rear end with those numbers. Untrue! I had a 2.73 ratio and the calculations done by myself and others on the board were incorrect.
The best thing to do is remove the cover and check, besides when was the last time the gear oil was changed anyway?