Best 1990 rear axle ratio for highway speeds, gas mileage and top speed.
#1
Best 1990 rear axle ratio for highway speeds, gas mileage and top speed.
Had a 1990 Vette that was far too busy at motorway speeds, so I'm looking at another car and want to know what axle has the lowest rpms at highway speeds.
#2
Team Owner
What year are you considering? Coupe or Convertible? Manual or auto trans?
Some C4's had a 2.59 as standard, a 2.73 was an option on some, and 3.07 was an option on some and some had that as standard. Most auto trans cars would have these ratios.
Some C4's had a 2.59 as standard, a 2.73 was an option on some, and 3.07 was an option on some and some had that as standard. Most auto trans cars would have these ratios.
#3
It's another 1990 auto.
So which is best at Highway speeds? E.g. Lowest rpm more comfortable cruising speeds.
Last edited by Tony Trans Am; 02-17-2018 at 01:55 PM.
#4
Le Mans Master
The lowest number makes for the lowest rpm.
So for the C4's the lowest rear axle in an automatic car is 2.59
The RPO codes can be found on the SPID label in the car.
I suspect that the following will be the ones you might see in a '90 automatic.
3.07 = G44
2.73 = GU2
2.59 = GM1
If you get another one make sure to let us know what you get.
Good luck.
So for the C4's the lowest rear axle in an automatic car is 2.59
The RPO codes can be found on the SPID label in the car.
I suspect that the following will be the ones you might see in a '90 automatic.
3.07 = G44
2.73 = GU2
2.59 = GM1
If you get another one make sure to let us know what you get.
Good luck.
#7
Melting Slicks
You had a transmission that was not shifting or the tach needs repair and a very common problem. What was your approximate mpg?
#8
What sort of rpm can I expect @ 70/80
Last edited by Tony Trans Am; 02-17-2018 at 04:54 PM.
#9
Pro
I'm not sure what your car was or what gears were in it but I have owned 5 C4 corvettes including a ZR1 and an LT4 and all have stock gearing from the highest gear to the lowest gear offered by GM and I never once thought on any car it was geared too low or had high cruising RPM that made me nuts. I currently have a 90 auto with 2.73 and it's as nice as could be to take a cruise. All I can say for sure is NONE of the stock gears produce a 4000 RPM cruise from a motor that has a 5500 RPM redline.
#10
Team Owner
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St. Jude Donor '05
x2 bet your trans wasnt hitting OD
You need to first see if that is a problem...once solved if you still dont like it find the gear you have now & go from there.
check out wallace racing calculators for starters
You need to first see if that is a problem...once solved if you still dont like it find the gear you have now & go from there.
check out wallace racing calculators for starters
#14
#15
Race Director
If it's like my SUV automatic, the transmission can shift but not necessarily achieve torque converter "lock-out". During the first 5 min of COLD operation with my SUV, lock-out won't occur. I assume it's for a combination of power and accelerated warm-up (emissions). Still...this post shows that would only make your rpms about 300 higher. Not enough to account for 4k rpms. Seems MORE likely you weren't getting to 4th!
Now...to answer your question:
1700 at 70mph with 2.59's means close to 2k rpms at 80mph (This will return the best MPGs!!!!)
Swap from 2.59's to 3.07 gears and you're around 2400rpm -- which was reported above.
Even with 3.54's (which is the highest "performance" stock rear gear I know of), 2700 rpms would be tops -- at 80mph.
Again...you'd have to have rear end ratio in the 5's to see 4k rpms. No one does that. Not sure you can even BUY anything much over 4.30 gears. Your last car needed service.
Last edited by GREGGPENN; 02-20-2018 at 12:53 AM. Reason: misspelling
#16
Burning Brakes
Absolutely 100% something was off about your last car, as the others above me have stated.
I have an A4 w/ 3.07 and no way is it at 4k RPM at highway speeds. Either the PO swapped out the rear end with WAY more gear, or your OD wasn't working. I've cruised mine at 80 MPH and its plenty mellow, just can't pin it down real low like the 6 speed guys can for example.
3.07s won't get the mileage of the 2.59's, but they get close enough and are much quicker cars.
I have an A4 w/ 3.07 and no way is it at 4k RPM at highway speeds. Either the PO swapped out the rear end with WAY more gear, or your OD wasn't working. I've cruised mine at 80 MPH and its plenty mellow, just can't pin it down real low like the 6 speed guys can for example.
3.07s won't get the mileage of the 2.59's, but they get close enough and are much quicker cars.
#17
Safety Car
(25.8 x 3.1416)/12 = 6.754 ft/rev or rear tires
(80 x 5280)/60 = 7040 ft/min traveled at 80 MPH
7040 / 6.754 = 1042.34 RPM of rear tires at 80 MPH
2.59, 2.73, 3.07 were the common factory auto ratio's available in 1990
1042.34 x 2.59 gear ratio x .7 = 1889 RPM of engine at 80 MPH (.7 is overdrive ratio)
1042.34 x 2.73 gear ratio x .7 = 1992 RPM of engine at 80 MPH
1042.34 x 3.07 gear ratio x .7 = 2240 RPM of engine at 80 MPH
To get 4000 RPM at 80 MPH in overdrive you would need a final drive ratio of (4000 / .7) /1042.34 = 5.48 rear ratio and that would not have been done by anyone.
If the car had 3.73 gears and was not shifting into overdrive it would be close to 4000 RPM at 80 MPH
1042.34 x 3.73 = 3887 RPM of engine at 80 MPH with aftermarket 3.73 gears and no overdrive, this is a possibility.
I actually believe you had an incorrect reading tach, that is a common problem in a 1990 corvette.
Math can help you trouble shoot all kinds of problems.
Last edited by bjankuski; 02-20-2018 at 08:34 AM.
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#19
SPID label location on a C4 varies. A '90 on the back-side of the console door. Stayed there through '94, '95 + in a rear compartment as was an '84 (if at all and '85), '86 thru '89 on the console door also.
#20
Burning Brakes
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My dad's 90 ZR-1 had an issue with the tach. It would read a 2500 idle speed. It would be pegged about .5" past the 9000 line and still never hit the rev limiter in the car. He has since remedied that with new components.