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.

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.

Hi,
It's another 1990 auto.
So which is best at Highway speeds? E.g. Lowest rpm more comfortable cruising speeds.

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.

What sort of rpm can I expect with a 2.59 car at 70/80 mph? The reason I sold my last 90 was it seemed ridiculously low geared, it was doing circa 4000+ @ 80mph.

:iagree:
I hope you understand that it's a trade-off. The lowest number (ratio) also makes for the slowest acceleration.

:cool:

You had a transmission that was not shifting or the tach needs repair and a very common problem. What was your approximate mpg?

I never measured the mpg, it was definitely that rpm, I could feel it through the car.
What sort of rpm can I expect @ 70/80

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.

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

My '88 A4 had a 2.59 and it was about 1700 rpm at 71 mph.

It also got about 30 mpg on the highway cruise at about 70 mph.

That's going to be hard for him to do since he states he sold it.

Mileage doesn't prevent problems. No way it could have been turning 4k at 80mp in 4th....even with an auto. If you're going to assume low miles guarantees proper operation, then assume it didn't have farm tractor gears. Gearing would have needed to be in the 5's.

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.

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.

Something was wrong with your car, or it was not factory stock. The RPM at 80 MPH will be as listed below if everything was working correctly with an auto:

(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.

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.

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.