ajusaf

I have a auto 90 crx dx. I get about 29-30 mpg. I am currently changing the 02 sensor anddoing a major tune up to improve mpg. I notice when i do 70 on the highway to get to work my Rpm is at about 32-3300 rpms so therefore I know iswasting alot of gas. On my mustang same speed on 5 gear i'm hitting 2000rpm So, I was thinking maybe using a HF manual tranny and throwing it in my car to improve MPG since witthe HF they get about 50-60 mpg. good idea or bad and how much work would it be for me to do?

augy

I don't know what year your mustang is or what engine is in it, but it's a very safe rule of thumb that the american-designed engines are higher displacement engines with a longer stroke, higher torque, and therefore, not nearly as high revving as a japanese engineered engine. Think about it-- a typical japanese motorcycle engine redlines at around 13000 (thirteen thousand) RPMs and a comfortable around-town RPM is 6000. My ex500 got 55+ mpg with these exact RPM numbers, even with my fairly aggressive riding. So, the point is, just because it's running at higher RPMs does NOT mean that it's using more gas or is less efficient.
As for using an HF tranny, the HF's rediculous gas mileage is due to it's engine tuning more than its transmission's gear ratios. So, if mpg is what you want, get the stock engine from an 89 crx HF (the 89 stock engine is tuned to get 50 mpg freeway with its hf tranny). The 90 crx hf gets lower mileage; 42 freeway if I remember correctly.
The gear ratios of the 89 tranny are
hf dx si
1st 3.250 3.250 3.250
2nd 1.650 1.894 1.894
3rd 1.033 1.259 1.259
4th 0.823 0.937 0.937
5th 0.694 0.771 0.771
Reverse 3.153 3.153 3.153

have fun.

Edit: Also note that an automatic will typically get slightly better mileage around town than a manual.