NewbVetteGuy

I've already replaced my C3's wheels and tires; I finally went back and looked at exactly what I bought: Kumho ECSTA PS31 255 45ZR18s; 27" high. (This was before there were really any reviews of the tires; apparently, the side walls are a bit soft for the performance enthusiasts but given the 1979 intertube balloon tires and 15" wheels that WERE on the car (literally the fronts were still the original, about-to-explode-at-any-time factory tires); it's an incredible improvement -no offense to any C3 balloon tire enthusiasts.)

Other info: rear axle gears-3.55; 4th OD gear in 4L60e trans =0.70

Now I have a few questions:

• How does the height of this wheel/tire combo compare to what was stock? Are these going to throw off my speedo by much? If so how much?
• If I've done the calculation right, the engine should be at 2166RPM @ 70, and 1702RPM @ 55 MPH -does this sound right?
• Will my lockup torque converter further drop the RPMs or does this calculation already assume zero slippage from the torque converter being locked? (if the former does anyone know how to calculate RPMs after TC lockup?)
• These low cruising RPMs mean that I should definitely select a stock / low RPM TC stall speed and mild cam that focuses on low to mid RPM torque so that I can actually get good fuel economy while cruising, right?

On another forum and thread someone indicated that overdrive would not improve my fuel economy unless the torque curve's max is at my cruise RPM, which honestly initially sounds reasonable but after closer thought sounds a bit crazy; I can't imagine that there's a car on earth that has such a low torque peak. -If that was the case I don't think cars would have overdrive at all.

I can understand if you want there still to be quite a bit of torque curve "meat" in your target cruise speed if you are trying to maximize fuel economy; I can understand that in a purely theoretical world perfectly matching your torque curve peak to your cruise RPM would give you some idealized fuel economy but I think that would be an awful, awful car to drive; no? (If I call CompCams and ask for a cam rated for 0-3,000 RPM, I'd be afraid they'd put me on speaker phone so the whole office could make fun of me; I have no desire for such a narrow ranged CAM, not that I think one really exists.)


Can anyone shed some light on what appears to be theoretical musing of one internet poster that's confusing me a bit?



Thanks,
Adam

resdoggie

Stock tires are 27". Wheel diameter doesn't matter. Speedo is fine.
RPM at speeds you noted sound about right in O/D.

NewbVetteGuy

Awesome! Thanks.


Any idea about how much RPMs might drop when the lockup TC kicks in?

Or about the idea of one internet poster that since these cars were built for high RPMs that lower RPMs won't actually help with fuel economy as the reduced cruise speed will be further from the torque curve peak? (Note: This guy appears to be a part of a cult that worships the TH350 and plans to drink it's transmission fluid when some comet passes by earth again.)

-My guess there is that as long as you're not lugging or dropping the RPMs crazy low or having to downshift because you don't have enough torque/hp to push the car through the air in OD you're still going to come out ahead with OD.


Adam

cv67

Cam converter and gear the car for the powerband you want NOT the cruise rpm in OD alone or youll be disappointed. It will be what it is

Not every motor is happiest or efficient at 2k or less.

rastafford3164

With regards to your question #1, RimsnTires has a calculator on their site that let's you try different wheel/tire combos versus stock and will tell you how it affects the speedo.