I am looking at decreasing my freeway rpm in 5th gear at 65-70 mph to some sort of comfortable level under 3000. The only way I thought of was going with a 15" wheel and a tall thin tire to increase the rolling distance. Has anyone else done this?
I figured I would make a few improvements to the power output of the engine to make up for the loss of power in 5th and hopefully increase the mpg while at it. And I also would find away to make the spedo correct.

Eric I did this on my Mazda; I went a slighter better ride and went to a taller profile tire and in the process dropped a little less than 100 RPM's which in my case got the motor out of an RPM band where it was rather boomy.

Look online there are final drive/speed RPM calculators all over the internet you can plug in tires sizes and see what size you'll need for a given RPM. The ultimate method of course is a 5 speed overdrive, you can lower the rear gear to 4:11 for acceleration and the overdrive will drop the revs..........but in the meantime play with the numbers online.

Now keep in mind an A-series engine will run at 5500 RPMs or more all day long..........if it is a case of engine noise inside the car but some insulation material / Thermal barrier materials made for cars. I think Thermo-Tec is a brand of insulation. Again the motor doesn't care and the gas milage is fine with stock gearing but interior noise on our cars is an issue.

Tom

Fit an LRR tire to that 15" wheel for the best mileage possible.

the best way (not cheapesed) to lower the RPM in 5th is to change the diff to a lower ratio, example; from a 4.1:1 ratio to a 3.7:1 ratio, that will defenatly bring it down but exelaration from start can be a bit slower.

not to put a spoke in the works legally you only can run a tyre 25mm higher or lower than standard .my tyre size is 588mm overall that is the biggest wheel and tyre legally I could run.

Do you think a 165/80 15 is out of the question? I would lower the car to make up for the height change from the tire / wheel combo if needed. I am going down to the auto yard today to scope out a 15 " inch wheel no wider than 5 (I hope), the local tire store said he would mount the 165/80 15 on it and check for clearance issues.

As per legallity, I don't think that would be an issue on the coast of Washington State, USA.

I am going to do some research as per suggested on the final drive/RPM calculators.

I am sure the engine is comfortable at those RPM's , but my sanity is not.

I might have not mentioned this is going on a 1978 stock GX with a 5 speed. I am not sure of the diff ratio.

Found the calculators. Stock tire size 55mph=2566rpm, 65=3032, 70=3265 - Tire size of 165/80 15 55=2300, 65=2718, 70=2927. I was surprized to find out that it takes little more than a 1/2" height to decrease over a 100rpm at speeds of 70mph.

Looked through the owners manual. The above figures are for a 78 B210 Hatchback GX with a 5 speed 3.7 diff, .854 (5th gear).

With this info I found that my speedometer is incorrect, I have been cruising around at about 62mph thinking I was going 55. Happily I have not been issued a citation. Anywho, any ideas of how to fix my odometer, especially if I go to a taller tire?

For the power issue, I just think I will shift into 4rth if needed. I do have a weber, I just haven't decided wether to install it or not.

Yes, there is no law in United States. We can run any DOT tire we want, different rim sizes, etc.

Yes, we been saying for years that your speedometer will be off 5-15%

See Tires and Speedometer

If your goal is to save fuel economy, forget spending hundreds on new tires, and spend $200 on a 5-speed (the going price in WA). It will increase fuel economy more than tires will as it is about 85% speed reduction in fifth gear.

As stated it is a 5spd. I am an old fart that is retired and bored, that just is looking to do something with his car. If I gain a few MPG (nose to the prius) so be it. But, the main goal is a smooth quite ride on the freeway. Me and the misses plan on going cross country with B210, so it needs to be a pleasant ride.

Then definitely get the LRR tires. They are good for something like 10% fuel economy on the Prius.