Thanx for the advise on the insulation, I had thought about that. I am going to try under the hood, inside the doors, all of engine compartment. I am triing to figure away out to insulate under the rear wheel vinyl arches, I think that a good amount of road noise is produced there.

I will try to post some pics of my progress.

Eric a couple of things you can add for cross country comfort: Taller profile tires besides lowering the RPM's added gas milage the softer sidewall will add to the ride. Also you can add an air dam. Also because our cars aren't long on insolation check McMaster-Carr or even Loews and add the material under the carpet at the firewall and floor. Adhesive backed closed cell rubber or foam inside the door skin. (pull of door panel roll up window and add to outer skin) On a 510 I had for a while I used 2 layers of the stock firewall insulation and 1/8 acoustic foam under the carpet..............loads better.

As for the speedo error, your car should have a 3.90 rear gear, use the online programs right down the RPM for 25, 35, 45, 55, 65, 70 &75 then before your trip you can make note of what the speedo says, then take little labels and stick em to the speedo face (On my B210 I used little red dots about 3mm in size) After a While you'll know that 27=25 or 55=62 etc.

Tom

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

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.

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.

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.

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.

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.

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.

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