[Steve R (Rizrtse)]

My car is running 3,500 to 3,700 RPM at 65 MPH and seems to get to 4,000 quickly at 70. A mechanic told me that running my 383 hp motor (rebuilt to stock specs except for a .040 bore and slighlty modified cam)with 3:55 gears with 14" rims and 60 series tires will produce this condition. He suggests getting 15" rims with 75 series tires to reduce the rpm by at least a grand. Another mechanic says change the gears back to the 3:23. Dodge certainly produced enough 3:55 or higher gear ratio cars with 14" tires. Could they all be "at risk" at high speeds for engine burnout. I need to go 30 miles to local shows at best. Any thoughts Dave or new friends?

[Dave-R (Roppa440)]

Who says you are doing 4000rpm at 70 mph? You looking at the factory tach? I have never seen one that did not read high.

I have this thing on my web site I call a maths Spreadsheet. If you have Microsoft Excel you can open it.

It has tables on there where you can plug in infomation like tyre size and gear ratio to see what speed you are doing etc. Have a look at it.

You do not say what your tyre size is but if you really were running 4000 rpm at 70 mph your rear tyres would have to be only around 22.5 inches in diameter. with 14" wheels and 60 series tyres they would have to be something as narrow as a 185/60/15 to fit the facts. They sure don't look that small!

Much more likely is that your rev counter reads high. However I would never use 60 series tyres on 14" wheels unless the wheels and tyres were very wide. They would look too small in the wheel arches otherwise. Use a 70 series tyre if you can or use 60 series on 15" wheels.

Using my spreadsheet you can work out that something like a 225/70/14 is 26.5" tall and that will give you an rpm of about 3,400rpm at 70mph. Your engine will live all day long no problem at that rpm.

[Steve R (Rizrtse)]

I have P245/60/14 series in rear and P235/60/14 in the front. With 3:55 gears the car seems to be reving to high at 3,700 - 4,000. I disagree on the look comment. My car sits nicely on the 60 series tires, all around. A 70 series on 15" wheels may require some spring arcing adjustments. My springs, like everythng else is new but the car rides a bit low but with the 60 series it works. Do you think I can save the 7"x14" rims and go with a taller tire and save some rpms? Or, do I have to peddle these beautiful rims and get the same ones in 15x7? I agree on the tach being off but the car feels like it has 4:11s!

[Dave-R (Roppa440)]

I suggested a 70 series tyre on 14" rims or 60 series on a 15" rim. Your car came from the factory with 70 or 75 series tyres.

The wider your tyre the taller the side wall is.
Remember the side wall is 60% the width of the tyre on a 60 series. That is why yours look ok but my preference would be for a taller tyre which would fill the wheel arch more. But that is down to personal taste.

Your tyres are about 25.5" tall. I would expect you to be pulling 3400-3500 rpm at 70 mph. With 4.10s you would indeed be pulling over 4000rpm but the engine will still live all day at that unless there was something wrong with it.

[Steve R (Rizrtse)]

I'll try a 70 series but unfortunately the widest 70x14 is a P225. A little thin for my tastes. Trying new tires will be cheaper than ordering new rims and rings that cost me $900. I just want to reduce the 3,800 or so RPM at 70mph to 3,200. This should do it. If I can get someone to but these this summer I'll order the 15x7s. Anyone interested? They are Stockton Wheel custom Magnums.

[Dave-R (Roppa440)]

225/70/14 is only 26.5" diameter so your rpm will be 3340 at 70 mph. Looks like either gigger wheels or the 3.23:1 rear gears.

With 3.23 gears and your existing wheels/tyres you would have 3150rpm at 70mph.