Tire Size vs Engine RPM vs Rear End Ratio

[Chatt69chgr]

I have been playing around with tire diameter and engine rpm calculators.  For 15 inch rims, I looked at 235 70 15 tires for the rear and tried 3.23 and 3.55 ratios.  Tire diameter came out around 28 inches.  My final transmission drive is 1:1.  I see at 70mph that for the 3.23, you have around 2600 or so rpm.  For the 3.55 you get around 3000 rpm.  These are not exact figures but just ballpark figures.  Now for a 60 series tire, say a 255, the tire diameter is around 27 inches or so.  Here you are running around 2800 and 3100 rpm respectively for the two different ratios.  My question is:

What is a good comfortable rpm to run down the interstate 75 miles to a meet when you would like to run 70 mph so as to not get run over on the interstate (440 engine).  I am thinking that if one wants to do anything that involves getting on the interstate and going 50-75 miles, you would want the 3.23 rear end and probably the 235 70 rear tires.  If you were strictly staying in town then the 355 or higher might make a lot of sense.  I am new to this and don't have any experience to fall back on.  What have the rest of you done? 

[daytonalo]

Keisler 5 speed , awesome 1/4 mile and killer top end , thats why I dumped my hemi box

[Chatt69chgr]

Keisler not an option as I have a 4-speed and bell housing already although I see how that would be the best solution if you had the money.  I guess I am looking to see what others that are keeping their 4-speeds have done in this area.  Thanks.

[firefighter3931]

The 3.23 is a popular hwy gear. Most guys use a 275/60/15 tire on the back which is 28in tall. Using a 28in tire will knock a 3.23 ratio down to 3.00 which is great for cruising at speed. As long as the engine is in it's powerband you will be fine. It really depends on the cam profile and specificly where the cam makes power. A big cam with lots of duration won't be happy cruising along at 2200 rpm....



Ron

[Chatt69chgr]

Thanks firefighter.  That's exactly what I wanted to know.  May I ask what the best choice of front tire would be to go with the 2.75/60/15 on the rear?  And would you happen to know what the best backspacing's would be?  I would assume we are talking about 15X7 rims on the front and 15X8 on the rear (correct me if I am wrong).  I am considering either Torque Thrust 11 or D's or Magnum 500's----- all 15 inch.

[firefighter3931]

Thanks firefighter.  That's exactly what I wanted to know.  May I ask what the best choice of front tire would be to go with the 2.75/60/15 on the rear?  And would you happen to know what the best backspacing's would be?  I would assume we are talking about 15X7 rims on the front and 15X8 on the rear (correct me if I am wrong).  I am considering either Torque Thrust 11 or D's or Magnum 500's----- all 15 inch.

Tire choice is subjective....some prefer a low/wide profile on the front while others like tall/narrow. Personally, i like the tall narrow look so a 225/70/15 tire is what i like to use. A 235 or 245 60/15 will work on the front as well but it will be wider than the 70 series tires.

For rear wheels ; a 15x8 with 4.5in backspace works well. On the front a 15x7 with 3.5-4.0 in backspace will get the job done.




Ron