So what's the ideal cruise RPM/gear ratio?

[JHan]

If a person were to put a 4bt into a light half-ton pickup with stock tires and a 5 speed, what would be the best RPM to shoot for at, say, 60 mph? With 235-75R-15's and a . 76 od (ZF transmission), I'm coming up with 1700ish RPM with 3. 00:1 rear gears. Is that about right, or is it too tall, and I'll be lugging and shifting all the time?

 

[Steve Graham]

I think it might be a little on the "tall" side. Heres how I came up with that.



I use tire dia derived from revs per mile. I use tire data from BF Goodrich tires.

Stock 94 Dodge Diesel 5sp, does 34. 1 mph in 5th per 1000 rpm. Stock 92 Dodge Diesel 5sp does 33. 9 mph per 1000 rpm. My Scout does 34. 8 mph per 1000 rpm, 5sp 3. 73 gears, 33/10. 50/15 BFG muds. The Scout weighs 4300 lbs. I calculate what you would have 36. 8 mph per 1000 rpm. Again with my data it says you would be doing 1620rpm at 60mph but at 70 you have 1920rpm. I would shrink the tire size a notch to get close to the 34 mph per 1000 rpm. IMHO.



Steve

 

[JHan]

I punched your #'s into the spreadsheet I was using, and came up with 57. 8mph at 1700 - does that sound right, or is my math skewed? Hmmm..... The pickup this's going in currently has 31-10. 50's with 3. 50 gears. I was all set to put different tires on it and change the gearing - guess what - it works out right now to 57. 61. Maybe this will be easier than I thought... ... .

 

[Steve Graham]

That last gearing you mentioned is real close to ideal, a bit on the high side IMHO. My spread sheet/data produces 57mph at 1700rpm, real close to yours. Thats 33. 5 mph per 1000 rpm. Your max economical speed is going to be 65 mph, approximately 1940 rpm. After 2000 rpm the fuel rate goes up real fast. Remember as your tires wear your rpms will become higher to maintain the same speed.

You can always change tire size.



Steve

 

[foxracer1]

This site is really helpful if you know all your ratios http://www.5speedtransmissions.com/calculators.html

 

[driverswanted]

This was the one I used on my 1/2 ton ram, nv4500 and 4BT.

http://www.geocities.com/z_design_studio/transmission.html



You can put in all 5 speeds into it, tire numbers, max RPM's, rear ratio.

It's also interactive in that you can move along the MPH scale and see all you potential shift point RPM's or move along the RPM scale and see your speed in any of the five gears. Easy way to pick a tire size if you're not ready to change rear end ratios.

Less than 1450 seams to lug for my 3. 23 rear, 31. 6" tires and the NV4500 in 5th.