Speed vrs tac

[olgraybeard]

I have 4.88 axle ratio with 38 x 13 super swampers 16" rims what speeds can I expect at 3000, 2500, 2000, my speedo is broken. You give me one I can figure the rest. also with this combo is it unusual to have almost no power going up hills, in 4th, 230cu in engine. Would shorter tires help alot or is it just the nature of the beast

[Charles Talbert]

I have 4.88 axle ratio with 38 x 13 super swampers 16" rims what speeds can I expect at 3000, 2500, 2000, my speedo is broken. You give me one I can figure the rest. also with this combo is it unusual to have almost no power going up hills, in 4th, 230cu in engine. Would shorter tires help alot or is it just the nature of the beast

I'd say you have pegged it fairly close, one should expect almost no power going up hill in 4th with that combo powered by a 230.

[olgraybeard]

does 50 mph at 2500 sound about right, anyone with a speedo and a tac, with this combo verify

[Chewie]

Just doing the math...

336/38=8.84
So 8.84 x 4.88gear x 50mph = 2156 rpm
x 55mph = 2372 rpm
x 60mph = 2588 rpm

Don't know for sure. Just sitting here with a calculator, but the numbers should be pretty close.

[W_A_Watson_II]

My calculations say you should be about 56 MPH at 2500 RPM.

My 5.83's and 29.5" tires put me at almost 49 MPH at 2500 RPM

[Frank Irons]

I also run the 38" Swampers on my M37. I am running 5.83s with an overdrive NV4500 and 50 mph is 1850 rpms and 60 mph is 2200 rpms. My son was here today for father's day and I calibrated my electronic speedometer with his help. What a nice present! I have been waiting for his help because I couldn't work the speedo and drive the truck at the same time.

Frank

[W_A_Watson_II]

Frank, WOW, I missed the Overdrive when I looked at your truck, nice. I always keep hunting for 5th gear. What spedo do you have?

[Frank Irons]

Frank, WOW, I missed the Overdrive when I looked at your truck, nice. I always keep hunting for 5th gear. What spedo do you have?

Will,

It is an electronic VDO and is calibrated by either inputing the number of pulses (if known or calculated), or driving a marked mile and letting the speedo calculate the pulses. Now that I know how many pulses my sender emits per mile (127000) I can fine tune up or down. I am running the 2 wheel Chevy tailshaft on my trans (shortest NV4500 configuration) which has a 40tooth speedo sender in it. I had calculated the number of pulses per mile and came within 200 pulses the auto calibration came up with.

Frank

[olgraybeard]

I found this calculator on the web it agrees with chewie and with the flow of traffic around here http://wahiduddin.net/calc/calc_speed_rpm.htm I'm going to have my son pace me to double check I made up this data plate for my dash board so I won't forget (I did it in black background with silver letters to match the other data plates)

1500 35... 2150 50
1700 40... 2400 55
1900 45 ...2600 60