RPM-Speed Relationship
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david_lee
Full Access Member
another really easy method of checking the rear gear ratio is to put a mark on your drivers rear tire and tie or tape a rope to the front of your drive shaft and lay it under the truck in a straight line out the back lined up with your driveshaft. then simply drive forward in a straight line watching the tire mark until your tire does a full revolution. put it in park and count the turns of the rope on the shaft. no jack required and is just as accurate.
MIDNIGHT RIDER
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Thanks for actually doing the math.
I can only speak for the after-market Autometer tach that has a sender that fastens onto the alternator.
The one I have is infinitely fine-tunable to match what the tach shows, be it fast or slow, to actual engine RPM.
That type of tach can be a little or a lot "fast".
Simp5782
SNOW TACKLER!
If your speedo doesnt work, and ur cant find a gps... just find someone with the new civics with the indash speedo that way u can look at it while u drive... did that in mine. just gotta catch em lol. I was turning 2600 almost in 5th with 4.10s and 245/75/16s.... now im running 3.55s and 235/85s and its like 2000 at 70.
just gotta catch em lol. I was turning 2600 almost in 5th with 4.10s and 245/75/16s.... now im running 3.55s and 235/85s and its like 2000 at 70.
icanfixall
Official GMM hand model
took you 12 miles to get that last 10 mph? lol also your not up to temp yet mister.....
icanfixall
Official GMM hand model
Wow..... Somebody is really "looking closely" at how long it took me to reach the faster speed..... Actually I was driving home from the last bar b que at Calvins (towcats). I was watching for the local chps that hide under overpass and behind them too. They love to hide and tag you for anything...... It really doesn't take 12 miles to increase 10 mph in my rig....
I have seen the speedo needle point almost back to the 15 mph mark several times.... Engine gets real quiet at whatever spped that is.... Really smooth too..... No feelings of unstability either.....
As for coolant temp... I was running at 190 degrees... You know those factory gauges can't be trusted... They are probably made by the French....
Actually I was driving home from the last bar b que at Calvins (towcats). I was watching for the local chps that hide under overpass and behind them too. They love to hide and tag you for anything...... It really doesn't take 12 miles to increase 10 mph in my rig.... I have seen the speedo needle point almost back to the 15 mph mark several times.... Engine gets real quiet at whatever spped that is.... Really smooth too..... No feelings of unstability either..... As for coolant temp... I was running at 190 degrees... You know those factory gauges can't be trusted... They are probably made by the French....
shall i scare u now???? i have had my rig with the big tires on it, up to 105mph. the needle was pointing a little past straight down. had a buddy follow me to see how fast it actually was. mine was surprisingly stable at that speed. havent done it since though. lol
Fordsandguns
Fords Rule
That's definitely getting up there. I know 85 in the Moose Truck feels like your about to break the sound barrier, and you really start thinking about the weight of that rubber whizzing around in your wheel wells. 
I wonder how many pounds a 1 foot square cross section of 36" X 12.5 tire weighs rotating at 66 RPM?
I wonder how many pounds a 1 foot square cross section of 36" X 12.5 tire weighs rotating at 66 RPM?
Did you also notice the fuel gage in those 12 miles? Looks like it's really sucking up the fuel.
no, that was still longbed. since it was shortened i havent had it on the highway yet. driveshafts are homemade and a little outa balance. in the process of getting ones made now. might acutally drive it to the rally.... maybe.
That's definitely getting up there. I know 85 in the Moose Truck feels like your about to break the sound barrier, and you really start thinking about the weight of that rubber whizzing around in your wheel wells.
i know them tires are HEAVY!. never weighed them but they weigh more than the 40's i have now.
OkieTruckerD
A Diesel Nut
To attach on the subject, my truck has 265/75R16, 3.55 rear end, and ZF5 box. my tach registers 2200 @ 65, approx. 2050 @ 60. What is the OD ratio, and is the speed/rpm reading accurate? Sorry for piggybacking the subject, instead of answering the current issue, I'll take the cane flogging shortly
Brianedwardss
Registered User
No Prob, I'll see if I can help.
265/75R16= 31.59" tire
31.59" tire = 639 Revolutions per mile.
639 X 3.55:1 rear end = 2268.45 driveline revolutions
2268.45 Revolutions X 0.76 ZF5 Overdrive ratio = 1724.022 This is your engine RPMs at 60 MPH. So...
RPM's at 60 MPH = 1724.022
RPMS at 65 mph = 1867.690
I see you did a ratio change in your rear-end. After going from 4.10 to 3.55, your speedo should've registered slow, very slow... Did the plastic speedo gear get changed in the tailshaft housing after the swap?
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