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Ask A Pro \  a couple questions...

a couple questions...

Ask A Pro Q & A
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replies 11
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granth   +1y
Hey Max....and anyone else that can help me. I put some wheels and tires on my truck that are smaller than my stocks were, and I was wondering if there was any way to figure about where my rpm's are at when I am going down the highwy. I know a tac would tell me lol, but I dont have one, and not where I want all new gauges. The miata.net calculator tells the revs/mile, but is there a way to actually tell anything from that?

Also, is there a way to calibrate the speedo, rather than doing gears or a taller tire to get the speed back to where it should be? Was told you can do that, but not sure?
TwistedMinis   +1y
Russ posted this a while back and I saved it. It may help you. Multiplying your tire diameter by pie, is also finding the circumference.

"With a tire that is 30.08 tall ... you multiply that by 3.1415927 ( Pie ) and get 94.499. That is roll out of the tire. ( how many inches it is around ... ) A mile is 5280 feet ... so multiply that times 12 ( inches in a foot ) and you get 63,360 inches in a mile. Divide the 63,360 by the tire rollout ( 94.99 ) and you get 667 revolutions of the tire in one mile. You have a 3.89 gear so multiply the 667 X 3.89 and the driveshaft turns 2,594.63 times in a mile The speedometer is supposed to turn 1000 times in a mile so you need a 2.59 to one ratio ... ( 2,594 divided by 1000 ) for the speedometer to be correct. IF the drive gear is a 7 ... (7 X 2.59 = 18.13 )"
granth   +1y
Edited: 4/15/2008 5:19:03 PM by 72bumpside

so it would need gears? o, and do u know stock ratios for te 2wd trucks? 4 speed 22r
bagdb2200   +1y
If you have a cable driven speedo you'll need to regear the speedo cable to match the new tire size. If you have an electric speedo you'll need to have it recalibrated.
granth   +1y
okay, so it IS possible to calibrate it somehow. im guessing its cable driven....its an 85 truck.
bodydropped85   +1y
http://www.ringpinion.com/Calculators.aspx

theres an rpm and tire height calculator there on randy's ring n pinion site.
granth   +1y
thanks for the info seth, and you too james, for the link.

seth, is there any chance u know redline on my truck? it sows 3200 rpm @ 70 mph and thought that sounded pretty good...
granth   +1y
also, that site says "fourth gear in a five speed" i take it, thats the same as 4th in a 4 speed?
bodydropped85   +1y
Edited: 4/15/2008 6:46:30 PM by bagged85

yes, 5th in a 5 speed is just overdrive.
granth   +1y
i think im safe around 3200 or so. its on the upper end of their ideal towing range.