a couple questions...

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?

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 )"

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

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.

okay, so it IS possible to calibrate it somehow. im guessing its cable driven....its an 85 truck.

http://www.ringpinion.com/Calculators.aspx

theres an rpm and tire height calculator there on randy's ring n pinion site.

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...

also, that site says "fourth gear in a five speed" i take it, thats the same as 4th in a 4 speed?

Edited: 4/15/2008 6:46:30 PM by bagged85

yes, 5th in a 5 speed is just overdrive.

i think im safe around 3200 or so. its on the upper end of their ideal towing range.