My Speedometer is reading way lower than actual. I could really use some advice on changing the drive gear at the transmission before I pick up some tickets and all the BS that comes with it. Indicated speed is about 82 % of actual, for example the speedo reads 45 MPH, but I'm really doing 55. If my speedo says 65 MPH, it's really closer to 84. This could get expensive! As far as I can tell, It appears to be a stock '72 Nova TH 350. The whole drive train may be based on that, but I'm running 27 inch diameter rear wheels/ tires. The axle ratio is unknown. I've got a 22 tooth drive gear. Do I need more or less teeth on the new gear? Any suggestions? What options are availible? Thanks in advance....Joe
You will need a driven gear (the one that the speedo cable attaches to) with less teeth. This will speed up the speedo. If your 20% off then you will need a gear with 20% less teeth. You will be limited to how much fewer teeth are available on the gears. If you can't put one in small enough then you will need to get a drive gear (the one on the output shaft) with more teeth.
Here ya go - http://www.tciauto.com/tc/speedometer-gear-calculator You can get the axle ratio close by marking the tire and drive shaft and spinning the tire one revolution while counting the drive shaft revolutions
Easy to change out, I suggest you take out the driven gear (speedo cable connection one) and shine a flashlight in there to look for color (both drive and driven) You then need to measure your tire size or just go online and determine it by the online sites. There are tons of speedo calculators out there. Try and get a gear combination that allows for some additional tweeking the driven gear high and low if you want to fine tune it. You should be able to get all the parts before taking the tail housing off. I got my parts at a trans shop (old set). On the drive gear there is a funny "J" clip that seems to get easily lost.
As above, But you need a bit more information about whats already in there. You need to raise your speedo ratio, but there is inconsistancies with your numbers. At 55 it's showing 45 which is 22% too slow[82%], but at 84 its showing 65 which is 29% too slow [77%]. Ratios are consistant [ or linear ] so the percentages shouldn't change You need to get the number more accurate [ try a GPS ] Here's a link to how to calculate and get the correct ratio if you already know what the existing ratio is. The calulation method is the same but you need to reverse the numbers to speed it up http://www.jalopyjournal.com/forum/showthread.php?p=7720853#post7720853
Mine does the same thing!! I can't seem to find a gear with less then 15 teeth, i think i need a 13. I just keep a tomtom in the car & use that for a speedo to calibrate speed at times!!lol Jimv PS on another note, i've been driving my car for 14 yrs and it has 61,000 miles on it, but also reads 1 mile when i go .8 of a mile so does that mean i have 72,000 miles on the car? ( i think thats how its reading, my speed is higher then my speedo says)
You're off 20%. T-350s are the easiest trans to dial in the speedo, lots of gear ratios, but some times you need one of those expensive custom adapters done up by a speedo shop.
If the magnet drive in the speedometer head is weak, the ratio can be off the faster you go. A better way to check is to use the odometer, its gear driven and the ratio will never change. Drive a steady speed on a marked highway, 10 miles or more. that will give you enough information to change out the gears. Joe
Thanks to all of you HAMBERS for the FAST responses! The car is up on the hoist for a lot of work to do to get it ready for the season, so the fast help really is great! Special thanks to Lubocrod for getting the basic "More Teeth or Less Teeth" concept straight in my head. And Huge thanks also to Dane for the TCI calculator link that had not just math but actual part numbers and colors! Now I can buy what I need and get the job done pronto. Aquarosco: I appreciate the tip to look inside with a flaslite. I'll be able to keep the Drive gear in the tranny as is. Kerrynzi/ Mike Hohnstein? Joe H: Thanks for the other tips including the GPS checking. Good useful stuff! I'll use it all. Once again, thanks for the timely help! Joe
I get the gears set up so the odometer reads correct, then if needed I play with the speedo calibration, there is a spring anchor adjustment you can make as well as adjusting where the needle points.