Can this be done without chanding the internal worm? I have a 3.75 diff - the donor had a 3.92 running on 185 65 14 tyres, I worked out that by using
185 70 13 tyres I would have a 2.2 mph over read on the speedo at 3000rpm. I could change to 185 60 13's and read 2mph under.
If I could change to a larger toothed drive, I could guarantee a SVA pass on the speedo, and use the tyres that I want to.
Also anyone know how many teeth as standard an '89 sierra 2.0i would have?
I am also looking into this at present ,as far as i know so far there are two drive worms and a choice of 4 pinions available which are suposedly
interchangable. I think the drive worms are the same diameter but with a different no of starts to them effectively giving a different gear ratio, the
problem is knowing which one you have fitted to start with.
I have been considering jacking up the back of the car and running the engine at a measured RPM and noting the speedo reading and then working out the
mPH per engine revs to give a check of speedo calibration. It would just need an accurrate tacho and a spreadsheet with diff ratio and rolling dia of
tyre.
Paul.
quote:
Originally posted by Mark Allanson
Can this be done without chanding the internal worm? I have a 3.75 diff - the donor had a 3.92 running on 185 65 14 tyres, I worked out that by using 185 70 13 tyres I would have a 2.2 mph over read on the speedo at 3000rpm. I could change to 185 60 13's and read 2mph under.
If I could change to a larger toothed drive, I could guarantee a SVA pass on the speedo, and use the tyres that I want to.
Also anyone know how many teeth as standard an '89 sierra 2.0i would have?
I've got a really nifty spreadsheet, which I modified from one gathered off the net. I put in a trye size calculator and the gear ratio's, and it will give the actual speeds in any gear at any rpm. All I need to know is the number of teeth on the worm & cog - either that or go for 185 60 13's!!
I also have a spread sheet off the net , it gives wheel and tyre sizes and one size of sierra drive gear you can modify it for different ones and i
found with a 3.89 diff and 18570 tyres i require a 25 tooth gear assuming the drive worm is as listed on the spreadsheet. I need a spreadsheet to tell
me MPH per 100 revs with a 3.89 diff and 185-70-13 tyres, i can then do a check to find what speedo drive ratio i have and what to change it to. it is
easy to work out the difference in percent of a 25 tooth gear as opposed to a 22.
Paul.
quote:
Originally posted by Mark Allanson
I've got a really nifty spreadsheet, which I modified from one gathered off the net. I put in a trye size calculator and the gear ratio's, and it will give the actual speeds in any gear at any rpm. All I need to know is the number of teeth on the worm & cog - either that or go for 185 60 13's!!