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Let’s talk rpm and ratios and help with tacho
https://www.auszookers.com/forum/viewtopic.php?f=5&t=57359		 Page 1 of 1
Author:  Shaungti [ Sat Sep 03, 2022 12:34 pm ]
Post subject:  Let’s talk rpm and ratios and help with tacho
Hi guys, just picked up a 94 model soft top with the dohc G13b running 245/60/r15 tyres
Car is in fair condition though the conversion is rough to say the least :?
I’ve got a rather large hole to repair in the firewall haha, and some wiring to be done

What I’m after some help with is whether I’ll need reduction gear’s and what i can expect to see change with them, currently tacho does not work so not sure wHere im sitting with revs
I’ve been offered some 6.5? :1 reduction gears and wondering if that will be beneficial
Author:  Gwagensteve [ Sat Sep 03, 2022 4:29 pm ]
Post subject:  Re: Let’s talk rpm and ratios and help with tacho
If your tyre size information is accurate (245 60 15 is an unusual size, only really for road cars) and the rest of the cars driveline is standard, you're road rpm will be 3560rpm at 100 km/hr, which is basically the same as standard. The car will be find on road pretty average off road, same as a standard Sierra.

However, if you're referring to the car in the photo, those tyres look more like a 245 75 16, which is a much more common size. That would take road rpm to 3215rpm at 100 km/h which is way too low for a 1.3 Sierra, especially with a GTi motor.

Reduction gears would be very beneficial.
Author:  Shaungti [ Sat Sep 03, 2022 8:32 pm ]
Post subject:  Re: Let’s talk rpm and ratios and help with tacho
So I got mixed up between the old spares tyres it had and the ones fitted haha, they’re actually 235/75 15

What would you expect rpm’s around 50-60km/h to be
Author:  Shaungti [ Sat Sep 03, 2022 8:47 pm ]
Post subject:  Re: Let’s talk rpm and ratios and help with tacho
Sorry for the confusion I actually have 235/75/15 on the car the spare wheels that came with it are as above haha

What would the revs be around 50-60km/h
Author:  sideways [ Sat Sep 03, 2022 9:54 pm ]
Post subject:  Re: Let’s talk rpm and ratios and help with tacho
It depends on which 5th gear you have, there's 0.795 and 0.864. I think yours should be 0.795 but anything is possible.

Here's my little chart.

Just drive it and see what you think. You're basically at stock ratio/tyre size for a JA SJ70 but the GTi engine has more power all the way through the curve. That's almost the same gearing as my Sierra and I find it great in high range. Unless you're planning on making it a touring vehicle don't worry that you're going to change the gearing and make the revs to high, you wont be overrevving that engine. With 6.5s your you'll be at 4000rpms at 110km/h, essentially your 5th will be quite similar to your current 4th.

The suitability of 6.5s will depend on your use. If you're driving rocks and ruts and then they will be nice but I'd say a bit overgeared for your tyre size. If you're into sand or mud then I reckon you'll find them too low.
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