Low ratio 25% reduction gears vvt auto gearbox qu

howdy.
was wondering if anyone could help me with some things tat are out of my league.

i have a 2012 jimny with an auto,
i have fitted 215 75 r15 bf goodrich km2 muddies,
i primarily use my zook for pig shooting and work, i travel some large kilometers.
and i have lost my top end.

these reduction gears are 25%, is anyone here able to work out what thr rpm will be at 100kmh? (60mph)
will they be to low or just right?

cheers for any help.

Lowering ratios will make it accelerate quicker and easier but at the expense of more top end.

You just don't have the power to do the speeds you did before. Lowering ratios effectively gives you more torque through the wheels but you'll not solve the lack of power by changing gears. You might be lucky and get the car into the right powerband at the right time to help you back to your old top speed but I doubt it.

With an auto or manual I would just accept you lose top speed.

With an auto I would make sure you are cooling the gearbox oil better than standard to avoid overheating it.

Kirkynut

power is a constant, so hasnt changed, the only variable change is the rolling diameter of the slightly larger tires.
the only way to correct it is gearing, producing "more" power will not make up for lack of gearing once larger tires are fitted.
ie, i can run 31's on a sierra with 6.5:1 transfer gears, which will bring it back to stock gearing performance with the bigger tires.

i will get a hold of the gearing calculator and show you what i mean, im terrible at explaining things on the interwebs.

www.1010tires.com/Tools/Tire-Size-Calcul.../205-75R15/215-75R15

than i have to find the gearing calculator and work out the difference once 25% lower reduction gears will do to those figures,
i pressume i will sit at 100kmh at around 3900 to 4000 rpm.

cheers
joel

so i sit on 3100 rpm at 100kmh, multiple that by 125% to accommodate the reduction gears, i will sit at 3850 rpm, a bit high...
maybe worth the risk.

Torque is the force that gets you going and power is the force that keeps you going. So when you are on the motorway going uphill and slowing down you can't keep it going at the same speed and thus don't have enough power.

Yes I agree that if you can bring everything back to factory ratios albeit by reducing the high ratio gearset by the same percentage you increased the tyre diameter you will be in the same position as a standard car. In theory anyway.

I think my mind may be stuck on rocklobsters where the main reduction is in the low ratio only.

You don't need a gearing calculator if the makers tell you it's reduced by a certain percentage, just a pen and paper to work out the maths of the tyre size to match, and perhaps a normal calculator if your maths isn't too good since you left school and stopped doing mental arithmetic like me! It was a long long time ago though!

Kirkynut