A different gear tire question
Thread Starter
JK Freak
Joined: Mar 2010
Posts: 517
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From: Encinitas, CA
A different gear tire question
I have a 08 4dr rubicon with 4:10 gears and auto I have researched the charts until i dream numbers, My question is if I go with a 35 s tire nominally 34.4 " what will my rpm be on a flat freeway at 75 ,mph. how can you determine it the rpm for different speeds.
Who is running 35/s without changing gears ratios? how bad is This only applies to 3.8 auto rubicons with 4:10 gears .. Thanks
Who is running 35/s without changing gears ratios? how bad is This only applies to 3.8 auto rubicons with 4:10 gears .. Thanks
JK Super Freak
Joined: Aug 2009
Posts: 1,821
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From: Wildomar ca
I have a 08 4dr rubicon with 4:10 gears and auto I have researched the charts until i dream numbers, My question is if I go with a 35 s tire nominally 34.4 " what will my rpm be on a flat freeway at 75 ,mph. how can you determine it the rpm for different speeds.
Who is running 35/s without changing gears ratios? how bad is This only applies to 3.8 auto rubicons with 4:10 gears .. Thanks
Who is running 35/s without changing gears ratios? how bad is This only applies to 3.8 auto rubicons with 4:10 gears .. Thanks
I ran 35s with 4.10 for a few years and didnt drive me nuts until I went 37s and knew I had to regear and man what difference if I knew what I know now I would have regear with the 35s
I just re read your post I don't have a rubi if that matters to you
Forum Tech Advisor
Joined: Jan 2008
Posts: 1,208
Likes: 11
From: Billings, MT
Transmission Ratio RPM
Enter R&P Ratio, Tire Height, Speed (mph), and Transmission Ratio, then click Solve to calculate RPM
http://www.ringpinion.com/Calculators/Calc_RPM.aspx
The actual tire size will be less than 34.4". It will probably be 33.75", or at most 34".
Your 4th gear transmission ratio is 0.69.
Since you have a Rubi, you want 5.38 gears. They will put you between 2751-2776 rpm at 75 mph. You'll get better fuel economy because your engine will produce enough power to avoid frequent transmission downshifting.
If you live at sea level, don't have a heavy rig, and don't drive hills or mountains, you can get by with 5.13s. That would put you at about 2624-2647 rpm at 75 mph.
Enter R&P Ratio, Tire Height, Speed (mph), and Transmission Ratio, then click Solve to calculate RPM
http://www.ringpinion.com/Calculators/Calc_RPM.aspx
The actual tire size will be less than 34.4". It will probably be 33.75", or at most 34".
Your 4th gear transmission ratio is 0.69.
Since you have a Rubi, you want 5.38 gears. They will put you between 2751-2776 rpm at 75 mph. You'll get better fuel economy because your engine will produce enough power to avoid frequent transmission downshifting.
If you live at sea level, don't have a heavy rig, and don't drive hills or mountains, you can get by with 5.13s. That would put you at about 2624-2647 rpm at 75 mph.