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· Master Story Teller & Fabricator
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17,656 Posts
Discussion Starter · #1 ·
I really haven't considered driving my car out to Vegas, but it's likely that such an adventure will be the last of this magnitude I'll ever do. My car is as new and spiffy as you can get.

But I've got my 3sp auto tranny and even though I've also got a 3:18 rear, I'm pretty sure I'll be turning 3500-4000 rpms for the whole 36 hour drive out there, and the additional 2000 miles we'll do while out there, plus 36 hours back. I presently have 205/50 R15's.

How much could I realistically reduce my rpms or increase my mpg by putting the largest diameter rear tires on my 8"x15" rims that I could reasonably fit in the wheel wells?

I did a quick diameter check at these sizes:

205 width x ??? height x 15" rim:

50 = 23.1" diameter(what I presently have)
60 = 24.7" diameter
70 = 26.3" diameter
80 = 27.9" diameter

195 width x ??? height x 15" rim:

60 = 24.2" diameter
70 = 25.7" diameter
80 = 27.3" diameter

This would be a special set up just for this one event.

Could I get a 500 or even a 1000rpm reduction?

Could I reasonably expect to get an extra 5 or more mpg if I went from a 23" tire to a 27" tire?

Git yer calculators out........

:veryhappy
 

· Super Moderator
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15,223 Posts
Gordo, not such a kooky idea. When Opels were my daily drivers I did this often for road trips. I had a set of 175/80-13's which were much taller than the 205/60-13's I normally ran.

Going from 23.1" tires to 26.3" tires is a 14% increase in diameter. You should see a similar rpm decrease.
 

· Registered
Joined
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486 Posts
I really haven't considered driving my car out to Vegas, but it's likely that such an adventure will be the last of this magnitude I'll ever do. My car is as new and spiffy as you can get.

But I've got my 3sp auto tranny and even though I've also got a 3:18 rear, I'm pretty sure I'll be turning 3500-4000 rpms for the whole 36 hour drive out there, and the additional 2000 miles we'll do while out there, plus 36 hours back. I presently have 205/50 R15's.

How much could I realistically reduce my rpms or increase my mpg by putting the largest diameter rear tires on my 8"x15" rims that I could reasonably fit in the wheel wells?

I did a quick diameter check at these sizes:

205 width x ??? height x 15" rim:

50 = 23.1" diameter(what I presently have)
60 = 24.7" diameter
70 = 26.3" diameter
80 = 27.9" diameter

195 width x ??? height x 15" rim:

60 = 24.2" diameter
70 = 25.7" diameter
80 = 27.3" diameter

This would be a special set up just for this one event.

Could I get a 500 or even a 1000rpm reduction?

Could I reasonably expect to get an extra 5 or more mpg if I went from a 23" tire to a 27" tire?

Git yer calculators out........

:veryhappy
Gordon-

Let the year of the Eagles live longer!!! Go for it regardless of the miles, wear and tear, and expense!!! Let the LEGEND OF GORDON FLY!!! Just imagine how bummed you will be after seeing some of the epic pics from those who were lucky enough to go!!
 

· "LT"
Joined
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82 Posts
Gordo you mention the two obvious benefits that you may reap. but the big drawback to larger tires is the inaccuracy reflected in the speedometer (ticket magnet) and a red car too. But I am sure you have taken this into consideration.
 

· Master Story Teller & Fabricator
Joined
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17,656 Posts
Discussion Starter · #7 · (Edited)
The RPM results after entering the following values:

Ring & pinion gear ratio = 3.18
Tire height (in.) = 23.1
Vehicle speed (MPH) = 80mph

Automatic in drive
3799
Automatic in overdrive
2660
3 speed in last (3rd) gear
3700
5 Speed or higher
manual in last gear
2960

Man, I sure wish my new 4sp overdrive tranny was already installed and working. On that note, Charlie tells me that my tranny will arrive at his house this week and I plan on driving down to Virginia this weekend to pick it up.

:banana:
 

· Registered
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2,631 Posts
That is "interesting"! Did I miss the page where the OD ratios were input (as well as the gear ratio for all the other gears)? I would think the 3 speed would be 1.00 (and possibly the 4 speed "last gear" as would the automatic but the RPM is different due to losses in the auto? I'm guessing the software "designer" is NOT a "car guy"!

Doug
 
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