need crx tire advice

[bwilson4web]

Originally Posted by larrybuck

I have an '87 CRX. I have 175/70/13's now.
. . . frustrated that I'm turning 2,200-2,300 rpms at a mere 50mph.
. . . I don't have the scratch for high dollar LR tires,
. . . I need at least 2 better tires to even be safe.
. . . a size common enough to be easily found used anywhere!

I've tried to reduce your requirements to a common set.

Sumitomo T4, low rolling resistance tires, Tire Rack price does not include shipping, mounting and balancing.

175/70R13 - 1,036 lbs - 51 psi - 927 rev/mile - $52/ea
185/70R13 - 1,135 lbs - 51 psi - 907 rev/mile - $60/ea

If you go with oversized, Sumitomos, you need to read the specs on the rim width to match your wheels; the tread width; and the overall diameter. Take a string and ruler and measure where your current tires sit relative to the wheel well. Make sure there is enough clearance before buying.

If there is any question, consider buying one tire to mount on a spare wheel and then put it on the rear and test drive over some speed bumps. Then swap to the front and again test over some speed bumps. The one tire test approach is somewhat expensive because if you by a second one, more shipping cost. Worse, if the tire doesn't work out, getting credit back will entail some loss. But then you'll know for the second one.

Double check the Sumitomo T4 reviews against your likely driving. They are not listed as being good on ice and snow but that is what winter tires and chains are for.

Having two tires that won't pass inspection suggests there may be an alignment problem. Start with an analysis of the wear pattern and follow-up with a measurement and alignment. Also, find out if your car has adjustments for:

• front toe - pretty much universal
• front camber - ?? sometimes requires a camber bolt
• rear toe - ?? fixable with metal tabs or EZ-Shim
• rear camber - ?? fixable with metal tabs or EZ-shim

If you do get an alignment, get a copy of the new values and start a fresh thread about '4 wheel alignment.' This is a different problem from the tires.

There are web sites that claim the best tires should be on the rear to prevent the car from spinning out in marginal conditions. However, I found that having a pair of different sized tires, they should be tested in one configuration and then swapped, front-and-rear. Check the handling and maximum effort braking in sane conditions and choose what works for you.

GOOD LUCK!
Bob Wilson

[California98Civic]

Originally Posted by bwilson4web

Sumitomo T4, low rolling resistance tires, Tire Rack price does not include shipping, mounting and balancing.

175/70R13 - 1,036 lbs - 51 psi - 927 rev/mile - $52/ea
185/70R13 - 1,135 lbs - 51 psi - 907 rev/mile - $60/ea

So the 185's rev a little more than 2% less than the 175s? That should mean the difference between these two tires in terms of MPG at 5th gear highway speeds would be a little over 1% maybe? In a car like an 80's CRX that's possibly a difference of 1.5-2.0mpg at highway speeds, no? If true and the 185s fit my CRX, that's what I would buy. SentraSE-R and others, such as arcosine, seem to have found that despite the larger sizes (worse aero) and heavier weights, that the taller gearing is a net benefit (on the highway, at least).

[bwilson4web]

Originally Posted by California98Civic

So the 185's rev a little more than 2% less than the 175s?

You mean fewer revs and the correction should be relative to the stock tire, assuming it is the 175/70R13:

(907-927)/927 = -2.16%

Originally Posted by California98Civic

That should mean the difference between these two tires in terms of MPG at 5th gear highway speeds would be a little over 1% maybe?

Only if GPS speed limits are being used. Changing the tire size also impacts distance measured and indicated speed. A slower turning tire will lead to a lower indicated speed so drivers are tempted to drive a little faster and into higher, aerodynamic drag.

Originally Posted by California98Civic

In a car like an 80's CRX that's possibly a difference of 1.5-2.0mpg at highway speeds, no? If true and the 185s fit my CRX, that's what I would buy. SentraSE-R and others, such as arcosine, seem to have found that despite the larger sizes (worse aero) and heavier weights, that the taller gearing is a net benefit (on the highway, at least).

I'd have to see their data but I don't see anything wrong as long as the same-speeds were used.

Bob Wilson

[Ryland]

Of course the stock size tire for the CRX HF is the 165/70 R13 and I've run 155/80R13 tires with good results, after all the car hardly weighs 2,000 pounds.

[bwilson4web]

Originally Posted by Ryland

Of course the stock size tire for the CRX HF is the 165/70 R13 and I've run 155/80R13 tires with good results, after all the car hardly weighs 2,000 pounds.

This makes it even better:

165/70R13 - 0,937 lbs - 51 psi - 949 rev/mile - $54/ea
175/70R13 - 1,036 lbs - 51 psi - 927 rev/mile - $52/ea - -2.32%
185/70R13 - 1,135 lbs - 51 psi - 907 rev/mile - $60/ea - -4.23%

If the 185/70R13 fits, there is potentially a 4% reduction in engine rpm.

Bob Wilson

[California98Civic]

Originally Posted by bwilson4web

This makes it even better:

165/70R13 - 0,937 lbs - 51 psi - 949 rev/mile - $54/ea
175/70R13 - 1,036 lbs - 51 psi - 927 rev/mile - $52/ea - -2.32%
185/70R13 - 1,135 lbs - 51 psi - 907 rev/mile - $60/ea - -4.23%

If the 185/70R13 fits, there is potentially a 4% reduction in engine rpm.

Bob Wilson

So at a steady speed comparison between the current 165s and a new set of 185s, with a 4.23% reduction in RPM, you might get an improvement of 2+mpg (again, assuming a consistent highway speed comparison of the tires). To minimize possible increased aero drag you could build tire spats for front and back tires. To minimize the small weight increase you could find a little weight in the car to remove.

Oh! and if these LRR tires are replacing high RR tires, the benefit should be much greater than 2mpg.

[bwilson4web]

Just remember that a larger diameter tire makes the speedometer show a lower speed than true and aerodynamic drag works on true speed. So at 50 mph true, the speedometer would show 50 * -4% = -2 mph or 50 - 2 = 48 mph. The driver would be tempted to run at an indicated 50 mph and be doing a true, 52 mph.

The only way larger diameter tires work is to use a GPS speedometer instead of the vehicle speedometer. Use sticky-tape over the speedometer to make sure the GPS reference is used.

Bob Wilson

[California98Civic]

Originally Posted by bwilson4web

The only way larger diameter tires work is to use a GPS speedometer instead of the vehicle speedometer. Use sticky-tape over the speedometer to make sure the GPS reference is used.

The Odometer would be off too, no? But can't they be recalibrated by changing the gear? Or I wonder if it is even possible just to create a new speedometer dial using a computer paint program of some sort and overlay it onto the OEM, having "calibrated" it to show what you need.

[bwilson4web]

What I do is use a calibration factor when I enter my miles and gallons. I know my larger tires need 106% to calculate the true distance. So I enter the corrected value in the miles but indicated miles and odometer in the comments.

In aviation, we had a magnetic compass correction card, usually just a few degrees, but over five hours at 120 mph, a few degrees is landing at the wrong airport. <grins>

Bob Wilson

[hondaguy72]

I own an 84 dx1300. Can confirm it's a 5 speed transmission with tall gearing. The trans is out right now being reconditioned for re-installation.

Here's a build link
1984 crx project - GasSavers.org - Helping You Save at the Pump Hypermiling and Fuel Efficiency Forum

Many tire shops carry inventories of used tires in good condition for cheap. Check around. Your gearing is taller than the si transmission, but would be improved with a 84 dx1300 transmission if you can find one.

Some useful info:
CRX Specs & Performance

These transmissions only weigh about 70lbs, so it can be swapped in a driveway with jack stands. Keep a look out on car-part.com! If you can get the money together for really good tires, the Michelin Destiny is a good choice. The best part is that all the tires in this size are very light to begin with. There would be less benefit to a LRR 13" due to this fact. Higher rated sidewall pressures are a good specification to look for.