Increasing car ground clearance - slightly taller tyres

[daverobev]

I have a driveway that is at a downwards angle and occasionally the bottom of our car touches the lip of it when leaving or entering our property. Tyre pressure is ok, it's just literally the weight difference when there is an extra person or two in the car makes the difference between clearing it or not.

I'm planning to grind off the edge of the concrete lip at some point, but I'm also thinking of getting slightly larger tyres just on the back, based on the following assumptions:

1. Don't need to worry about the speedometer changing because I assume that's measured by the front wheels?

2. There is a larger space between the rear tyres and the wheel arch (9-10cm at rest flat) than there is for the front tyres (about 7cm), plus no risk of rubbing when turning the steering

3. I'm thinking of going 185/50 R16 to 185/55 R16 - which will give me almost 1cm at the back, and hence roughly 5mm in the middle of the car where it touches

Any reason I shouldn't do this? I'll keep the current rear tyres and put them on the front when the fronts are worn out.

[GilesMM]

Can you cross the low spot at a different angle? That usually works for me.

[daverobev]

Can you cross the low spot at a different angle? That usually works for me.

High spot, it's a straight line and we're crossing at 90 degrees already, so unfortunately not.

[RWD]

That is a pretty small change so it'd probably be fine. Potential downsides include reduced fuel economy (bigger tires are heavier), worse braking performance, and handling detriments. Ride quality could be affected too, though hard to say which way.

[daverobev]

That is a pretty small change so it'd probably be fine. Potential downsides include reduced fuel economy (bigger tires are heavier), worse braking performance, and handling detriments. Ride quality could be affected too, though hard to say which way.

The reduction in fuel economy is the biggest issue I guess, as it's an EV. I'm guessing handling and braking would be minimal considering they are the rear pair.

Presumably each tyre isn't going to be more than a few hundred grammes heavier either, which is likely negligible?

I think these cars originally came with 14 inch rims, I'm sure I read somewhere that smaller rims improved range marginally. But obviously I want to raise the car up, and I'm not sure the EV version ever came with anything below 15 inchers (possibly more space needed for larger brakes because being an EV it's heavier than the petrol version).

[Financial.Velociraptor]

Using tires other than the manufacturer specs is most likely going to throw off your alignment.  Be prepared to have weird tire wear shorter tire life, unless you align yearly.

[Paper Chaser]

Is the vehicle AWD? With an ICE with AWD, it's pretty critical to have tire diameter the same on all 4 corners. Most EVs obviously don't have mechanical connections between the wheels, but it could still cause some unforeseen side effects with torque vectoring, cruise control, or other such stuff.

[daverobev]

Using tires other than the manufacturer specs is most likely going to throw off your alignment.  Be prepared to have weird tire wear shorter tire life, unless you align yearly.

Interesting thanks, but why would alignment need changing yearly rather than just initially?

[daverobev]

Is the vehicle AWD? With an ICE with AWD, it's pretty critical to have tire diameter the same on all 4 corners. Most EVs obviously don't have mechanical connections between the wheels, but it could still cause some unforeseen side effects with torque vectoring, cruise control, or other such stuff.

This one is FWD. Foolishly I didn't get cruise control on it (it needed to have parking sensors as well to get that, and it seemed expensive... a mistake on my part).

What I'm hearing is that 'it's not ideal' but most likely 'it's a small change and probably won't cause major problems' (being as it is less than a cm to the radius, of a wheel of 30cm radius, so ~3%?).

I probably won't do it, just grind the edge of the drive down to a nice curve.

[oldladystache]

Could you put a board or something on the driveway to raise the rear of the car while the low part is going over the peak? Does that make sense? Not sure how to explain my idea?

[Financial.Velociraptor]

Using tires other than the manufacturer specs is most likely going to throw off your alignment.  Be prepared to have weird tire wear shorter tire life, unless you align yearly.

Interesting thanks, but why would alignment need changing yearly rather than just initially?

The alignment is engineered with assumed constant pressures delivered at precise constant direction of the applied force(s).  If you change the geometry/pressure, the alignment will start to drift over time.  Driving with one tire severely deflated for an extended period can do the same thing.   

You are basically introducing torque at an angle that is contra where the adjustments are placed to counteract.

[daverobev]

Could you put a board or something on the driveway to raise the rear of the car while the low part is going over the peak? Does that make sense? Not sure how to explain my idea?

Yes, it'd have to be cement though. I'll smooth off the lip a little and see how I go.

It's possible that it's just tyre wear (they have plenty of life left in them, but they have done 20,000km) bringing the car down ever so slightly. Hmm and actually there is some gravel stuff on the pavement/sidewalk that has also generally been swept away over time - I've replenished that somewhat from loose stuff elsewhere which also I think has helped things.

The natural gas pipes were upgraded I guess a couple of years back and they put crappy loose stuff on the sidewalk where previously it was concrete.

Probably a combination of everything.

[daverobev]

Using tires other than the manufacturer specs is most likely going to throw off your alignment.  Be prepared to have weird tire wear shorter tire life, unless you align yearly.

Interesting thanks, but why would alignment need changing yearly rather than just initially?

The alignment is engineered with assumed constant pressures delivered at precise constant direction of the applied force(s).  If you change the geometry/pressure, the alignment will start to drift over time.  Driving with one tire severely deflated for an extended period can do the same thing.   

You are basically introducing torque at an angle that is contra where the adjustments are placed to counteract.

Right so the long and short of it is, either do all four tyres or none, and it'd be best to do none. Ok, message received, thank you.

[Sibley]

This seems like a lot of hassle to workaround the real problem: the concrete has a high spot. Just get it ground down. Probably be a lot cheaper than trying to modify the car.

[Just Joe]

I've changed the tire sizes on cars many times over the years. As long as it is a small difference, you probably won't face any car problems from it. In an EV, you might lose a few KM to inefficiencies. It might even help the ride quality of the car though the handling might be worsened a tiny bit.

I would alter the driveway before I altered the car much though. Change all for tires at the same time.

Our Honda CRV came with 205/70-R15 tires. I changed it to 205/75-R15 about a decade ago with zero problems. On some cars a change like this can cause the tire to rub the chassis when making sharp turns at low speeds. All the driver needs to do is lessen the steering angle with the steering wheel. No harm in the short term. In the long term it could rub the paint off of the chassis leading to a rusty spot.

The change to the CRV was purely aesthetic though b/c I could not see a real functional change. The tires filled the wheel wells slightly better and didn't look so small. I had plenty of room for diameter changes, not as much for width changes. The speedometer read ~3 mph slow since the factory. This change made the speedometer more accurate to actual ground speed. The speedometer reads from a gear or sensor that attaches to the transmission. It won't be aware of you tire size change, only how many times the tires rotate per unit of time. Your phones GPS can show you the difference in actual speed.

This can be useful too: https://www.miata.net/garage/tirecalc.html

It applies to all cars and all tires. 

[ChpBstrd]

I have the same issue because my car has a uhaul trailer hitch which can rub the ground when loaded.

Instead of grinding out the high spot, can you dump a few bags of concrete on the low spot, near the curb? On my driveway, this is where the wheel is located when the frame is dragging on the slope of the driveway. Make that spot higher and it lifts the whole car. Plus the driveway fix applies to all vehicles, present and future, whereas the tire fix applies to one car for the life of those tires.

[JLee]

Using tires other than the manufacturer specs is most likely going to throw off your alignment.  Be prepared to have weird tire wear shorter tire life, unless you align yearly.

Interesting thanks, but why would alignment need changing yearly rather than just initially?

The alignment is engineered with assumed constant pressures delivered at precise constant direction of the applied force(s).  If you change the geometry/pressure, the alignment will start to drift over time.  Driving with one tire severely deflated for an extended period can do the same thing.   

You are basically introducing torque at an angle that is contra where the adjustments are placed to counteract.

Wheels/tires are under tremendously variable forces depending on what's going on -- I'm surprised to read that a small difference in tire diameter would result in drifting alignments.  The only case I'm aware of this being a notable issue is for vehicles subjected to unusual stresses - track cars, offroad driving, etc, where there are extreme forces on the alignment hardware.

[innkeeper77]

I would really be careful here. Maybe replace all four tires- the speedometer will be off but the car will think everything is fine. I do this on my stupid adventure mobile (off road truck) for ground clearance.

If you replace only two wheels, the sensors for traction control etc will be very likely to be VERY unhappy. Semi modern and modern vehicles count the revolutions of each wheel, and this will throw off the programming significantly. Keep all four tires spinning at the same speed while driving straight.

Grind it down, or just make ramps to raise the wheels before you hit! You could even start with just a couple pieces of 2x4 to test the theory.

[Boll weevil]

You should also look into what technology is used for the tire pressure sensors if your car has them. I’ve been told that some of the newer cars don’t have an independent sensor but instead somehow calculate the pressure based on wheel speed and diameter. You may be setting up a scenario where the vehicle is not properly calculating the pressures.

[Just Joe]

We had a couple of old GM products in the family that compared tire revolutions to determine if a tire was low. Used the ABS sensors I think.